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The Kenyan hippo
Kanga, Erustus Mutembei
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The Kenyan Hippo
Population dynamics, impacts on riparian vegetation and
conflicts with humans
The research for this thesis was carried out in the Community and Conservation
Ecology Group at the Center for Ecological and Evolutionary Studies (CEES) of
the University of Groningen, The Netherlands
Lay-out and figures:
Cover:
Photos:
Printed by:
Dick Visser
Erustus Kanga
Erustus Kanga, Felix Micheni (p. 18)
Van Denderen BV, Groningen
ISBN: 978-90-367-5132-2
ISBN: 978-90-367-5133-9 (digital version)
RIJKSUNIVERSITEIT GRONINGEN
The Kenyan Hippo
Population dynamics, impacts on riparian vegetation and
conflicts with humans
Proefschrift
ter verkrijging van het doctoraat in de
Wiskunde en Natuurwetenschappen
aan de Rijksuniversiteit Groningen
op gezag van de
Rector Magnificus, dr. E. Sterken,
in het openbaar te verdedigen op
vrijdag 14 oktober 2011
om 14.30 uur
door
Erustus Mutembei Kanga
geboren op 23 december, 1969
te Meru, Kenia
Promotor:
Prof. dr. H. Olff
Co-promotor:
Dr. J.O. Ogutu
Beoordelingscommissie:
Prof. dr. A.J. Hester
Prof. dr. H. de Kroon
Prof. dr. J.P. Bakker
Contents
Chapter 1
General Introduction and Study Rationale
7
Chapter 2
Human-hippo conflicts in Kenya during 1997-2008: Vulnerability of a
megaherbivore to anthropogenic land use changes
19
Chapter 3
Population trend and distribution of the Vulnerable common hippopotamus
Hippopotamus amphibius in the Mara Region of Kenya
33
Chapter 4
Hippopotamus and livestock grazing near water points: consequences for
vegetation cover, plant species richness and composition in an
East African Savannah
47
Chapter 5
Hippopotamus and livestock grazing: influences on riparian vegetation and
facilitation of other herbivores in the Mara Region of Kenya
67
Chapter 6
Hippopotamus as agents of change on riparian-edge communities: A synthesis
87
References
97
Summary / Samenvatting
111
Acknowledgements
121
List of publications
125
Affiliation of co-authors
126
Curriculum Vitae
127
Chapter one
1
General Introduction and Study Rationale
Introduction
Many countries in sub-Saharan Africa host large and world-famous conservation
estates some of which are hotspots of unique biodiversity and ecosystem processes
and offer excellent opportunities for ecotourism. Nonetheless, numerous changes
have markedly modified both protected and non-protected areas in many of these
countries in recent decades with significant consequences for biodiversity conservation and human well-being. Marked increases in human populations, shifts in
cultural traditions and land tenure arrangements and rising demand for land for
human settlements, crop and livestock production and other uses have fundamentally altered land use patterns (Meyer and Turner 1994; Foley et al. 2005). The
impacts of these land use changes are further compounded by climatic changes due
to global warming, thereby posing major challenges to efforts to conserve and
sustain biodiversity, key ecosystem processes and services in protected and nonprotected areas, especially in dryland savannas (Ogutu and Owen-Smith 2003,
Holmgren et al. 2006; Li et al. 2006). These changes often have asymmetrical effects
on neighboring protected and non-protected ecosystems and thus alter the nature
and strength of critical ecosystem connectors and flows.
Marked impacts of land use changes are evident in Kenyan ecosystems regardless of their conservation status and include loss of wildlife habitats in savanna
rangelands, degradation, fragmentation and loss of wildlife migratory corridors,
restricted access to water, spiraling human-wildlife conflicts, loss of biodiversity
and declining wildlife abundance (Prins 1992; Verlinden 1997; Homewood et al.
2001; Serneels and Lambin 2001). The Masai Mara Region of Kenya (Mara), which
forms the northernmost part of the Serengeti-Mara Ecosystem, exemplifies
savanna rangelands experiencing major perturbations in recent decades. Although
the Maasai and their livestock have long coexisted harmoniously with wildlife in
the Mara; that harmony is now being increasingly threatened by human population
growth and expansion of settlements, diversification of livelihood options, privatization of formerly communally owned lands, among other changes (Lamprey and
Reid 2004; Ogutu et al. 2009). The changing land tenure has encouraged cultivation,
permanent settlements, including development of urban centers, and intensification of land use with the result that traditional wildlife habitats have contracted
and wildlife numbers and diversity declined (Thompson and Homewood 2002;
Ottichilo et al. 2001; Serneels and Lambin 2001).
African savannas support diverse assemblages of indigenous large herbivores
supplemented by livestock (Skarpe 1991) whose grazing and browsing activities
shape many ecosystem processes, including creating and maintaining spatial
heterogeneity in landscapes (McNaughton 1984, 1985; Illius and O’Connor 2000;
Ogutu et al. 2010). Human activities and location of water sources additionally
influence ecosystem structure and function, in particular the spatial and temporal
8
Chapter 1
distribution of wild herbivores and livestock (Western 1975; Verlinden 1997, de
Leeuw et al. 2001; Redfern et al. 2003; Ogutu et al. 2010). This is especially true
for the Mara, in which water availability becomes progressively limited and water
sources become points of contacts and conflicts between wild herbivores, the
pastoral Maasai and their livestock during the dry season. However, although the
Serengeti-Mara is the most well studied savanna ecosystem in Africa in both its
ecological and human dimensions, with detailed background knowledge available
on community and ecosystem processes, the ongoing sendentalization of the
pastoral Maasai and increased livestock concentrations, especially around water
sources, have exposed glaring gaps in our knowledge and understanding of how
these processes alter biodiversity, particularly in riparian-edge habitats, that are
key resources zones during dry seasons.
Within the riparian habitats of the Mara, the common hippopotamus Hippopotamus amphibious Linnaeus 1758 and livestock are the main resident grazers. Due
to their strong water dependence, both hippos and livestock heavily utilize the
riparian habitats year-round and so have the potential to compete for grazing
resources. Furthermore, because hippo and livestock grazing can differentially
modify vegetation structure, they may have contrasting effects on plants and other
herbivore species (Lock 1972; Walker et al. 1987; Harrington et al. 1999;
Eltringham 1999; Fleischner 1994; Thrash 2000).
I thus undertook this study to analyze (1) human-hippo conflicts in Kenya over
the 12-year period covering 1997-2008, (2) hippo population dynamics in the Mara
during 1958-2006 and (3) the pattern and consequences of hippo and livestock
grazing in the riparian habitats of the Mara on vegetation structure, species richness and composition and herbivore abundance and diversity under protection in
the Masai Mara National Reserve and traditional pastoralism in the adjoining
pastoral ranches.
Mara Region
The riparian habitats that formed the main focus of this study are part of the Mara
River Basin that covers some 13,750 km2, and is located between 37.78°E and
0.43°S in southwestern Kenya and 33.78°E and 1.48°S in northern Tanzania. The
basin is shared between Kenya and Tanzania, with an upper basin area of about
8,941 km2 (65%) in Kenya and a lower basin area of about 4,809 km2 (35%) in
Tanzania (WREM 2008). The basin is the catchment area for the Mara River and is
also an important habitat for people and wildlife (Fig. 1.1). The basin is characterized by a diversity of land use patterns ranging from natural forests in the upper
reaches to large-scale mechanized farms, smallholder subsistence farms, communal
pastoral grazing lands, protected open savanna grasslands and wetlands. The upper
Introduction
9
AFRICA
N
50 km
Figure 1.1 Site map of the trans-boundary Mara River Basin, showing the Mara River with its
tributaries.
reaches of the basin is at about 2,915 m asl, and comprises escarpments that constitute the Mau Forests in Kenya, receiving an average of 1,400 mm of rainwater
annually. This rainwater infiltrate through the soil into Enapuyapui swamp to form
the Nyangores and Amala rivers, that are the source and only perennial tributaries
of the Mara River (WREM 2008; Fig. 1.1). The Mara River meanders through
Maasai pastoral ranches and the Masai Mara National Reserve (MMNR). In the
MMNR, two other main but seasonal tributaries, the Talek and the Sand Rivers,
join the Mara River. The mainstream Mara River continues flowing through the
savannah grasslands of the northern Serengeti National Park (SNP) before entering
the Mara Swamp and discharging into Lake Victoria (Fig. 1.1). Thus, the Mara River
is part of the Lake Victoria drainage system and the greater Nile River Basin.
The lowlands and wooded savannah grasslands that form the Maasai pastoral
ranches, MMNR and northern SNP receive an average annual rainfall ranging from
about 500 mm in the southeast to 1200 mm in the northwest (Norton-Griffith et al.
1975), hence the flow of Mara River provides the only permanent source of surface
water for the Maasai pastoralists, their livestock and wildlife. In addition, the Mara
River sustains one of the greatest spectacles of the natural world; the annual
10
Chapter 1
migration of wildebeest Connochaetes taurinus Burchell 1823, Burchell’s zebra
Equus burchelli Gray 1824 and Thomson’s gazelle Gazella thomsoni Günther 1884
that arrive in the Mara Basin during the dry season in search of water and forage
(Pennycuick 1975; Maddock 1979). Consequently, the Mara River also sustains a
thriving tourism industry built around this natural phenomenon. In addition, the
Mara River supports livelihoods of the pastoral communities and their livestock,
and is an important shelter and day-living habitat for the common hippopotamus
resident in the Mara River Basin. However, the basin is facing serious and
numerous environmental problems primarily created by wide spread encroachment
on protected forests and the savanna rangelands for settlement and cultivation
(Mutie et al. 2005; Mati et al. 2008). These specifically include: (i) deforestation
resulting from encroachment and human settlement in the Mau forests; (ii) soil
erosion and high sediment loads in the Mara River; (iii) human-wildlife conflicts
resulting from small and large-scale farming that has extended into wildlife range
and corridors; (iv) increased frequency and intensity of floods and droughts due to
climate variability and land use change; (v) declining water quality and quantity
due to poor agricultural practices and excessive water abstractions.
The focal species
The common hippopotamus, commonly referred to as hippo, is an unmistakable
species, with a barrel-shaped, almost hairless body weighing about 1500 to 3000 kg
(Kingdon 1982; Eltringham 1999). It is adapted to semi-aquatic habitats, and therefore it is never found far from water. Hippos have featured in human affairs since
at least the time of Pharaohs, where they were venerated as gods and have been
portrayed in art down the ages (Eltringham 1999). Therefore, it is surprising that in
an intensely studied savanna like the Serengeti-Mara ecosystem, the hippo has
largely been overlooked (see Sinclair and Norton-Griffiths 1979; Sinclair and Arcese
1995; Sinclair et al. 2008). Nonetheless, the hippo is an exceptional megaherbivore
(Owen-Smith 1988) within the Mara River Basin, and it differs from other megaherbivores in having a dual requirement of a shelter and day-living space in water and
an open grazing range often visited at night (Eltringham 1999).
In coining the term megaherbivore, Owen-Smith (1988) grouped together all
terrestrial large herbivores with an adult body weight greater than 1000 kg. Other
megaherbivores within the Mara include the African Elephant Loxodonta africana
Blumenbach 1797, Black Rhino Diceros bicornis Drummond 1826 and giraffe
Giraffa camelopardalis Linnaeus 1758. Owen-Smith (1988) emphasized that the
large body size of megaherbivores renders them largely immune to non-human
predation and they can generally tolerate food of a lower quality than that required
by other herbivores. Owen-Smith (1988) further suggested that megaherbivores
Introduction
11
would therefore be less affected by predation or environmental fluctuations like
drought and that their populations would be maintained at high densities causing
heavy sustained impacts upon their environments. Nevertheless, hippos maintain
short grazing lawns in areas that they graze, suggesting that they also enjoy high
quality food. In addition, hippos can be vastly affected by environmental fluctuations especially prolonged droughts and above-average rainfall (Marshall and Sayer
1976; Smuts and Whyte 1981).
Historically, hippos were found throughout sub-Saharan Africa, but most populations have greatly declined in size while others have disappeared, with the largest
populations remaining in East Africa (Eltringham 1999; Lewison and Oliver 2008).
Although not strictly nocturnal, hippos typically forage for food at night, and spend
the day digesting their food, sleeping and socializing (Klingel 1991). Hippos have a
chambered stomach and are referred to as ‘pseudo-ruminants’, and their digestive
system can effectively ferment grasses and other low quality foods (Eltringham
1999). Various studies claim that their diet consists predominantly, or solely, of
grasses (Field 1970; Oliver and Laurie 1974; Mackie 1976; Scotcher et al. 1978;
Kingdon 1982; Eltringham 1999) but recent studies have challenged this assumption and described their diet as variable (Mugangu and Hunter 1992; Boisserie et al.
2005; Cerling et al. 2008), with a few incidences of scavenging on carcasses
reported (Dudley 1996). The primary threats to hippos are loss of essential grazing
lands to cultivation and encroaching human settlement, unregulated or illegal
hunting (Weiller et al. 1994; Eltringham 1999; Williamson 2004; Conservation
2006; Lewison and Oliver 2008) and the growing pressure on fresh water resources
across Africa (WWC 2003) that has often resulted in loss of their habitats. Due to
increasing human population and agricultural expansion and development in and
around wetlands, hippos often run into frequent conflict with people. In addition,
hippos have notorious crop-raiding tendencies and can extensively damage the
range through grazing and trampling when they occur at high densities (Lock 1972;
Thornton 1971; Mkanda and Kumchedwa 1997; Eltringham 1999). The protection
of riparian and wetland habitats is currently therefore a pressing priority for
hippopotamus conservation, including measures to prevent the drying-up of water
courses and loss of riparian-edge grazing ground.
Hippos are important to the ecology of permanent wetlands in Africa and exert
significant environmental impacts (Olivier and Laurie 1974). Their regular movements from water pools used during the day to adjacent grazing areas utilized at
night create trails, modify river channel geomorphology, and assist in developing
micro-topography (Naiman and Rogers 1997; McCarthy et al. 1998; Deocampo
2002), with consequences for other organisms. Their trampling and grazing impacts
directly or indirectly control the availability of resources for other organisms along
the riparian habitats, through physical modification, maintenance, or creation of
micro-habitats. At moderate densities, hippos can be beneficial to the ecology of an
12
Chapter 1
area through their maintenance of habitat mosaics while at high densities they can
cause intense grazing, soil erosion and may decrease plant and other wildlife abundance and diversity (Field 1970; Thornton 1971; Lock 1972; Olivier and Laurie
1974; Eltringham 1999; Arsenault and Owen-Smith 2002). In addition, the short,
hippo-grazed grasses along the riparian habitats create biological barriers to fire,
further influencing biodiversity of riparian communities.
Hippos pond and create water pools that are important refugia for aquatic
organisms. They stir the water, prevent development of anoxic conditions, while
their dung fertilizes water, and alter the dynamics of nutrients and particulate
matter, promoting primary production, especially fish life which in turn feed crocodiles and birds and numerous other aquatic organisms (Naiman and Rogers 1997;
Gereta and Wolanski 1998; Wolanski and Gereta 1999; Mosepele et al. 2009). Thus
even though they are less studied in the Serengeti-Mara ecosystem, hippos may
have significant non-trophic impacts on the structure, function and biodiversity of
this ecosystem.
The creation and modification of riparian habitat structures by hippos is an
important mechanism generating landscape-scale heterogeneity and is synonymous
with physical ecosystem engineering (Jones et al. 1994, 1997; Write et al. 2002).
However, there has been little recognition of the importance of hippos and their
influence in shaping the riparian habitat mosaics and environments across Africa
(but see Olivier and Laurie 1974). Here I present a study on the grazing and trampling effects of hippos within a savanna riparian community and explore the feedback from the abiotic environment along rivers to hippo grazing, the responses of
plants and other herbivore species to the changed abiotic conditions, in the
protected reserve and the adjacent pastoral system in the Mara Region of Kenya. I
assess differences in patterns of responses of plant and herbivore species abundance and richness to hippo and livestock grazing and how these patterns are
modified by protection versus pastoralism.
Research Hypotheses
H1: Vegetation height and cover will increase with distance from water in
response to declining grazing intensities
This hypothesis predicts that vegetation height and cover will increase with
increasing distance from water sources, due to declining grazing intensities from
water (Fig. 1.2). The consequences of this hypothesis are that hippopotamus grazing
will further contribute directly to patchiness in grasslands through defoliation and
trampling within a restricted strip along the riparian zone, aiding spatial heterogeneity of landscapes at intermediate distances from rivers.
Introduction
13
intermediate grazing
intensity distance
grass >30 cm
percentage cover
grass <10 cm
grass 10–30 cm
forbs+shrubs
distance from rivers
Figure 1.2 Hypothesized response of vegetation cover components, grass <10 cm high, grass
10–30 cm, grass >30cm and forbs and shrubs to distance from rivers as a function of declining
grazing intensity.
H2: Plant species richness will increase with distance from rivers in
response to declining grazing intensities
This hypothesis predicts that plant species richness will increase with increasing
distance from rivers in response to declining grazing and trampling intensity. The
heavily grazed and trampled areas close to river banks will have low plant species
richness except for forb species. The distribution and abundance of forbs is
expected to respond positively to grazing intensity and the species abundance and
richness of forbs to decline with distance from rivers (Fig. 1.3). Grass species will
increase with distance from rivers as grazing intensity declines while shrub species
will establish in areas of high grazing intensity and decline with distance from
rivers as grazing intensity declines.
number of species
grass
forbs
shrubs
distance from rivers
Figure 1.3 Hypothesized response of plant species, forbs, grass and shrubs with distance from
rivers as a function of declining grazing intensity.
14
Chapter 1
H3: Herbivore abundance and species richness will increase with increasing
distance away from rivers due to a corresponding increase in forage availability
This hypothesis predicts spatial variation in herbivore abundance and distribution
with distance from rivers and along the distance-to-river gradient. Hippo grazing
along riparian areas will facilitate some herbivore species and cause competitions
with others. In addition, hippo grazing will influence predation risk for herbivores,
as the cover of tall vegetation increases with distance from rivers, with varying
effects on different herbivore foraging guilds. The dry seasons will further force
herbivores to congregate close to rivers, amplifying habitat stress especially in the
pastoral ranches grazed heavily by livestock. Herbivore use of the riparian habitats
in the pastoral ranches will be constrained by human presence and livestock
herding. Herbivores that are more water dependent and require high food quality
will decline with distance from rivers. Herbivores that are bulk feeders will
increase with distance from rivers. Herbivores that are less water dependent will
be uniformly distributed relative to distance to rivers. In general, a humped distribution will describe the distribution of the abundance of most herbivores along the
distance-to-river gradient (Fig. 1.4; Ogutu et al. 2010).
bulk feeders
food quality
quantity/quality
water
independent
response
generalized
herbivore
response
distance from rivers
Figure 1.4 Hypothesized response of herbivores that are, water dependent and sensitive to food
quality, bulk feeders, and water independent and generalized herbivore response along grazing
gradients from rivers.
Research Questions
I attempted to find answers to the following three questions. (1) How does the
pattern of human-hippo conflicts throughout Kenya vary over time and what does
this variation imply for the future of hippo conservation in Kenya? (2) How do
Introduction
15
long-term hippo population dynamics respond to changing land use and climatic
variability and compare with the dynamics of other megaherbivores in Kenya? (3)
What are the impacts of hippo grazing on vegetation and other herbivores and how
are these impacts modified by land use? In particular, how does the impact of
hippopotamus grazing and trampling on (1) bare ground cover, (2) vegetation cover
and height, (3) plant species richness and composition, (4) herbivore abundance and
species richness vary along the distance-to-river gradient and with protection and
pastoralism?
Thesis outline
This thesis is organized in 6 chapters as follows. Chapter 1 serves as an introduction and provides brief background information on anthropogenic effects on the
distribution and abundance of herbivores in savannah ecosystems. The chapter also
provides background information on the Mara River Basin that is the focus landscape for most of this study. This chapter also highlights the ecological importance
of hippos in African wetlands. Finally, the chapter presents the three main
hypotheses and research questions explored in greater detail in subsequent chapters of this thesis. Chapters 2 and 3 cover the conservation and management of
hippopotamus in Kenya. Chapter 2 presents an analysis of human-hippo conflicts
throughout Kenya during 1997 to 2008. Very few studies have directly addressed
the problem of human-hippo conflicts in Kenya, thus, there has been major gaps in
information on this topic. Given the importance that the Kenya Wildlife Service
(KWS) attaches to wetlands and threatened species conservation, analysis of the
long term monitoring data that KWS collects on human-hippo conflicts in Kenya is
clearly useful. This chapter thus examines key management issues on human-hippo
conflicts, identifying the extent, severity and distribution (spatial and temporal) of
hippo-related damages to crops and how retaliatory killings of hippos are threatening and undermining hippo conservation efforts in Kenya. This chapter highlights
the fact that conflicts between people and hippopotamus in Kenya probably cannot
be entirely eliminated but can be mitigated, by discouraging agricultural activities
associated with high human density on lands bordering riparian habitats and
promoting conservation and sustainable use of wetlands.
Chapter 3 explores the population status and conservation of hippopotamus at
one of Kenya’s important conservation area, the Masai Mara. Despite its imposing
size and formerly large abundance, the common hippopotamus has been much less
studied in Kenya compared to other Megaherbivores. Thus, this chapter also fills
this important gap in our knowledge. The chapter reveals how hippopotamus populations can increase rapidly and expand their range even in a context of considerable climatic variability and against a background of deteriorating habitat
conditions. Increased anthropogenic activities, especially land-use changes and
livestock herding are predicted to adversely affect hippopotamus conservation
16
Chapter 1
efforts, with significant spill over effects on other mammalian grazers dependent
on hippo grazing impacts on vegetation along riparian habitats.
The ecological implications of hippopotamus grazing and trampling activities
and their relations with the environment are covered in Chapters 4 and 5. Although
herbivore grazing is an important evolutionary force shaping vegetation biodiversity and structure in African savanna ecosystems, most studies have generalized
herbivore grazing effects, with few studying the effects of specific herbivore
species, and especially along sensitive riparian habitats. Chapter 4 explores the
interactive effects of hippopotamus and livestock grazing on vegetation structure,
herbaceous plant species composition and richness along a riparian habitat in
protected and pastoral systems of Masai Mara, and demonstrates the important
engineering impacts of hippopotamus that enables establishment and co-existence
of plant species, culminating in increased species richness in areas experiencing
intermediate grazing levels, especially within protected areas. Chapter 5 documents
the facilitative role of hippopotamus grazing on habitat use by other wild herbivores along the Mara riparian habitat, demonstrating that hippopotamus grazing
activities led to a shifting mosaic of patches that differ in vegetation structure, that
enhance structural heterogeneity of vegetation and attract a diverse and abundant
herbivore assemblage at intermediate distances from rivers. Lastly, chapter 6
synthesizes the results from chapters 2 to 5 and concludes that hippopotamus are
keystone ecosystem engineers able to profoundly modify ecosystems and facilitate
other herbivores in the Mara River Basin but the future of their conservation is
threatened by dramatic land use changes in the basin, water abstraction from the
Mara River, and rising levels of conflict with people throughout Kenyan wetlands.
This chapter recommends urgent preparation of a species management plan for
Kenyan hippos that aims to reduce human-hippo conflicts and promote peaceful
interactions between hippos and local communities.
I have tried to make these chapters as independent of each other as possible so that
each can be read essentially independently of the others, while retaining a sequence
from the very general to the very particular, in what I consider to be a natural
order for this thesis.
Introduction
17
Chapter two
2
Human-hippo conflicts in Kenya
during 1997–2008: Vulnerability of
a megaherbivore to anthropogenic
land use changes
Erustus M. Kanga, Joseph O. Ogutu, Hans-Peter Piepho and Han Olff
Published in Land Use Science (2011), DOI: 10.1080/1747423X.2011.590235
Abstract
Rising human population and the associated demand for more land, water
and other natural resources is intensifying conflicts between people and
wildlife worldwide. We investigated the nature, intensity, seasonality,
spatial and temporal patterns in human-hippo conflict incidences reported
from wildlife stations Kenya-wide, over a 12-year period spanning 1997 to
2008. Overall, 4493 human-hippo conflict incidences were recorded, representing a mean rate of 4.46±0.29 incidences per month. The conflict incidences increased by 1285% from 1997 to 2008, resulting in 937 peak
incidences reported in 2008. Number of conflict incidences differed among
conservation regions, with incidences increasing during severe droughts and
over time. Crop damage was the most commonly reported type of conflict.
Wildlife managers attended to 90% of all reported conflict incidences. Hippo
mortality increased linearly with increasing conflict incidences, portending
a precarious future for hippos outside protected areas of Kenya. This
dramatic rise in human-hippo conflicts is a consequence of fundamental
land use changes around wetlands and riparian-edge habitats.
20
Chapter 2
Introduction
Human population growth and demand for more land, water and other natural
resources are intensifying conflicts between people and wildlife worldwide (IUCN
2003). Human-wildlife conflicts (HWC) arise from direct and indirect negative
interactions, leading to economic losses to agriculture through destruction of crops,
human fatalities and injuries, depredation of livestock and retaliatory killings of
wildlife (Hill 1997; Siex and Struhsaker 1999; Tourenq et al. 2001; Treves and
Karanth 2003). In the African savannas, intensification of land conversion to cultivation and/or human settlement is a key factor driving people into more direct
contacts with wildlife (Sala et al. 2000; Thuiller et al. 2006).
The pattern of increasing conflicts between people and wildlife is evident in
Kenya (Musau and Strum 1984; Mizutani 1993; Thouless 1994; Kiiru 1995; Sitati et
al. 2003; Patterson et al. 2004). Land use changes especially within the pastoral
systems of Kenya, driven by rapid expansion of cultivation, land subdivision and
privatization of land tenure (Kimani and Pickard 1998; Lamprey and Reid 2004;
Okello and D’Amour 2008), are largely responsible for the escalating human wildlife
conflicts. As a result, expanding settlements and cultivation are exerting increasing
impacts on the distribution and abundance of wildlife at both the population and
community levels (Verlinden 1997; Prins 1992; Serneels and Lambin 2001). Consequently, wildlife grazing areas are increasingly dwindling in size, migration corridors are being lost or modified and wildlife access to water sources is getting
increasingly blocked, resulting in elevated human wildlife conflicts and wildlife
population declines (Serneels and Lambin 2001; Thuiller et al. 2006). The information required to understand and potentially to mitigate these conflicts has not been
adequately collated and analyzed in Kenya, especially for the common
hippopotamus (Hippopotamus amphibius Linnaeus 1758), except for an unpublished thesis on the Lake Victoria Basin (Post 2000).
Megaherbivores (weighing over 1000 kg, Owen-Smith 1988), such as hippopotamus, Elephant (Loxodonta africana Blumenbach 1797), and large carnivores rank
among the most problematic and lie at the heart of human wildlife conflicts
because they are dangerous to humans. Despite the fact that megaherbivores often
cause major devastation to crops and are often a physical threat to humans, most
research has focused only on the elephant and neglected hippopotamus, yet the
latter are involved in numerous conflicts with people in many parts of Africa
(Mkanda and Kumchedwa 1997; Eltringham 1999). Nonetheless, substantial information on human-hippo conflicts is scattered in office files and thus less widely
accessible.
Hippos differ from other megaherbivores in having a dual requirement of daily
living space in water and an open grazing range often visited at night (Eltringham
1999). This requirement affects the manner in which hippos utilize resources and
Human-hippo conflicts
21
survive in areas dominated by high human population densities and continuous
land use changes. While most studies on human wildlife conflicts have concluded
that conflicts are intense on the periphery of protected areas (Naughton-Treves
1998; Saj et al. 2001), this may not necessarily be true for hippos since they inhabit
wetlands that often extend outside protected areas into agricultural landscapes.
Being mainly wild grazers (Cerling et al. 2008), hippos destroy crops cultivated
close to wetlands (Mkanda and Kumchedwa 1997; Eltringham 1999) and pose physical threats to local communities. However, like most other hippo-range states in
Africa, Kenya has done little to evaluate the type, extent and consequences of
human-hippo conflicts, even though local communities report numerous complaints on hippo damages regularly.
Substantial human-induced environmental changes pose a serious challenge to
biodiversity conservation (Cincotta et al. 2000; Thaxton 2007). It is probable that
significant proportions of threatened and vulnerable species of conservation
concern, like the common hippopotamus (Lewison and Oliver 2008), rely on or
utilise agricultural landscapes and experience conflicts with humans (Siex and
Struhsaker 1999; O’Connell-Rodwell et al. 2000; Tourenq et al. 2001; Green et al.
2005). Therefore, to effectively address human wildlife conflicts, it is necessary to
consider both the effects of damage caused by wildlife as well as the impacts of
mitigating actions on the conservation status of target species. We thus investigated the nature, intensity, seasonality, spatial and temporal patterns in humanhippo conflicts over a 12-year period spanning 1997 to 2008, to inform decisions on
best-practice management of human-hippo conflicts in Kenya. We also attempted to
establish correlates of the patterns in human-hippo conflicts, using human-wildlife
conflict data collected from a network of 69 wildlife stations and 23 outposts, and
interpret the implications of our findings for hippo conservation and management,
in the wake of rising human population pressure on wetland habitats across Kenya.
Materials and Methods
Study area
The Republic of Kenya (East Africa) lies between latitude 4° N to 4° S and longitude
34° E to 41° E, with the equator running approximately through the middle of the
country. The country covers an area of about 582,646 km2, 8.2% of which comprise
22 terrestrial national parks, 4 marine national parks, 28 terrestrial national
reserves, 6 marine national reserves and 5 national sanctuaries administered by the
Kenya Wildlife Service. Generally, Kenya’s landuse is largely pastoral in semi-arid
zones and agricultural in the moist and humid zones. Rainfall is bimodal in most
parts of the country, with short rains normally occurring during October-December
and the long rains during March-June. Kenya experienced extreme El Niño floods
22
Chapter 2
N
300 km
Figure 2.1 Map of Kenya, showing conservation regions, protected areas and KWS stations that
reported human-hippo conflicts during 1997–2008.
in 1997–1998, mild El Niño floods in 2001–2004 and 2007, and severe droughts in
1999–2000, 2005–2006 (Ogutu et al. 2007) and in 2008–2009. Human population
size in Kenya grew rapidly during the past half century, from 8 million in the 1960s
to about 37 million by 2007, at an average annual rate of 2.8% (CBS 2001; Thaxton
2007); an increase associated with rising demand for more land for settlements and
cultivation, with increased contacts between people and wildlife, especially close to
water sources (UNEP 2009).
The Kenya Wildlife Service mandate is to conserve and manage wildlife in
Kenya, and has therefore stratified the country geographically into Conservation
Regions (Figure 2.1), based on broad ecosystems and landscape characteristics for
ease of biodiversity conservation administration. Wetlands, the main hippopotamus
habitats, cover about 2–3% of the country’s surface area and support a substantial
proportion of the Kenya’s biodiversity resources. These wetlands are diverse in type
and distribution but face numerous threats, including pollution, cultivation,
Human-hippo conflicts
23
reclamation for settlements and unsustainable exploitation (Crafter, Njuguna and
Howard 1992). Conversion of wetlands to agriculture and the impact of farming on
riparian habitats are the main threats to hippopotamus conservation in Kenya,
followed by illegal poaching for meat and their canine teeth ivory (Weiler, DeMeulenaer and Vanden-Block 1994; Williamson 2004). Although hippo population
numbers are known to be declining in most parts of Africa, comparable information
on hippo population trends is still lacking for Kenya (Eltringham 1999; Lewison and
Oliver 2008; Kanga et al. 2011), despite the fact that human encroachment on
wetlands has greatly interfered with hippopotamus ecology and heightened humanhippo conflicts Kenya-wide.
Methods
We collated information on human-hippo conflict incidences from Occurrence
Books in which human-wildlife conflict incidences were recorded daily at 69
wildlife stations and 23 wildlife outposts Kenya-wide during 1997-2008. Records in
the Occurrence Books are made whenever members of the public reported
complaints or Kenya Wildlife Service personnel make field visits in response to
communities’ distress calls. Conflicts were grouped into crop damage, human
mortality and injury, livestock mortality, hippo mortality and physical threat categories. Conflict incidences that could not be assigned to any of these five categories
were lumped together as unclassified. Physical threats denoted incidences where
hippos intruded on people, threatened their safety or constrained their free movements.
Statistical data analysis
We first stratified the conflicts dataset geographically by conservation regions and
then modelled temporal trends in the number of conflict incidences reported for
each region per year using the zero-inflated negative binomial regression, assuming
a negative binomial distribution of conflict incidences. The aim of this modelling
was to smooth climate related inter-annual fluctuations in order to understand
seasonal and long term trends over the study period. Further, we modeled temporal
trends in the expected monthly frequencies of conflict incidences in each region
over the 12-year period and accounted for seasonality, using a quadratic month
effect, and regional differences in the incidences. The zero-inflated model was
selected to account for excess or structural zeros in the data arising potentially
from underreporting of actual conflict incidences or random zeros corresponding to
zeros expected under the negative binomial probability model. We first established
that the data were over-dispersed relative to the Poisson distribution, a baseline
model for count data, and that the zero-inflated negative binomial model gave a
better fit to the conflict incidences than the zero-inflated Poisson model using the
Vuong test (Vuong, 1989). We also compared the mean number of conflict incidences
24
Chapter 2
per station per year and per station per month among regions using one-way
ANOVA. We regressed the number of conflict incidences summed over all regions on
year and accounted for serial autocorrelation of error terms using the first-order
autoregressive model. However, the model with serial autocorrelation had weaker
support in the data than a model assuming complete independence based on the
Akaike Information Criterion (Burnham and Anderson 2002), which we therefore
used for final inference. Finally, we used the negative binomial regression model to
relate hippopotamus mortality to the level of conflict incidences in Kenya. All
models were fitted in STATA (StataCorp 2001).
Results
Temporal patterns in conflict incidences
A total of 4493 human-hippo conflict incidences were reported between 1997 and
2008, corresponding to a mean rate of 4.46±0.29 (1SD) incidences per month. The
highest number of conflict incidences reported per year was 937 in 2008, representing an increase of 1285% from 41 incidences reported for 1997. Conflict incidences were initially low until the severe drought of 1999–2000 after which the
incidences rose markedly to over 400 per annum during 2000–2002. Thereafter, the
number of incidences dropped by more than a half during the high rainfall period of
2003–2004. The conflict incidences began to rise again, albeit more gradually
during the severe drought of 2005–2006, and were distinctly higher than expected
in the drought of 2008. Marked increases in conflict incidences were thus contemporaneous with severe droughts and conflicts increased significantly linearly over
time (Figure 2.2, LN(conflicts) = –420.86 +0.213 × year, p = 0.008).
7.0
R2 = 0.476
6.5
LN (conflicts)
6.0
5.5
5.0
4.5
4.0
3.5
1996
1998
2000
2002
2004
2006
2008
year
Figure 2.2 Trends in human-hippo conflicts in Kenya during 1997–2008 (Upward pointing arrows
indicate dry while downward pointing arrows indicate wet periods).
Human-hippo conflicts
25
A
B
5
Coast
4
3
2
1
Central
0
3
2
1
0
4
Southern
3
2
0
4
Mountain
LN (conflicts + 1)
1
3
2
1
0
3
Tsavo
2
1
Northern
0
3
2
1
0
5
Western
4
3
2
1
0
1996
1998
2000
2002
year
2004
2006
2008
1 2
3
4
5
6
7
8 9 10 11 12
month
Figure 2.3 Long-term (A) and seasonal (B) trends in human-hippo conflicts in Kenya during
1997–2008.
26
Chapter 2
The expected conflict incidences differed significantly among conservation
regions (Z = –4.52, p < 0.001), with a significant increase in conflict incidences over
time evident in the Coast (Z = 3.68, p < 0.001), Central Rift (Z = 2.29, p = 0.022),
Southern (Z = 2.46, p = 0.024) and Tsavo (Z = 2.72, p = 0.007, Figure 2.3) regions.
Frequency distribution of conflict incidences by region
The Coast region contributed the highest number of conflict incidences (46%),
followed by Western (19%), Southern (12%), Tsavo (10%), Mountain (6%), Central
Rift (5%), and Northern (2%) regions (Table 2.1). Multiple pairwise comparisons of
mean conflict incidences per station per year and per station per month across regions
revealed significant regional distinctions (F6, 6617 = 51.9, p < 0.001, Table 2.2).
Table 2.1 Summary of descriptive statistics of monthly human-hippo conflict incidences in Kenya
during 1997–2008. STD =1 standard deviation; IQR = Interquartile range; N= Total number of
months; TS= Total number of all KWS stations; CS=Number of stations that recorded conflicts.
Region
Mean
STD
Median
IQR
Min
Max
N
TS
CS
Coast
14.2
17.9
6.5
23.5
0
91
144
14
6
Western
5.7
9.0
2
8.5
0
71
144
11
7
Southern
3.9
6.7
1
5
0
32
144
7
6
Tsavo
3.0
4.1
1
5
0
20
144
7
3
Mountain
2.0
3.7
1
2
0
29
144
9
6
Central Rift
1.7
2.8
0
2
0
13
144
11
10
Northern
0.73
1.4
0
1
0
9
144
11
8
Table 2.2 Mean number of human-hippo conflict incidences per station per year and per station
per month in each conservation region in Kenya during 1997–2008, and pairwise comparisons of
regional differences. N1 is the number of stations times the number of years and N2 is the number
of stations times the number of months in the period 1997–2008..
Conservation
Region
Station and year
Station and month
Mean
SE
N1
Mean†
SE
N2
2.03
0.48
120
0.17 a
0.02
1440
28.01
8.62
72
2.33 d
0.26
864
Mountain
3.81
0.89
72
0.32 abc
0.04
864
Northern
0.95
0.27
96
0.08 ab
0.01
1152
Southern
7.72
2.93
72
0.64 c e
0.09
864
11.92
4.01
36
0.99 e
0.13
432
9.77
2.31
84
0.81 e
0.09
1008
Central Rift
Coast
Tsavo
Western
†Means
with similar letters were not significantly different at P = 0.05
Human-hippo conflicts
27
Frequency distribution of conflict incidences by type
The most widely reported type of hippo conflict was crop damage (62.1%), followed
by physical threat (15.0%), hippo mortalities (13.3%), human fatality and injury
(3.4%), and livestock mortality (1.1%). The remaining 5.1% were unclassified
conflict reports. The 745 conflict incidences reported in 2008 and involving both
crop damage and physical threats suggest that land use changes within
hippopotamus habitats had elevated conflict incidences by 2,659% relative to 27
incidences reported in 1997. The temporal distributions of crop damage also varied
considerably among regions and accounted for 50, 18, 12, 9, 6, 3 and 3% of all the
incidences reported for the Coast; Western; Southern; Tsavo; Mountain; Central
Rift and Northern regions, respectively.
Seasonality in conflict incidences
There was a significant difference in expected monthly conflict incidences among
regions (Z = 4.73, p < 0.001), implying that the impact of human land use practices
on hippos differed regionally. However, conflict incidences were seasonal in most
regions with a peak generally evident during June-August (Figure 2.3). The seasonality in conflict incidences was significant for the Coast, Mountain, Southern and
Western regions (Table 2.3), and was influenced strongly by seasonal variation in
crop damage that accounted for 63% of all the conflict incidences, consistent with
reports that hippos ate and trampled on a variety of crops.
Relationship between conflict incidences and hippo mortality
Hippo mortalities increased significantly linearly with increasing number of
reported conflicts (R2 = 0.736, Z = 11.90, p < 0.001), implying that elimination of
individual animals at conflict points through problem animal control is a major
cause of hippo deaths in Kenya. On average, Kenya Wildlife Service attended to 90%
of human-hippo conflict incidences over the 12-year period, including 93, 84, 70,
89, 89, 92, 87, 94, 94, 98 and 95% of all reported incidences each year from 1997 to
2008, respectively.
Discussion
This study revealed four distinct patterns in the spatial and temporal distribution
of human-hippo conflict incidences in Kenya. First, conflicts occurred throughout
the year but with a seasonal peak apparent during the transition from the late-wet
(May-June) to the early-dry (June-August) season, a period of crop ripening in most
parts of the country. Second, although conflict incidences occurred in all conservation regions, there was an apparently higher concentration of incidences in the
Coast (46%) and Western (19%) regions. Third, about 63% of human-hippo conflicts
28
Chapter 2
Table 2.3 Results of analyses of monthly variation (seasonality) in human-hippo conflicts across
conservation regions in Kenya during 1997–2008.
Conservation Region
Effect
Coast
Intercept
Month
Mountain
6.15
<0.001
3.14
0.002
–3.31
0.001
Intercept
0.81
0.52
1.57
0.117
Month
0.23
0.18
1.32
0.187
Month × Month
–0.02
0.01
–1.88
0.060
Intercept
–0.60
0.44
–1.36
0.172
0.41
0.16
2.53
0.011
Month × Month
–0.02
0.01
–1.86
0.063
Intercept
–0.83
0.52
–1.59
0.113
0.32
0.19
1.65
0.098
–0.03
0.02
–1.77
0.077
0.27
0.49
0.56
0.573
Intercept
Month
Month × Month
Intercept
0.56
0.19
2.97
0.003
–0.04
0.01
–2.96
0.003
2.07
0.40
5.2
–0.19
0.14
–1.38
0.169
Month × Month
0.01
0.01
1.28
0.202
Intercept
0.86
0.40
2.16
0.031
Month
0.37
0.14
2.66
0.008
–0.02
0.01
–2.18
0.029
Month
Western
0.34
0.01
Month × Month
Tsavo
2.12
0.12
Month
Southern
Z
0.36
Month
Northern
P >|Z|
SE
–0.03
Month × Month
Central Rift
Estimate
Month × Month
<0.001
in Kenya arose from agricultural- related causes (i.e. crop damage and livestock
mortalities). Lastly, about 78% of the conflict incidences (agricultural and physical
threats) are associated with human-induced land use changes. The management
responses to these conflicts led to considerable losses of hippos annually through
problem animal control responses.
The dramatic rise in human-hippo conflicts across Kenya is a consequence of
both natural and human-mediated disturbances of wildlife habitats (Lamprey and
Reid 2004), including wetlands (Crafter et al. 1992). The high conflict incidences
recorded during 2001–2002 may have been largely influenced by the 1999–2000
drought while the high incidences during 2007-2008 can be attributed to the effects
of both drought and land use changes that continued to perturb hippopotamus
habitats. The scale and distribution of conflict incidences varied between conservation regions, underlining regional differences in land use patterns. Conflicts were
Human-hippo conflicts
29
high in regions where hippopotamus coexists with high human population densities, particularly in close proximity to major wetlands and rivers. This was true for
the Coastal and Western regions, characterized by high human populations, practicing subsistence crop farming mostly on wetlands and river banks, unlike in the
Northern region where pastoralism is the main economic activity, with minimal or
no crop cultivation (UNEP 2009).
Human-hippo conflicts occurred throughout the year in all areas but showed an
overall seasonal pattern, with an annual peak generally during June-August, a
period of crop ripening in most parts of the country. This pattern is consistent with
the observation that over 63% of all the conflict incidences were related to agriculture and is similar to that reported for Malawian hippos (Mkanda and Kumchedwa
1997). The probable proximate causes of the observed pattern of human-hippo
conflict are linked to increasing human population and the associated increase in
demand for agricultural and settlement space, especially in areas close to water,
and the fact that hippos, humans and their livestock compete for resources along
wetland margins. In addition, drought and competition with livestock may be
forcing hippos to forage further from their daily living space (Field 1970; Lock
1972), increasing their probability of contacts with humans. However, it remains
unclear whether the seasonality we observed in hippo conflict incidences reflects
seasonal restrictions in food availability or is simply an opportunistic response to
greater availability of an alternative, more nutritious and easily accessible food
type (Cerling et al. 2008).
Similar to other wildlife species, hippos caused serious socio-economic losses to
rural Kenyan farmers through crop raids, human mortalities and physical injuries
(Mizutani 1993; Butler 2000; Patterson et al. 2004). However, the large number and
increasing incidences of crop damage and physical threats posed by hippos in Kenya
imply possibly greater economic and social consequences to the livelihoods of
affected communities than those caused by other megaherbivores. These consequences are however confounded by the fact that rural communities are increasingly using wetlands for multiple cropping agriculture for subsistence and that
such crops are more vulnerable to wildlife damage, especially hippopotamus.
The hippopotamus mortalities recorded were highly correlated with the rise in
conflict incidences, portending a bleak future for hippos outside protected areas,
especially given that no actions are currently being undertaken to stem their persecution. Moreover, actual hippo mortalities could have been higher than reported if
injured hippos wandered off and died later. When faced with increased humanwildlife conflicts in Kenya, wildlife managers commonly react by killing the
offending animals as a problem animal control measure, in the hope of eliminating
the problem animals. Hippopotamus are easy target in wetlands for such control
measures. Thus, unregulated problem animal control can adversely affect their
population status in the long-term, given that there are likely many unrecorded
30
Chapter 2
incidences of systematic poaching of hippos, further aggravating their plight in
Kenya.
Conclusions and recommendations
Conflicts between people and hippopotamus in Kenya probably cannot be entirely
eliminated but can be mitigated, by discouraging agricultural activities in areas
with high human densities on lands bordering riparian habitats and promoting
sustainable use of wetlands, including through wildlife conservation, with the
overall goal of enhancing human wellbeing. Although hippos cause serious damage
to crops and physically threaten people, their continued persecution through unregulated problem animal control will drastically reduce their population abundance.
If this trend continues unchecked and the conflicts are not well managed,
hippopotamus populations in unprotected lands in Kenya may soon become unviable. Human-hippo conflicts could be reduced through effective public education,
promotion of communal conservancies that provide tourism revenues and land
rents directly to communities, selective problem animal control programs and
translocation of hippo populations in intensively farmed areas. However, given the
rapidly expanding human population and frequent food shortages linked to recurrent droughts in Kenya and the fact that riparian-edge areas are often some of the
best for agriculture, successfully balancing the need for national food security and
conservation of hippos will undoubtedly require considerable ingenuity and
resources. A Kenya-wide distribution and status survey for hippopotamus is needed
as a basis for developing effective management and conservation interventions for
hippos outside protected areas.
Acknowledgements
We thank Walter Mususi and Moses Maloba for assistance with the collation of the conflicts
dataset. We also thank the Kenya Wildlife Service for logistical support and for availing the
conflict data. EK was supported by the Netherlands Fellowship Program (NFP) and the
University of Groningen through the Government of Kenya and by the Frankfurt Zoological
Society (FZS).
Human-hippo conflicts
31
Chapter three
3
Population trend and distribution
of the Vulnerable common hippopotamus
Hippopotamus amphibius in the Mara
Region of Kenya
Erustus M. Kanga, Joseph O. Ogutu, Han Olff and Peter Santema
Published in Oryx (2011) 45: 20-27
Abstract
The common hippopotamus Hippopotamus amphibius can significantly
influence the dynamics of ecosystems and engender serious conflicts with
people but, in Kenya, one of the species strongholds, it has been little studied
or monitored. We surveyed the hippopotamus population in the Masai Mara
National Reserve and the adjoining pastoral ranches in 2006 using foot
counts along 155.3 km of the main rivers. We counted 4,170 hippopotamuses in 171 schools. Comparisons with earlier surveys suggest that this
population increased by 169.6% between 1971 and 1980 within the reserve
and, although it did not increase within the reserve during 1980–2006, it
increased by 359.4% outside the reserve during this period against a background of deteriorating habitat conditions. The overall density in 2006 was
26.9 hippopotamuses per km of river, equivalent to a biomass of 26,677
kg/km of river. The ratio of calves to 100 adults was 9:100 inside the
reserve, 10:100 outside the reserve and 6:100 along tributaries of the Mara
River, implying that the population is either increasing or that its spatial
distribution is being compressed because of range contraction. The apparent
increase in the hippopotamus population contrasts with marked contemporaneous declines in the populations of most other large mammalian herbivore species in the Reserve. We discuss possible reasons underlying the
increase in the hippopotamus population.
34
Chapter 3
Introduction
The common hippopotamus Hippopotamus amphibius is a semi-aquatic artiodactyl
of sub-Saharan Africa (Kingdon 1979; Eltringham 1999) and, historically, was
widely distributed throughout the region (Eltringham 1999; Feldhake 2005;
Lewison and Oliver 2008). Five evolutionary units have been described based on
morphological differences (Ansell 1971; Grubb 1993; Eltringham 1999) but only
three of these are genetically different (Okello et al. 2005).
The range of the hippopotamus has become increasingly restricted in recent
decades, a contraction that has been accompanied by substantial declines in abundance, with the most recent estimates suggesting population declines of 7–20%
during 1996–2004 (Eltringham 1999; Lewison and Oliver 2008). Although the
causes of these declines are documented and well understood (habitat loss,
exploitation and conflicts with people), they continue to operate and appear
unlikely to be eliminated in the near future, amplifying the need to develop effective conservation measures for the hippopotamus.
Recent estimates suggest that 125,000–148,000 hippopotamuses currently occur
in fragmented populations in rivers, lakes and other wetlands of eastern, western
and southern Africa (Oliver 1993; Lewison and Oliver 2008). Of the 36 countries
where the common hippopotamus is known to occur, 20 have confirmed declining
populations, seven have populations of unknown status, nine have stable populations and three (Algeria, Egypt and Mauritania) have experienced recent extinctions
(Lewison and Oliver 2008). Based on the estimated global population, coupled with
intensifying threats of poaching for meat and ivory (Weiller et al. 1994; Williamson
2004; Conservation 2006), progressive habitat loss and persecution because of
conflicts with people, the hippopotamus was categorized as Vulner-able on the
IUCN Red List in 2006 (Lewison and Oliver 2008).
Despite its Vulnerable status and ecological significance, and rising conflicts
with people, the hippopotamus has not been well studied or monitored in many
parts of its range, including Kenya where the species has been officially protected
since the 1920s (Kenya Game Department, 1953). Monitoring is necessary to understand the factors underpinning population dynamics and hence to develop an
understanding of how the hippopotamus influences, and is influenced by, changes
in riparian habitats (Field 1970; Thornton 1971; Lock 1972; Eltringham 1999) and
responds to land-use changes, climate change and variability, and conflicts with
humans. The paucity of data on hippopotamus population status and dynamics in
Kenya is due in part to the difficulty and high costs of counting a nocturnal, semiaquatic mammal inhabiting river systems that are often fringed by dense riparian
woodlands. It is this difficulty that has primarily limited hippopotamus population
monitoring in the Mara Region of Kenya, where regular aerial monitoring and occasional ground counts of other herbivores have been conducted over the last 3
Hippopotamus in the Mara region
35
decades (Stelfox et al. 1986; Broten and Said 1995; Ottichilo et al. 2000; Homewood
et al. 2001; Serneels et al. 2001a; Reid et al. 2003; Ogutu et al. 2009). Hippopotamuses were counted only five times in the Masai Mara River systems between 1958
and 2006 compared to 50 times for the other large herbivores of the Mara Region
(Talbot and Stewart 1964; Ottichilo et al. 2000; Ogutu et al. 2009; Kenya Wildlife
Service, unpubl. data; Department of Resource Surveys and Remote Sensing of
Kenya, unpubl. data). These counts reveal important patterns of temporal variation
in the population abundance of the hippopotamus in the area and emphasize its
importance for the dynamics of the Serengeti-Mara ecosystem (Darling 1961;
Olivier and Laurie 1974; Karstad 1984; Reid et al. 2003).
Our objectives in this study were to establish the current status of the
hippopotamus population in the Mara Region and to investigate how the population
has changed both spatially and temporally. We present data on hippopotamus population abundance in the Mara Region since 1971, and compare the temporal
patterns to those of sympatric megaherbivores (elephant Loxodonta africana, black
rhino Diceros bicornis and giraffe Giraffa camelopardalis) over the same period. We
evaluate the effects of land use on this hippopotamus population and examine the
implications of continuing changes in land use on the conservation and management of the species in the Serengeti-Mara ecosystem.
Study area
The Mara Region in south-west Kenya is bounded by the Serengeti National Park in
Tanzania to the south and the Siria escarpment to the west (Fig. 3.1). This region
forms the northernmost limit of the Serengeti-Mara ecosystem straddling the
Kenya-Tanzania boundary. The ecosystem comprises several wildlife conservation
administrations and conservation-pastoralist multiple land-use zones in the two
countries (Sinclair and Arcese 1995). The 5,500 km2 Mara includes the c. 1,530 km2
Masai Mara National Reserve and the adjacent pastoral ranches of Koyiaki,
Olkinyei, Siana, Lemek and Ol Chorro Oiroua with a combined total of 4,000 km2.
The Mara receives a mean total annual rainfall of 600 mm in the south-east rising
to 1,200 mm in the north-west (Norton-Griffiths et al. 1975). Rainfall is bimodal,
with the short rains falling during November-December and the long rains during
January-June, although January and February are often dry. The vegetation is
predominantly grassland, with isolated scrublands and woodlands, especially along
drainage lines and on hilltops (Epp and Agatsiva 1980).
Several rivers and numerous streams drain the Mara but the Mara River,
traversing both Kenya and Tanzania, is the only river that flows all year. The Sand,
Talek and Olare Orok rivers are the main tributaries of the Mara River and are
largely seasonal. The Mara River is 396 km long and its flow through the Masai
36
Chapter 3
Ethiopia
N
ua
ro
Kenya
Somalia
Uganda
Sudan
rro
ho
Oi
Lemek
C
Ol
rok river
Nairobi
Olkinyei
Mare
river
Olare-O
Tanzania
Se
ren
get
iN
Talek
river
MMNR
Sa
atio
nal
river system
international border
Koyiaki
Pa
rk,
Tan
z
nd
Siana
rive
Keekorok Pool
r
ani
a
Masai Mara National Reserve
20 km
Figure 3.1 Masai Mara National Reserve and the adjoining pastoral ranches, showing the Mara
River and its tributaries.
Mara National Reserve and Serengeti National Park sustains a large variety of
abundant wildlife, including hippopotamus, crocodile Crocodylus niloticus, wildebeest Connochaetes taurinus, Burchell’s zebra Equus burchelli and Thomson’s
gazelle Gazella thomsoni. This wildlife assemblage supports a robust tourism
industry in the Mara. However, wildlife populations there are facing many waterrelated problems, including increasing water shortages and declining water quality
linked to expanding irrigated cultivation, unregulated water extraction, and deforestation of the Mau Forest catchments of the Mara River (Mutie et al. 2005).
Marked declines in herbivore numbers in the Mara have been attributed to their
progressive exclusion from pastoral ranches by land-use changes, including expansion of mechanized and subsistence agriculture and settlements, which have
affected 8% of the Mara and caused land cover changes on up to 36% of the
adjoining pastoral ranches (Homewood et al. 2001; Serneels et al. 2001b; Lamprey
and Reid 2004; Mutie et al. 2005). These changes have probably intensified competition between livestock and wild herbivores on the pastoral ranches of the Mara.
Hippopotamus in the Mara region
37
Moreover, settlement of the formerly semi-nomadic Masai pastoralists (Kimani and
Pickard 1998; Western et al. 2009) and the associated intensification of land use
and grazing by large numbers of livestock on the pastoral ranches has accelerated
range degradation and fragmentation, including along riparian habitats. Rising
temperatures and recurrent droughts (Ogutu et al. 2007) have amplified herbivore
mortalities in the Serengeti-Mara ecosystem.
Methods
Ground, boat and aerial survey methods have been used to count hippopotamuses
(Petrides and Swank 1965; Olivier and Laurie 1974; Marshall and Sayer 1976;
Viljoen 1980; Karstad 1984; Tembo 1987; Norton 1988; Smart 1990; Bhima 1996)
but accurate counts have been obtained only with ground surveys (Tembo 1987;
TAWIRI 2001). We conducted ground counts of hippopotamus along the Mara
River of Kenya, its three main tributaries and one major water pool during
September–November 2006. This period was chosen because it spans the late dry
season when water levels in the rivers are lowest and visibility of hippopotamuses
in the water is highest.
We divided the study area into river sections using a 1:50,000 map. Three
observers walked quietly along the river banks and, upon sighting individuals or
groups of hippopotamuses, recorded the total number, group size and number of
calves, with the aid of binoculars. We distinguished calves by their small body size
relative to adults and subadults. We assumed that the number of individuals
counted in a group accurately represented the size of the group, based on calibration trials conducted at the Keekorok Pool with a known number of hippopotamuses. Locations of all individuals and groups were determined with a global
positioning system.
We tested for differences in expected group sizes between regions of the Mara
River within the Masai Mara National Reserve, pastoral ranches, and Mara River
tributaries within the Reserve using a negative binomial regression model
assuming a negative binomial error distribution and log link function (Edwards and
Berry 1987). We performed multiple pair wise comparisons of expected group sizes
between regions and used simulation adjustment for multiplicity. We synthesized
the denominator degrees of freedom for Wald F-tests using Kenward and Roger’s
(1997) method for small sample sizes. All models were fitted with the SAS procedure GLIMMIX (SAS Institute 2006). To compare the expected percentage composition of calves inside and outside the Reserve as well as in the tributaries, we used a
logistic regression assuming binomial error distribution and a logit link function.
We conducted multiple comparisons of the expected percentage composition of
calves across regions and adjusted the tests for multiplicity using simulation adjust-
38
Chapter 3
ment. We used a χ2 goodness-of-fit test to examine differences in hippopotamus
densities between regions and years and analyzed temporal trends in abundance by
computing percentage change in abundance between consecutive counts.
Results
We counted 4,170 individuals along 155.3 km of the Mara River system and at one
pool. As we walked long sections of the same river in a day the chances of double
counting, although hard to eliminate entirely, were probably low. Counts made in
1971 (Olivier and Laurie 1974) and 1980 (Karstad 1984) within the Masai Mara
National Reserve indicated that the population increased by 169.6% during this
time, i.e. a mean annual growth rate of 18.8% (Table 3.1). Between 1980 and 2006
the population did not increase within the reserve but increased outside the reserve
by 359.4%. The overall increase of the population in the Mara River inside and
outside the reserve combined was 49.9% for this period, i.e. a mean annual growth
rate of 3.1%, during which new groups apparently spread upstream to the pastoral
Table 3.1 Number of hippopotamuses counted (with percentage increase compared to previous
count), km of river surveyed and density of hippopotamus per km of river in the Mara Region (Figs
3.1 and 3.2) between 1971 and 2006. Blank cells indicate absence of surveys.
Olivier and Laurie (1974)3
River section
No.
km
Density
(km-1)
Mara River
inside reserve
738
60
12.3
Karstad (1984)1, 4
No.
(% increase) km
Density
(km-1)
53.3
36.1
1,571
(359.4%)
45.7
34.4
Talek River
158
46.1
3.4
Olare Orok River2
490
10.2
48.0
Keekorok Pool2
27
4,170
155.3
(49.9%;
Mara River
only)
26.9
Total
74.2
24.1
342
50.2
6.8
No.
Density
(% increase) km (km-1)
1,924
(-3.3%)
Mara River
outside reserve2
1,990
(169.6%)
This study (2006)
2,332
124.4
17.1
1Karstad
used both ground and aerial counts and this enabled comparison of his data with ours.
groups established after 1982
3Counts made in July and August 1971
4Counts made in August, September and October 1980
2New
Hippopotamus in the Mara region
39
A
Old Mara Bridge
B
Mara Safari Club
Koyiaki
N
Masai Mara
National Reserve
er
re
riv
Lemek
Ma
Olare-O
rok river
Ol Chorro
Oiroua
Ma
re
riv
er
iver
kr
Tale
10 km
Koyiaki
river junction
for Mara and Talek
Hippo group size
1–5
6 – 30
Old Mara Bridge
31 – 60
61 – 132
reserve/ranch borders
New Mara Bridge
MMNR
river system
Figure 3.2 Spatial distribution of hippopotamus Hippopotamus amphibius in the Masai Mara
National Reserve (A) and Pastoral Ranches (B) in 2006.
ranches (Fig. 3.2). Our count in 2006 represents a biomass of 26,677 kg/km of river,
assuming a unit weight for a hippo of 1,000 kg (Coe et al. 1976).
The density of hippopotamuses in 2006 was 36.1 and 34.4 hippos/km of river
within and outside the Masai Mara National Reserve, respectively. A χ2 goodnessof-fit test indicated a significant difference in densities between these two stretches
of the Mara River (χ23 – 0.0001, P > 0.05) and over time in the Reserve (χ22 – 0.0001,
P > 0.05, Table 3.1). We counted 171 groups of 1–132 individuals (Fig. 3.3). Although
group sizes appeared large in the Mara River tributaries (95% confidence limits
18.7–42.5 compared to the Mara River within, 20.2–31.7, and outside, 17.3–27.5,
the Reserve), differences in group sizes among the three regions were not significant (F2, 168 = 0.73; P = 0.48).
The ratio of calves to 100 adults was 9 : 100 (Reserve), 10 : 100 (pastoral
ranches) and 6 : 100 (tributaries). Even though the percentage composition of calves
was lower for tributaries (95% confidence limits 3–9%), compared to the Reserve
(7–10%) or the pastoral ranches (8–10%), logistic regression analysis showed that the
expected percentage composition of calves did not differ across regions (F2, 147 =
1.87; P = 0.59) nor were there significant pairwise differences among regions.
40
Chapter 3
Discussion
frequency distribution of group sizes (%)
The hippopotamus population size reported here represents the minimum number
of individuals in the part of the Mara River we surveyed. The estimated apparent
annual growth rate is high, and may suggest immigration from outside, but is
biologically achievable for the hippopotamus. Similar growth rates have been
reported for the hippopotamus in Luangwa River in Zambia (Marshall and Sayer
1976) and Lundi River in Zimbabwe (O’Connor and Campbell 1986). This apparent
growth rate could be partially attributed to protection in the Masai Mara National
Reserve and the opportunity for range expansion upstream in the Mara River to the
pastoral ranches. The hippopotamus typically has an adult male : female ratio of 1 :
1 (Smuts and Whyte 1981), compared to 1 : 2 typical of most large mammals, a
gestation period of 8 months (Marshall and Sayer 1976; Smuts and Whyte 1981),
which is short given its large body size, and a high fecundity of 0.55 (on average, a
mature female hippo can produce a calf every 21.8 months; Laws and Clough 1966;
Smuts and Whyte 1981). These factors all contribute to a high potential for rapid
population increase when conditions are favorable. The high rate of apparent population growth reinforces the observation that hippopotamus populations are not
often limited by diseases or predation but rather by the availability of suitable
habitat and forage (O’Connor and Campbell 1986), with the latter being relatively
abundant in less disturbed sections of the Mara (Boutton et al. 1988; Onyeanusi
1988).
Hippopotamus density increased three-fold in the Masai Mara National Reserve,
from 12.3 hippos/km of river in 1971 to 36.1 hippo/km of river in 2006 but with
marked differences in densities between regions (Reserve = 36.1, pastoral ranches
= 34.4, Talek River 5 3.4 and Olare Orok River = 48.1 hippos/km of river). These
50
Mara River in Reserve (n = 76)
Mara River outside Reserve (n = 72)
tributaries (n = 23)
40
30
20
10
0
1–5
6–19
20–50
51–70
>70
Hippo group sizes
Figure 3.3 Percentage frequency distribution of hippopotamus group sizes in the Mara region in 2006.
Hippopotamus in the Mara region
41
densities are likely to vary between seasons; elevated water levels allow groups to
spread out as pools enlarge without a corresponding change in the overall hippo
density.
The estimated density of 36.1 hippo/km of river within the Masai Mara National
Reserve is higher than the 20.2 hippos/km reported for Liwonde National Park in
Malawi in 1993 (Bhima 1996) and the 21.6 hippo/km reported for Luangwa River in
Zambia in 1970 (Marshall and Sayer 1976) but similar to the 39.7 hippos/km of river
reported for the Luangwa River in 1983 (Tembo 1987). However, similar direct
comparisons with estimates of hippopotamus densities in other water bodies are
often complicated by differences in habitat suitability (Olivier and Laurie 1974;
Eltringham 1999).
The similarities in expected group sizes across regions complicate identification
of high-quality hippopotamus habitat; nonetheless, the relatively large group sizes
in the tributaries and pastoral ranches may indicate overcrowding because there
are few suitable pools (Olivier and Laurie 1974; Klingel 1991; Viljoen 1995) or
compression by land-use changes, humans and their livestock. Although we
counted at the end of the dry season when most surface water dries out in the
Mara and the distribution of hippopotamuses is more restricted, the counts suggest
that the Mara River offers more favorable water conditions for hippopotamuses
during the dry season compared to its tributaries, as indicated by the moderate
group sizes and wider distribution of the groups encountered, suggesting less
crowding. However, land-use changes and livestock herding may be adversely
affecting hippopotamuses in the section of the Mara River within the pastoral
ranches and its tributaries, where hippopotamuses form relatively large groups
(Mara River 17.3–31.7; tributaries 18.7–42.5).
The ratio of calves to adults represents the apparent juvenile recruitment rate,
although not all calves and adults are likely to be counted in any census and some
subadults grouped with adults may not have reached breeding age. However, a high
calf : adult ratio is not necessarily indicative of a fast growing population because
the actual rate of population growth is not only a function of this ratio but also of
juvenile and subadult recruitment and mortalities. The observed ratio for the Mara
hippopotamuses of 9 : 100 is almost twice the 4.8 : 100 reported for Luangwa River
in Zambia in 1983 (Tembo 1987). Although the percentage composition of calves did
not differ among regions the tributaries had a noticeably lower ratio, probably
because of overcrowding that may depress population growth.
The numerical and spatial changes in the Mara hippopotamus population thus
suggests the population is expanding but may also indicate increasing concentration of hippopotamuses in the Mara River because of contraction and compression
caused by deterioration and truncation of their habitats from human activities.
Moreover, if destruction of the Mara River catchments and increasing water
extraction has progressively reduced overall water availability in the watershed,
42
Chapter 3
thus increasing the possibility of sighting hippopotamuses and yielding higher
counts, then the changes are unlikely to indicate a true population increase.
The density of the hippopotamus in the Masai Mara National Reserve in 2006
was similar to that reported for 1980, implying a stable population in the Reserve.
But significant population expansion was evident in the pastoral ranches, where
hippopotamus density increased five-fold from 6.8 to 34.4 hippos/km of river.
Various authors (Sayer and Rakha 1974; Marshall and Sayer 1976; Smuts and
Whyte 1981; O’Connor and Campbell 1986) have suggested that improved environmental conditions, most notably above average rainfall, improve conception and
subsequent calf survival and recruitment of the hippopotamus through provision of
adequate food and suitable shelter, resulting in high population growth. Our results,
however, indicate that the Mara hippopotamus population increased against a
background of deteriorating habitat conditions related to recurrent droughts, rising
temperatures and progressive habitat desiccation (Ogutu et al. 2007), and fundamental land-use changes (Homewood et al. 2001; Serneels et al. 2001a; Lamprey
and Reid 2004; Mutie et al. 2005; Ogutu et al. 2009).
The observed increase in the hippopotamus population is inconsistent with the
contemporaneous declines in the populations of mammalian herbivores weighing
less than 1,000 kg in the Mara (Ottichilo et al. 2000; Homewood et al. 2001;
Serneels et al. 2001a; Ogutu et al. 2009). Of the three other sympatric megaherbivores (animals weighing 1000 kg, Owen-Smith 1988) in the Mara, only the elephant
population has increased (Dublin 1995; Kenya Wildlife Service, unpubl. Data); black
rhino and giraffe populations have declined. The increase in elephant numbers has
been attributed to an influx of elephants escaping from poachers in the Serengeti
from the late 1970s to early 1990s (Dublin and Douglas-Hamilton 1987). The
declines in giraffe and rhino populations have been attributed to declining woodland cover (Dublin 1995, Lamprey and Reid 2004), poaching (Walpole et al. 2001),
habitat alteration, fragmentation and loss because of privatization of land tenure,
expanding settlements, cultivation, and settlement of the formerly semi-nomadic
Masai and consequent intensification of land use (Homewood et al. 2001; Serneels
et al. 2001a; Ogutu et al. 2009). Displacement of giraffe and rhinos by pastoral livestock in the Mara might also have contributed to the declines (Mukinya 1973;
Walpole et al. 2001; Ogutu et al. 2009). The increase of the hippopotamus population on the pastoral ranches of the Mara, although probably reflecting population
growth, could also reflect exclusion of hippopotamuses from parts of their former
range in the adjacent pastoral areas because of progressive habitat loss and
declining water levels in the Mara River. Given that the hippopotamus is a grazer
whereas giraffe and black rhinos are browsers and elephant is a mixed grazerbrowser, hippopotamuses could respond differentially to environmental changes
compared to other megaherbivores. However, habitat degradation, fragmentation
and loss due to land-use change and poaching are unlikely to be the cause of the
Hippopotamus in the Mara region
43
increase in hippopotamuses because many other mammalian grazers have declined
in the Mara, suggesting that contrasting feeding styles alone are insufficient to
account for these differences.
It is unlikely that rainfall was responsible for the expansion of the hippopotamus population in the Mara because, despite fluctuating widely, rainfall did not
increase consistently between 1971 and 2006 (Ogutu et al. 2007). A potential
contributing factor could have been conversion of woodlands into grasslands in the
Mara by elephants and fire (Dublin 1995) and humans (Lamprey and Reid 2004) and
maintenance of these grasslands by hippopotamuses and other herbivores. Together
with protection in the reserve and pastoral ranches this could have offered suitable conditions for hippopotamuses to thrive. However, this does not explain why
populations of sympatric herbivores declined concurrently. Given the persistent
declines in numbers of the other herbivores, deteriorating habitat conditions and
increasing pressure on water and other habitat resources, the high density of the
hippopotamus is most likely indicative of range contraction and compression in the
Mara.
The increase in hippopotamus density in the pastoral ranches suggests these
areas play an important role in sustaining the population. However, land-use
changes on the ranches and destruction of forests in the Mau catchments of the
Mara River will, unless regulated, restrict the distribution of the hippopotamus
population, and reduction in the volume and quality of water in the Mara River and
its tributaries will lead to a decline in density. This may have significant spillover
effects on other mammalian grazers that are dependent on the grazing lawns maintained by hippopotamuses. Regular monitoring of the Mara hippopotamus population is therefore required to improve our understanding of the response of the
species to land-use and climate changes. Because the Mara River is transnational,
effective management, conservation and monitoring of the hippopotamus in the
Serengeti-Mara ecosystem require close collaboration between relevant institutions
in Kenya and Tanzania.
Acknowledgements
We thank Charles Matankory and Sospeter Kiambi for assistance with fieldwork and for
arranging field logistics, the Kenya Wildlife Service rangers for providing security during field
work, wardens of the Masai Mara National Reserve and the management of the Koyiaki and
Lemek pastoral ranches for allowing us unlimited access to these areas, and Dr Richard H.
Lamprey for useful discussions that helped improve this article. EK was supported by the
Netherlands Fellowship Programme and the University of Groningen, through the Government of Kenya, and by the Frankfurt Zoological Society.
44
Chapter 3
Hippopotamus in the Mara region
45
Chapter four
4
Hippopotamus and livestock grazing near
water points: consequences for vegetation
cover, plant species richness and
composition in an East African Savannah
Erustus M. Kanga, Joseph O. Ogutu, Hans-Peter Piepho and Han Olff
Abstract
Grazing large mammals shape landscape heterogeneity and the composition
and structure of vegetation in savannas. The riverine systems of the Masai
Mara National Reserve of Kenya are inhabited and grazed by the common
hippopotamus all year, which seems to be especially important in vegetation
modification. However, Maasai pastoralists inhabiting pastoral rangelands
adjoining the Mara reserve have been progressively settling down from a
formerly semi-nomadic lifestyle, thus increasing concentration of livestock
grazing along the local rivers, potentially altering these effects of hippo on
the riparian vegetation. We examined if landscapes grazed predominantly by
hippopotamus and livestock differ in vegetation structure, plant species
richness and composition along distance from water sources in the wet and
dry seasons. We used 25 transects, each 5 km long and having 13 sampling
plots each measuring 10 × 10 m2 and radiating from rivers in the reserve
(n = 16) and pastoral ranches (n = 9). In each plot, we measured the height
and visually estimated the percent cover of grasses, forbs, shrubs and bare
ground, plant species composition and richness and grazing intensity. The
riverine areas in pastoral ranches were more heavily and homogonously
grazed than the reserve. The mean plant species richness was similar in
both landscapes but varied in response to spatial gradients in grazing intensity, mostly due to variation in forb and shrub species. By contrast, the plant
species composition differed strikingly between the reserve and the ranches,
while species similarity indices declined with increasing distance from
water, reflecting differential modification of vegetation structure and spatial
heterogeneity by gradients in grazing intensity. Intense and sustained
grazing shifted vegetation composition towards annuals and invasive weedy
species and grasses with short stature that characteristically exclude fires,
thus stimulating densification of woody species. Although the precise differences between hippopotamus and livestock grazing were complex and
varied with landscape and seasons, impacts of livestock grazing altered
plant species composition, the structure and spatial heterogeneity of vegetation and declined less rapidly with increasing distance from water relative
to hippopotamus grazing.
48
Chapter 4
Introduction
Grazing large herbivores influence and are reciprocally influenced by vegetation, an
interaction that shapes landscape heterogeneity, nutrient cycling, vegetation
composition, productivity and structure (McNaughton 1985; Eckert and Spencer
1987; Whicker and Detling 1988; Belsky 1992; Ritchie and Tilman 1995; Frank et
al. 1998; Olff and Ritchie 1998; Dahlberg 2000; Eccard et al. 2000; Shackleton 2000;
Thrash 2000). The impact of grazing on vegetation varies with the species, density,
body size and spatial and temporal distribution of mammalian herbivores (Belsky
1983; Bakker 1985; McNaughton 1986; Olff and Ritchie 1998; Ritchie and Olff
1999). Continuous and intense gazing can exert greater negative impacts on vegetation than can intense but periodic grazing that allows sufficient time for vegetation
recovery. Traditional pastoralists in East Africa have long been aware of this and
thus distributed the impacts of their livestock grazing by moving them periodically
between pastures and allowing sufficient time for vegetation recovery from heavy
utilization. Such mobility also enabled livestock herds to more efficiently exploit
grazing resources distributed unevenly in space and time (Homewood and Rodgers
1991; Oba et al. 2000).
Intense herbivore grazing and trampling creates and widens gaps in vegetation,
changes species colonization matrix of rangelands and differentially influences vegetation structure and species composition (Whicker and Detling 1988; Fusco et al.
1995; Zerihun and Saleem 2000; Oba et al. 2001; Thrash 2000; Van Wieren and
Bakker 2008). Moreover, the effect of grazing on species richness varies regionally
with soil productivity (Olff and Ritchie 1998), such that ecosystems with moderate to
high soil productivity have higher plant species diversity than those with lower
productivity (Olff and Ritchie 1998; Bakker 1998; Milchunas et al. 1988, 1998; Proulx
and Mazumder 1998; Bakker et al. 2006). Plant species diversity can be enhanced by
low to moderate grazing intensity (Bakker 1989; Van Wieren 1995; Hodgson and
Illius 1996) but reduced by high grazing intensity (Milchunas et al. 1988; Hobbs and
Huenneke 1992). As a result, moderate disturbances induced by intermediate grazing
intensities can support high levels of diversity in grassland communities (Grime
1973; Zeevalking and Fresco 1977; Milchunas et al. 1988; Puerto et al. 1990; Hobbs
and Huenneke 1992). Other important consequences of grazing include trampling
that enhances creation of niches for colonizing species and propagule dispersal (Olff
and Ritchie 1998; Bullock and Marriott 2000; Bakker and Olff 2003). As well, grazing
by mixed herds of different herbivore species can enhance vegetation productivity
and plant species diversity, with the effect varying depending on the complement of
herbivore species involved and the status of resources limiting plant growth, such as
water, nutrients and light (Bell 1971; Jefferies et al. 1994; Ritchie at al. 1998; Pausas
and Austin 2001). Thus, the effect of grazing on vegetation can be complex and vary
with the environment, production system, spatial and temporal scales.
Contrasting effects of hippo and livestock grazing
49
Riparian savanna habitats grazed heavily by hippopotamus (Hippopotamus
amphibious Linnaeus 1758) or livestock, experience ecological stresses through
depletion of herbaceous vegetation and increased denudation which can be detrimental to habitat suitability for other herbivores (Lock 1972; Eltringham 1999;
Fleischner 1994; Thrash 2000). Hippopotamuses and livestock sharing the same
landscape may compete for grazing resources, especially close to water, differentially modify vegetation structure and produce contrasting impacts on riparian
habitats as a result. However, despite grazing being an important evolutionary
force shaping vegetation biodiversity and structure in African savanna ecosystems
(McNaughton 1979; 1983; 1984; 1988), a comparative evaluation of the effects of
hippopotamus and livestock grazing on herbaceous plant species richness and
composition in protected and pastoral landscapes has received relatively scanty
attention. Consequently, we investigated how grazing by hippopotamus and livestock modify vegetation structure, species richness and composition along distance
from water sources in a protected and adjoining pastoral landscapes in the Mara
Region of Kenya.
We expected hippopotamus to cause fewer patches of bare ground than livestock
because of differences in their grazing strategies. Hippopotamus pluck grasses and
create patches of short grass lawns whereas livestock pull up shallow-rooted
grasses as a result of their bulk-feeding style. We further expected the varying
intensity of hippopotamus and livestock grazing to exert differential impacts on
vegetation, resulting in differences in vegetation structure, plant species abundance, richness and composition and dominance of particular plant species. More
particularly, we expected vegetation height and cover to increase with increasing
distance from water sources, due to declining grazing intensity away from water, in
both hippopotamus and livestock dominated areas. In contrast, we expected plant
species richness to increase with increasing distance from water, owing to declining
grazing intensity farther from water, and to be higher in areas grazed predominantly by hippopotamus than by livestock. Finally, we expected both hippopotamus
and livestock grazing to encourage the establishment and spread of invasive and
annual plant species through competitive release, especially in heavily grazed
areas. Testing predictions of these hypotheses is essential to enhancing our understanding of the long-term consequences of the ongoing sedenterization by pastoralists and their livestock on the biodiversity of riparian habitats, and the associated
forage resources in savanna rangelands of the Mara and possibly elsewhere.
Materials and Methods
Study area
The Mara Region (Mara) is located in south western Kenya, between latitudes
50
Chapter 4
a
o
orr
Kenya
Ol
rou
Oi
Ch
N
Lemek
20
Nairobi
19
Tanzania
18
Olkinyei
15
24
23
14
21
17 9
25
13
7
11
Koyiaki
22
12
4
10
Se
5 km transects
ren
1
get
iN
atio
5
6
3
2
nal
Pa
rk,
MMNR
Tan
z
ani
a
16
Siana
Kenya
20 km
Figure 4.1 Map of Masai Mara National Reserve and the adjoining pastoral ranches showing the
transects (numbered) radiating from rivers, sampled during 2007–2008. The study area encompassed the protected Masai Mara National Reserve and the Koyaki, Lemek and Ol Chorro Oirua
pastoral ranches.
34º45´ E and 36º00´ E and is bounded by the Serengeti National Park (SNP) in
Tanzania to the south and Siria escarpment to the west (Fig. 4.1). This region forms
the northernmost limit of the Serengeti-Mara ecosystem, covering some 25,000 km2
and straddling the Kenya-Tanzania boundary. The ecosystem comprises several
wildlife conservation administrations and conservation-pastoralist multiple land
use zones in each of the two countries (Sinclair and Arcese 1995). The Mara covers
about 5,500 km2, with the Masai Mara National Reserve (MMNR) covering some
1,530 km2 while the adjacent pastoral ranches, including Koyiaki (931 km2),
Olkinyei (787 km2), Siana (1316 km2), Lemek (717 km2) and Ol Chorro Oiroua (59
km2) cover a combined total of about 4,000 km2. The Mara receives an annual rainfall of about 600 mm in the south east, rising to about 1300 mm at the north
western edge (Norton-Griffiths et al. 1975; Ogutu et al. in press). Rainfall is
bimodal, with the short rains falling from November to December and the long
Contrasting effects of hippo and livestock grazing
51
140
long term (1990–2005)
sample period (2006–2008)
mean rainfall (mm)
120
100
80
Figure 4.2 Periodic and seasonal
rainfall distribution for the Mara
region of Kenya, during the
sampling period and long term
averages.
60
40
20
0
early dry
late dry
early wet
late wet
seasonal periods
rains from January to June, though January and February are often dry. During
the sampling period, rainfall approximated the multiyear norm (Fig. 4.2). The Mara
plains are blanketed by extensive flows of Tertiary phonolitic lava, but dark,
nutrient-rich deep clay vertisols, also called black cotton soils, are more extensive
(Williams 1964; Glover 1966). The vegetation is predominantly grassland, with
isolated scrublands and woodlands, especially along the drainage lines and on hill
tops (Epp and Agatsiva 1980).
The Mara has had a long history of harmonious coexistence between pastoralists and wildlife dating back to several millennia (Marshall 1990; McCabe 2003).
Traditional pastoralism remains the major form of land use to date, and the Mara
still supports a robust and diverse assemblage of large herbivores, with livestock
numbers having been largely stable between 1977 and 2002 despite marked
seasonal fluctuations (Broten and Said 1995; Serneels and Lambin 2001; Lamprey
and Reid 2004). However, marked declines by large wild herbivores in the Mara
during 1977-2003 have been attributed to progressive exclusion from the pastoral
ranches by land use changes, including expansion of large-scale mechanized
commercial and small-scale subsistence agriculture and settlements, human population growth, illegal wildlife harvests and marked climatic variability (Homewood
et al. 2001; Serneels and Lambin 2001; Ottichilo et al. 2001; Ogutu et al. 2009).
These changes progressively degrade, fragment and contract wildlife habitats,
thereby intensifying competition between livestock and wild herbivores in the
pastoral ranches. Together with land subdivision and privatization of land tenure
(Kimani and Prickard 1998) and sedenterization of the formerly semi-nomadic
Maasai pastoralists and the associated intensification of land use in Masailands
(Western et al. 2009), these processes progressively compress wildlife and livestock
distributions and heighten conflicts among the competing and incongruent land
uses, including along riparian habitats.
52
Chapter 4
Sampling design
We selected two landscapes; a protected conservation reserve, the Masai Mara
National Reserve (MMNR), and the adjoining community pastoral ranches of
Koyiaki, Lemek and Ol Chorro Oiroua (Fig. 4.1). Livestock grazing is prohibited in
the reserve except for illegal incursions but livestock and wildlife graze together in
the pastoral ranches. We established 25 random transects, each 5 km long and radiating from the Mara, Talek and Olare Orok Rivers. Sixteen transects were located in
areas grazed by hippopotamus and other wild herbivores, while another nine transects were placed in areas grazed by livestock, hippos and other wild herbivores
(Fig. 4.1). However, along the 5 km riparian strip, hippos and livestock are the main
resident grazers in the MMNR and the pastoral ranches, respectively. Topography
increased rather gently away from rivers within the 5 km distance sampled by
transects in both the reserve (range 1668 m to 1718 m) and the pastoral ranches
(1773 m to 1836 m).
Along each transect, we established 13 sampling plots each measuring 10 × 10 m2
at distances of 0, 100, 250, 500, 750, 1000, 1250, 1500, 2000, 2500, 3000, 4000 and
5000 meters from rivers. Because we expected vegetation to change more rapidly
close to, than far from rivers, we reduced the intensity of sampling away from
rivers to capture this gradient. In each plot, we visually estimated the percent cover
of three growth forms of vegetation (grasses, forbs and shrubs) and bare ground.
Grasses were further subdivided into three height classes: less than 10 cm tall,
10–30 cm, and greater than 30 cm. The cover measurements provided a simple,
quick and efficient method for assessing rangeland conditions. We estimated plant
species richness and abundance by identifying and counting individuals of each
plant species in each plot. Plant specimens that could not be definitively identified
in the field were taken to the East African Herbarium for further identification.
Plant nomenclature follows Agnew and Agnew (1994) and Beentje (1994). Additionally, we recorded and scored the intensity of grazing within a radius of 30 m from
the centre of each plot. The grazing score was set on a scale of one (heavily grazed)
to four (no clear evidence of grazing), based on visual assessment. Finally, because
hippopotamus can deplete herbaceous vegetation and increase denudation in areas
that they heavily utilize, we counted all hippopotamus trails within a 50 m radius
from the center of each plot. Field samplings were carried out during the early dry
(July-August) and late dry (September-October) seasons of 2007 and 2008 and in the
late wet season (March-April) of 2008. We were unable to access the study area to
obtain samples for the early wet (January-February) season of 2008 as scheduled
due to the outbreak of widespread post-election violence in Kenya at the time.
Transects were treated as the unit of replication. The total of 650 samples (n = 325
plots × 2 seasons) dropped to 634 as 16 samples (n = 8 for the reserve and n = 8 for
the ranches) were discarded because the associated plots were either burned in the
dry season, or were inaccessible in the wet season due to heavy rainfall. The 16
Contrasting effects of hippo and livestock grazing
53
transects used in the reserve therefore produced 406 samples during the wet and
dry seasons combined whereas the 9 transects used in the pastoral ranches
produced 228 samples over the same period.
Statistical analysis
Percentage cover of the five vegetation components and bare ground were arcsine
square-root transformed prior to analyses. We then used Mann-Whitney U-test to
relate grazing intensity scores to distance from water and landscapes. Further, we
used a multivariate general linear model to relate the proportions of bare ground,
vegetation cover and species richness to distance from water, landscapes, seasons
and their interactions. We calculated the mean frequency of each species at each
sampling distance from water per landscape and estimated the contribution of each
plant life form to the total species richness between landscapes, seasons and along
distance from water. To estimate the proportion of plant species shared between
landscapes, we used a simple binary measure of species presence and absence, the
Jaccard’s coefficient. This index measures the similarity in species composition
between two communities (A and B), as C = j/(a+b-j), where j is the number of
species common to both communities and a and b are the numbers of species occurring only in communities A and B, respectively. The index ranges from zero, for two
communities with no species in common, to one, for two communities with identical sets of species (Magurran 1988). Lastly, we used gradient analysis to examine
plant species compositions between the two landscapes based on species frequency
of occurrence along distance from water. First, a Detrended Correspondence
Analysis (DCA) was used to determine the length of the gradient, which was 1.3,
and therefore a Principal Components Analysis (PCA) was used for further analyses
(ter Braak 1985). Preliminary analyses showed that samples from plots on river
banks had strong effect on ordination and we therefore removed these samples
from our analysis, to show more continuous effects of gradients. We used grazing
intensity, number of hippopotamus trails and distance from water as passive environmental variables to correlated species composition along axes. All models were
fitted in Statistica (version-8, StatSoft 2007) and in PC-ORD (McCune and Mefford
2006).
Results
Grazing intensity, distribution of hippopotamus trails and vegetation cover
Grazing intensity was higher in the pastoral ranches (mean rank = 397.33, n = 228)
than in the MMNR (mean rank = 272.67, n = 406; Mann-Whitney U = 28083,
P < 0.001). The intensity of grazing did not change significantly along distance from
water in the pastoral ranches (Kendall’s tau_b = -0.069, P > 0.05, Fig. 4.3), but
54
Chapter 4
mean grazing intensity
(increasing factor 1–4)
4
3
pastoral ranches
2
1
Masai Mara National Reserve
0
0–0.75
1–2
2–3
Figure 4.3 Mean grazing intensity
as a function of distance from
water in the Mara region of Kenya
during 2007–2008. Solid and
dashed lines denote the Masai
Mara National Reserve and
pastoral ranches, respectively.
4–5
distance from water (km)
declined significantly with distance from water in the MMNR (Kendall’s tau_b =
-0.401, P < 0.001, Fig. 4.3). The mean number of hippopotamus trails was significantly higher in the MMNR (F1, 618 = 17.6, P < 0.001; 0.31±0.02, n = 406) than in
the pastoral ranches (0.14±0.01, n = 228) and declined significantly with distance
from rivers (F3, 618 = 36.2, P < 0.001), with the decline being steeper for the MMNR
(P = 0.013). Hippopotamus actively utilized a strip within 2.5 km on either side of
the rivers in both landscapes, and this strip was patchily grazed in the MMNR.
The mean percentage cover of the five components of vegetation differed significantly across seasons (F6, 616 = 11.0, P < 0.001), landscapes (F6, 616 = 19.7,
P < 0.001) and along distance from water (F18, 1742 = 8.8, P < 0.001, Fig. 4.4). The
mean percentage cover of bare ground was similar between the dry and wet
seasons but varied between landscapes and with increasing distance from water
(P < 0.001, Fig. 4.4A), such that it was lower in the pastoral ranches than in the
MMNR within 750 m from rivers but became higher in the pastoral ranches than
the MMNR at greater distances from rivers. The mean percentage cover of grasses
shorter than 10 cm was higher in the dry (0.81±0.02, n = 312) than the wet
(0.59±0.02, n = 322, P < 0.001) season and in the pastoral ranches (0.92±0.03, n =
228) than the MMNR (0.57±0.02, n = 406, P < 0.001). It declined significantly with
increasing distance from rivers (P < 0.001), but the decline was only significant for
the MMNR (Fig. 4.4B). The mean percentage cover of grasses 10–30 cm tall was
similar in both seasons and landscapes and increased significantly with distance
from water similarly in both landscapes (P < 0.001, Fig. 4.4C). The mean percentage
cover of grasses taller than 30 cm was higher in the wet (0.59±0.03, n = 322) than
the dry (0.32±0.02, n = 312, P < 0.001) season, as well as in the MMNR (0.58±0.03,
n = 406) than in the pastoral ranches (0.24±0.03, n = 228, P < 0.001). It also
increased with increasing distance from rivers (P < 0.001), but the increase was
only significant for the MMNR (Fig. 4.4D). The mean percentage cover of forbs was
higher in the wet (0.14±0.01, n = 322) than the dry (0.11±0.01, n = 312, P = 0.05)
season but similar in both landscapes and declined with increasing distance from
Contrasting effects of hippo and livestock grazing
55
water similarly in both landscapes (P < 0.001, Fig. 4.4E). The mean percentage
cover of shrubs was comparable between the dry and wet seasons but was higher
in the pastoral ranches (0.07±0.01, n = 228) than in the MMNR (0.04±0, n = 406,
P = 0.012). Furthermore, it declined significantly with distance from water, similarly in both landscapes (P < 0.001, Fig. 4.4F). A multivariate test of significance
showed that grazing intensity was significantly related to percent cover of bare
ground and vegetation categories (Wilks’ Lambda = 0.19, F18, 1768 = 76.8, P <
0.35
bare ground
0.30
0.25
acd
0.6
acd
a
0.4
d
a
d
a
a
a
0.2
0.05
0.00
mean cover (%)
b
cd
0.10
0.0
grass <10 cm
B
c
0.20
forbs
E
a
abc
abc
0.16
ab
ab
bd
bc
0.8
ab
bc
0.12
0.6
cd
d
0.08
d
0.2
0.04
0.0
0.00
grass 10–30 cm
C
0.5
pastoral ranches
Masai Mara National Reserve
0.4
cd
0.2
bcd
bcd
d
0.08
abc
bd
0.12
shrubs
F
a
abc
a
0.06
bc
ab
c
0.04
c
c
0.02
0–0.75
1–2
2–3
distance from water (km)
c
0.10
bcd
0.1
0.0
bcd
d
0.4
0.3
b
ac
0.15
1.0
b
0.8
a
D
1.0
0.20
1.2
grass >30 cm
A
b
4–5
0.00
0–0.75
1–2
2–3
c
4–5
distance from water (km)
Figure 4.4 Mean percent cover of vegetation and bare soil and interactions between landscape and
distance from water in the Mara region of Kenya during 2007–2008. Grey and white bars denote
the pastoral ranches and the Masai Mara National Reserve, respectively. Bars with similar letters
in each category denote means that did not differ significantly.
56
Chapter 4
0.001), such that the cover of bare ground (P < 0.001), grasses shorter than 10 cm (P
< 0.001), forbs (P < 0.001) and shrubs (P < 0.001) increased significantly with
increasing grazing intensity, whereas the cover of grasses between 10-30 cm (P <
0.001) and taller than 30 cm (P < 0.001) declined with increasing grazing intensity.
Species richness and floristic composition
We recorded 145 plant species belonging to 98 genera and 40 families and
comprising 31% forbs, 28% grasses, 23% trees and 18% shrubs. Plant species richness varied between landscapes, seasons and along distance from water (F13, 620 =
9.35, P < 0.001, Table 4.1). The mean number of plant species per 100 m2 plot was
higher in the wet (15.44±0.25, n = 322) than the dry (11.89±0.26, n = 312) season.
There was no significant difference in plant species richness between the MMNR
(13.86±0.21, n = 406) and the pastoral ranches (13.74±0.32, n = 228; Table 4.1;
Fig. 4.5). However, the number of plant species declined with distance from water
in both landscapes (P = 0.012, Fig. 4.5) and was significantly influenced by grazing
intensity (P < 0.001). In particular, species diversity was lower in patches of dense
vegetation cover or bare ground than in patch mosaics supporting mixed short,
medium and tall vegetation.
Growth form characterization
The mean number of forb, grass, shrub and tree species differed between landscapes (F4, 617 = 9.04, P < 0.001), seasons (F4, 617 = 29.66, P < 0.001) and along
distance from water (F12, 1632 = 4.06, P < 0.001). Grazing intensity significantly
modified the number of forb species (P < 0.001) but only marginally influenced the
grass species (P = 0.085, Table 4.2). More forb species were recorded in the wet
Table 4.1 Results of statistical tests of the effects of season, landscape, distance from water and
their interactions on plant species richness in the Mara Region of Kenya. NDF is the numerator and
DDF the denominator degrees of freedom.
Effects
NDF
DDF
Intercept
1
620
Landscape
1
620
Season
1
620
F
P
10697.12
631.17
<0.001
62.46
3.69
0.055
1294.13
76.36
<0.001
MS
Distance
3
620
62.69
3.70
0.012
Landscape × Season
1
620
15.17
0.89
0.345
Landscape × Distance
3
620
18.94
1.12
0.341
Season × Distance
3
620
12.80
0.76
0.520
Grazing intensity
1
620
183.38
10.82
0.001
Contrasting effects of hippo and livestock grazing
57
mean species richness (n/10 m2)
16
15
Figure 4.5 Mean number of
plant species per 10 m2 as a
function of distance from water
in the Mara region of Kenya
during 2007–2008. Solid and
dashed lines denote the Masai
Mara National Reserve and
pastoral ranches, respectively.
14
13
12
Masai Mara National Reserve
pastoral ranches
11
0
0.1 0.25 0.5 0.75 1 1.25 1.5
2
2.5
3
4
5
distance from water (km)
(4.54±0.13, n = 322) than the dry (3.61±0.09, n=312, P < 0.001) season, in the
MMNR (4.26±0.10, n = 406) than the pastoral ranches (3.78±0.13, n = 228,
P < 0.001) and the number of forb species declined with distance from water, similarly in both landscapes (P = 0.010, Table 4.2). The number of grass species was
higher in the wet (9.37±0.15, n = 322) than the dry (7.08±0.10, n = 312, P < 0.001)
season, marginally higher in the MMNR (8.40±0.14, n = 406) than the pastoral
ranches (7.98±0.16, n = 228, P = 0.046, Table 4.2) and did not change with
distance from water in either landscape. The number of shrub species was similar
in both seasons, higher in the pastoral ranches (1.68±0.12, n =228) than the MMNR
(1.00±0.05, n = 406, P = 0.001) and declined significantly with distance from
water in both landscapes (P < 0.001, Table 4.2). The decline was more marked in the
pastoral ranches (P = 0.008). The mean number of tree species was similar between
landscapes but declined significantly with increasing distance from water (P =
0.018) and the rate of decline was steeper for the pastoral ranches (P = 0.025, Table
4.2).
Species similarity index and grazing impact signatures
The reserve and the pastoral ranches had an overall plant species similarity index
of 0.57. This index declined consistently with distance from water such that it was
0.50, 0.49, 0.41 and 0.29 at 0–0.75, 1–2, 2–3 and 4–5 km from water, respectively.
The first two ordination (PCA) axes explained 31% of the variation in plant species
composition between the two landscapes. Grazing intensity was the most important factor influencing species composition along the first axis independent of the
number of hippo trails, while distance from water and number of hippo trails influenced species composition along the second axis (Fig. 4.6). The PCA ordination space
clearly separated the reserve and pastoral ranches based on plant species composition, with four species clusters apparent: (a) species associated with high number of
hippo trails and close to water, were exemplified by Aristida adoensis, Loudetia
58
Chapter 4
simplex, Sida ovate and Tragus berteronianus ; (b) species associated with distance
from water in areas of low number of hippo trails clustered together and included
Eragrostis racemosa, Acacia drepanolobium, and Athroisma psylloides; (c) species
responding to heavy grazing irrespective of number of hippo trails and distance
from water were represented by Cynodon dactylon, Microchloa kunthii, Sida
massaica and Solanum incanum; and (d) species common in low or moderately
grazed areas irrespective of distance from water were exemplified by Chloris
gayana, Cymbopogon spp and Setaria incrassate (Fig. 4.6).
Table 4.2 Results of statistical tests of the effects of landscape and distance from water and their
interactions on the mean number of forb, grass, shrub and tree species in the Mara region of
Kenya. NDF is the numerator and DDF the denominator degrees of freedom.
Growth form
Effects
Forbs
Shrubs
Grasses
Trees
NDF
DDF
F
P>F
Intercept
1
633
165.99
<0.001
Landscape
1
633
18.11
<0.001
Season
1
633
37.85
<0.001
Distance
3
633
3.81
0.010
Landscape × Distance
3
633
0.69
0.582
Grazing level
1
633
15.40
<0.001
Intercept
1
633
46.54
<0.001
Landscape
1
633
12.44
Season
1
633
0.005
Distance
3
633
9.53
0.001
0.936
<0.001
Landscape × Distance
3
633
3.93
0.008
Grazing level
1
633
0.48
0.492
Intercept
1
633
738.66
<0.001
Landscape
1
633
4.08
0.046
Season
1
633
101.54
<0.001
Distance
3
633
1.01
0.386
Landscape × Distance
3
633
0.50
0.684
Grazing level
1
633
2.97
0.085
Intercept
1
633
5.26
0.016
Landscape
1
633
0.32
0.495
Season
1
633
1.64
0.108
Distance
3
633
3.28
0.018
Landscape × Distance
3
633
3.18
0.025
Grazing level
1
633
2.79
0.121
Contrasting effects of hippo and livestock grazing
59
R_0.5
R_0.25
Hippo trails
Axis 2
R_0.1
MMNR
Pastoral ranches
TraguBER
SidaOVA
LoudeSIM
R_1.25
Landscape legend
AristADO
R_0.75
SidaMAS
R_1
R_1.5
MicroKUN
CymbopSP
SolanINC
P_0.5
P_0.1
Axis 1
R_2
P_0.75
ChlorGAY
SetarINC
P_1 Grazing intensity
P_1.25 CynodDAC
P_0.25
P_3
P_1.5
P_2.5
R_2.5
R_3
R_4
Distance
P_5
P_4
P_2
AcaciDRE
R_5
AthroPSY
Eragr.RAC
Figure 4.6 Ordination diagram (PCA) based on plant species frequency, showing the influence of
distance from water, grazing intensity and number of hippo trails on plant species composition in
the Mara region of Kenya. Abbreviation refer to sampling locations and species abbreviation [R and
P refer to reserve and pastoral ranches respectively and the preceding number is the location (km)
from water, while species are abbreviated with the first 5-letter and first 3-letters in caps for Latin
names].
Discussion
The high grazing intensity recorded close to water in both landscapes is characteristic of gradients created by herbivores in areas of concentrated resource utilization
(Thornton 1971; Lock 1972; Eltringham 1999; Fleischner 1994; Thrash 1998;
Thrash 2000; Oba et al. 2001), such as areas close to water, especially in the dry
season when water is limiting. However, grazing intensity declined steeply away
60
Chapter 4
from water in the reserve but not in the pastoral ranches, indicating more homogeneous distribution of grazing impact in the pastoral than the protected landscape.
The heavily grazed strip right next to water had the highest number of
hippopotamus trails in the reserve due to concentrated utilization by hippopotamus
leaving and returning to water daily. In contrast, the more widespread impact of
heavy grazing in the pastoral ranches was due to year-round grazing by resident
livestock, occurring at high densities in the pastoral ranches. The difference in
grazing intensity between the reserve and the pastoral ranches suggest that the
impact of grazing by wild herbivores is more diffusely distributed over the landscape and fades faster away from water sources, resulting in the lower grazing
intensity farther from water recorded for the reserve. However, the intense grazing
impacts within the 750 m radius from rivers in the reserve implicate repeated
grazing and trampling by hippopotamus (Thornton 1971; Lock 1972; Eltringham
1999). Higher concentrations of livestock reduced vegetation cover and plant height
over longer distances from water in the pastoral ranches than the reserve.
Repeated grazing and trampling by hippopotami leaving and returning to water
daily in the reserve created sacrificial zones devoid of most vegetation cover and
extending for nearly 750 m from river banks, similar to findings of earlier studies
(Lock 1972; Eltringam 1999; Thrash and Derry 1999). These sacrificial zones were
characteristically denuded and had the highest grazing intensity scores and percent
cover of short grasses and forbs. In contrast, the higher percent cover of bare
ground beyond 750 m from water in the pastoral ranches than the reserve can be
largely attributed to heavy and sustained grazing and trampling by large herds of
livestock. Grass height was markedly shorter closer to water in both landscapes
and declined significantly with increasing distance from water in the reserve but
not in the pastoral ranches, where short grasses dominated up to 5 km from water.
Forbs were well represented 2-3 km from water in both landscapes but their contribution to ground cover decreased sharply thereafter, implying that forbs favored
short grass lawns created and maintained by heavy grazing close to water. Shrub
cover was higher closer to water in both landscapes and farther from water in the
ranches than in the reserve, implying that heavy grazing enhanced thickening and
spread of shrubs, through suppression of fires and reduced competition with
grasses (Moleele and Perkins 1998; Thrash 1998; Oba et al. 2000; Sharp and Whittaker 2003; Lunt et al. 2007).
Grazing patterns and impacts varied spatially, creating discernible spatial
heterogeneity in vegetation structure and species composition (McNaughton 1983;
Olff and Richie 1998), especially in the reserve. Notably, areas closer to water were
typically more heavily grazed than distant areas in the reserve, similar to other
observations of utilization gradients emanating from points of countertraded use
known as piospheres (Andrew 1988; James et al. 1999). The differences in the
patterns of spatial heterogeneity in vegetation structure away from rivers between
Contrasting effects of hippo and livestock grazing
61
the two landscapes could reflect differences in hippopotamus and livestock grazing
styles and their resultant impacts on vegetation. Although high grazing intensity
maintained grass cover at relatively low levels, grasses nevertheless constituted the
major fraction of the herbaceous layer. Also, the observed seasonal variation in
vegetation cover, suggest that forage availability is limiting in the dry season as
indicated by intensification of impacts of grazing close to water.
Despite their differences in grazing intensity and vegetation structure, both
landscapes had similar plant species richness. Species richness was highest at intermediate grazing intensity scores in the reserve, consistent with predictions of the
intermediate disturbance hypothesis for mesic savannah rangelands (Grime 1973),
suggesting that moderate grazing pressure enabled coexistence of many plant
species thereby enhancing herbaceous species richness (Whicker and Detling 1988;
Olff and Richie 1998; Oba et al. 2001). However, at high and low grazing intensity
scores, species richness declined in the reserve due to domination by a few species,
as has also been observed elsewhere (Del-Val and Crawley 2005; Lunt et al. 2007).
This was not the case for the pastoral ranches where the intermediate level of
grazing intensity was not evident but species richness still followed a humped
distribution from water. This contrast further reflects the contrasting grazing
strategies of hippopotamus and livestock, the two dominant constituents of the
grazing large mammal community in the riparian-edge habitats of Masai Mara. In
the reserve, the impact of hippopotamus grazing created discrete shifting mosaics
of tall, medium and short vegetation cover that promoted the coexistence of herbaceous species, especially at moderately grazed locations from water. In the pastoral
ranches, by contrast, livestock grazing was heavy, creating wide patches of bare
ground and homogenized the structural vegetation pattern, thus enhancing the
establishment of annual and invasive species, which also elevated species richness
at intermediate distances from water similar to patterns found elsewhere (Zerihun
and Saleem 2000). Therefore, grazing of the riparian zone by hippopotamus and
livestock in the Mara had contrasting impacts but allowed diverse herbaceous
species to coexist.
The numerical predominance of forbs within heavily grazed patches is consistent with findings of several other studies (Whicker and Detling 1988; Del-Val and
Crawley 2005), while the high number of plant species recorded in the wet season
probably reflects germination of annual species (Zerihun and Saleem 2000). The
number of forb species reduced with increasing distance from water implying that
grasses outcompeted forbs in areas of low grazing intensity (Oba et al. 2001; Del-Val
and Crawley 2005). Surprisingly, the number of grass species did not change along
the distance-to-water gradient in both landscapes, suggesting that it was insensitive to average vegetation height, and was only marginally responsive to grazing
intensity. Woody species (shrubs and trees) were consistently more abundant closer
to water and in the pastoral ranches, implying that intense grazing close to water
62
Chapter 4
by hippopotamus in the reserve and livestock in the ranches kept grasses short all
year, thereby suppressing fires and stimulating encroachment of woody species
(Moleele and Perkins 1998; Oba et al. 2000; Sharp and Whittaker 2003; Lunt et al.
2007). Our results thus demonstrate that both high and low intensities of grazing
can reduce plant species richness, most notably by altering the species composition
of forbs and shrubs.
Although plant species richness was similar between the reserve and the
pastoral ranches, differences in species composition sufficiently separated the two
landscapes in the PCA ordination space. Additionally, the Jaccard’s similarity index
showed that both landscapes shared 57% of the species recorded; the similarity
index also declined considerably away from water in response to differences in
grazing gradients between the landscapes. Variations in species composition in the
reserve are explained by distance from water, number of hippo trails and grazing
intensity. In contrast, variations in species composition in the pastoral ranches are
less explained by distance from water but mostly by grazing intensity. Notably,
areas far away from water (4–5 km) in the pastoral ranches have similar species
composition to areas close to water (0.1–0.5 km) in the reserve, while species
compositions within 1–5 km in reserve and 0.1–3 km in the pastoral ranches are
distinctive to each landscape (Fig. 4.6). The differences in species composition
between the reserve and pastoral ranches are clearly discernable; and are probably
because of the added effects of heavy year-round livestock grazing in the pastoral
ranches. Large herds of migratory herbivores only use the reserve half-year,
allowing sufficient time for vegetation recovery. Therefore, we attribute differences
in species composition to the different modes and intensities of hippopotamus and
livestock grazing on vegetation structure and spatial heterogeneity between the
reserve and ranches (Thrash 2000).
Certainly, different factors are driving plant species composition between the
reserve and ranches resulting to species clusters in the landscapes. Heavily grazed
areas especially in the pastoral ranches retained species like Cynodon dactylon
which is a grazing lawn species, Microchloa kunthii which establishes well in
degraded areas, an increaser species like Sida massaica and invasive species like
Solanum incanum (Fig. 4.6). Areas highly utilized by the hippos and mostly in the
reserve had typical species such as Aristida adoensis which is common in highly
grazed areas; an increaser species like Sida ovate, weeds such as Tragus berteronianus, and a low palatable grass Loudetia simplex. Species associated with distance
from water and in areas less utilized by hippos but commonly in the pastoral
ranches included Eragrostis racemosa, Athroisma psylloides and Acacia drepanolobium. However, areas in the reserves that were moderately grazed irrespective of
distance from water had a typical species cluster that included Setaria incrassate
which only survives in moderately grazed areas; Cymbopogon spp and Chloris
gayana, a high quality pasture grass. Therefore, the data demonstrate that differen-
Contrasting effects of hippo and livestock grazing
63
tial modes and intensities of grazing by hippopotamus and livestock along riparianedge habitats of Masai Mara have significantly and differentially modified plant
species composition. Furthermore, changes in vegetation structure, plant species
richness and composition induced by hippopotamus and livestock created habitat
and forage diversity that may influence the suitability of riparian habitats for other
mammalian herbivore species in the Mara.
Conclusion
We assumed that the effects of soils and other herbivores on vegetation structure
and grazing intensity would be comparable between the reserve and pastoral
ranches because both landscapes are broadly similar, are adjacent to each other and
are not separated by physical barriers to animal movements. Therefore, patterns in
grazing intensity and vegetation structure detected along the riparian zones in the
reserve and the pastoral ranches could be, either directly or indirectly related to
impacts of hippopotamus and livestock grazing. The gradients in impacts of grazing
we observed altered vegetation structure, species richness and composition.
Despite more intense grazing and associated reduction in vegetation cover in the
pastoral ranches, species richness was similar between the reserve and the ranches,
suggesting that vegetation structure and species composition responded differentially to grazing impacts. Species richness indices concealed important shifts in
plant species composition that occurred along the distance-to-water gradient in
response to varying impacts of grazing. The increasing sedenterization of the Masai
pastoralists and their livestock in the pastoral ranches of the Mara, will progressively increase both the intensity and extent of the impact of livestock grazing,
thereby shifting the vegetation composition towards annuals and invasive weedy
species and further suppressing fires and promoting the establishment and spread of
woody species encroachment, and lowering the quality of the pastoral rangelands
for wildlife. The long-term consequences of these impacts are still unclear and hard
to predict but continued densification of woody species may reduce the suitability
of the pastoral landscapes for livestock and other grazing wild herbivores.
Acknowledgements
We thank Charles Matankory and Sospeter Kiambi for assistance with field work and for
arranging field logistics. We also thank the Kenya Wildlife Service rangers for providing
security during field work, wardens of the Masai Mara National Reserve and the management of the Koyiaki, Lemek and Ol Chorro Oiroua pastoral ranches for allowing us unlimited
access to the study area. EK was supported by the Netherlands Fellowship Program (NFP)
and the University of Groningen through the Government of Kenya and by the Frankfurt
Zoological Society (FZS).
64
Chapter 4
Contrasting effects of hippo and livestock grazing
65
Chapter five
5
Hippopotamus and livestock grazing:
influences on riparian vegetation and
facilitation of other herbivores in the Mara
Region of Kenya
Erustus M. Kanga, Joseph O. Ogutu, Hans-Peter Piepho and Han Olff
Published in Landscape and Ecological Engineering (2011), DOI: 10.1007/s11355-011-0175-y
Abstract
Riparian savanna habitats grazed by hippopotamus or livestock experience
seasonal ecological stresses through depletion of herbaceous vegetation, and
are often points of contacts and conflicts between herbivores, humans and
their livestock. We investigated how hippopotamus and livestock grazing
influence vegetation structure and cover and facilitate other wild herbivores
in the Mara Region of Kenya. We used 5 km-long transects, each having 13
plots measuring 10 × 10 m2 and radiating from rivers in the Masai Mara
National Reserve and adjoining community pastoral ranches. For each plot,
we measured the height and visually estimated the percent cover of grasses,
forbs, shrubs and bare ground, herbivore abundance and species richness.
Our results showed that grass height was shortest closest to rivers in both
landscapes, increased with increasing distance from rivers in the reserve but
was uniformly short in the pastoral ranches. Shifting mosaics of short grass
lawns interspersed with patches of medium to tall grasses occurred within
2.5 km from rivers in the reserve in areas grazed habitually by hippos.
Hence, hippo grazing enhanced structural heterogeneity of vegetation but
livestock grazing had a homogenizing effect in the pastoral ranches. The
distribution of biomass and species richness of other ungulates along
distance from rivers followed the quadratic pattern in the reserve,
suggesting that hippopotamus grazing attracted more herbivores to the
vegetation patches at intermediate distances from rivers in the reserve.
However, the distribution of biomass and species richness of other ungulates
followed a linear pattern in the pastoral ranches, implying that herbivores
avoided areas grazed heavily by livestock in the pastoral ranches, especially
near rivers.
68
Chapter 5
Introduction
African savannas support a diverse indigenous herbivore assemblage besides livestock production by pastoral communities (Skarpe 1991). Understanding the spatial
and temporal dynamics of savannas used by wild herbivores, livestock and people
is essential for their effective management for wildlife conservation and for
promoting human well-being (Coughenour 1991; Baily et al. 1996). The distribution
of herbivores within landscapes is influenced by the composite effects of biotic
factors such as competition, species composition, forage quality and quantity, and
abiotic factors such as topography and distance to water (Milchunas and Laurenroth 1993; Bailey et al. 1996; Illius and O’Connor 2000; Adler et al. 2001; Landsberg
et al. 2003; Redfern et al. 2003). In particular, the distribution of herbivores in arid
and semi-arid savannas is strongly influenced by the location of surface water and
nutritious forage, especially during the dry season, when water becomes progressively limiting and water sources become points of contact and conflict between
herbivores, humans and their livestock (Western 1975; Fryxell and Sinclair 1988;
Illius and O’Connor 2000).
Forage production in savannahs is primarily limited by rainfall, which varies
considerably in space and time, producing patchiness in green forage and
ephemeral water availability (Deshmukh 1984; Boutton 1988). However, African
herbivores have adapted to the seasonal variability in forage and water by regular
seasonal migrations or irregular and unpredictable dispersal movements between
water and forage resources (Fryxell and Sinclair 1988; Fryxell et al. 1988). Furthermore, herbivore distribution patterns in response to resource variability reflect
trade-offs between satisfying their water and forage requirements and minimizing
predation risk (Bergman et al. 2001; Bailey et al. 1996). Herbivore functional groupings based on body size, dietary guild, foraging behavior and digestive physiology,
may further explain variations in patterns of their distributions (Jarman 1974;
Demment et al. 1985; Wilmshurst et al. 2000). Thus, among the more waterdependent herbivores, large-sized animals should travel further distances from
water sources than small animals to satisfy their forage quantity requirements. The
less water dependent herbivores such as browsers are less constrained by distance
to water sources (Western 1975; Redfern et al. 2003). Nevertheless, during dry
seasons, more rapid depletion of forage occurs near water sources.
At the landscape scale, radial gradients in vegetation characteristics originating
from areas of concentrated resource use provide evidence for how herbivores influence vegetation patterns. Herbivore grazing impacts in savannas are higher closer
to water points, creating utilization gradients termed piospheres (Lange 1969;
Andrew 1988; Thrash 1998, 2000; Thrash and Derry 1999). However relatively
little is known about the development of piosphere gradients in ecosystems
supporting diverse assemblages of large wild herbivores, livestock and pastoralists,
Hippo and livestock grazing, facilitation of other herbivores
69
such as the semi-arid savanna ecosystems of East Africa. Riparian savanna habitats
in such ecosystems, if also grazed heavily by hippopotamus (Hippopotamus
amphibious Linnaeus 1758) or livestock, may experience seasonal ecological
stresses through depletion of herbaceous vegetation and increased denudation
(Thornton 1971; Lock 1972; Fleischner 1994; Eltrigham 1999; Oba et al. 2000).
While most wild herbivores are highly mobile and distribute their grazing impacts
more evenly over the landscape, hippos and pastoral livestock are typically centralplace foragers because hippos must leave and return to water whereas pastoral
livestock must leave and return to pastoral settlements daily. This creates zones of
attenuating impacts from water and settlements (Ogutu et al. 2010), which, in turn,
affect the use of riparian habitats and pastoral landscapes by other herbivores.
Hippo grazing can be potentially destructive to vegetation due to a combination of
their large daily food requirements and characteristic grazing style of plucking
grass (Lock 1972; Eltrigham 1974; Thornton 1971). Similarly, heavy livestock
grazing can be detrimental to wildlife habitats (Jones 1981; Quinn and Walgenbach
1990; Fleischner 1994), except under well-managed grazing conditions (Vavra
2005). Fleischner (1994) underscored this point by asserting that the ecological costs
of livestock grazing include the general loss of biodiversity, manifested in reduced
population densities of a wide variety of taxa, as well as aiding the spread of alien
and weedy species; disrupting ecosystem functions, including nutrient cycling and
succession; changes in community organization and vegetation stratification and
damage to soils.
Hippos not only pluck grass but also create and maintain short grass lawns in
areas where they preferentially feed (Olivier and Laurie 1974; Eltringham 1999;
Arsenault and Owen-Smith 2002). The mosaics of closely cropped grass lawns interspersed with areas of long grass alter vertical vegetation structure and create
patchy landscapes of varying vegetation height and cover. This increases spatial
heterogeneity in vegetation structure (Hobbs 1996; Adler et al. 2001), which is
important to other wildlife through its indirect effects on competition, facilitation
and predator-prey relationships (Prins and Olff 1998; Murray and Illius 2000). The
enhanced structural diversity of vegetation patches can facilitate other herbivores,
that differentially select vegetation patches with intermediate biomass and highquality forage (Wilmshurst et al. 2000; Prins and Olff 1998; Olff et al. 2002; Arsenault and Owen-Smith 2002), and avoid patches with higher predation risk, such as
tall grasslands, and other potential predator ambush sites (Hernandez and Laundre
2005; Verdolin 2006). In contrast, relatively few systematic investigations have
found positive benefits of livestock grazing to other wild herbivores (Belsky et al.
1999). As such, the effects of livestock grazing on wildlife populations is an important conservation concern (Fleischner 1994; Prins 2000).
In recent decades, human-induced land use changes, excessive resource extraction, and erection of artificial barriers have increasingly threatened savanna
70
Chapter 5
ecosystems through reduction of grazing areas and disruption of access to water
sources. Consequently, declining savanna rangelands and sedentarization of
pastoralists (Kimani and Prichard 1998; Homewood et al. 2001; Lamprey and Reid
2004; Western et al. 2009) and the associated expansion of settlements and cultivation and intensification of livestock grazing could fundamentally modify the spatial
distribution and movement patterns of herbivores and heighten competition
between livestock and wildlife (Prins and Olff 1998). This could accelerate degradation and fragmentation of rangelands and cause declines in wild herbivore populations (Verlinden 1997; Serneels et al. 2001). If such savannah habitats are utilized
by both hippos and livestock, they may be expected to compete for limiting grazing
resources, especially close to water points. Furthermore, because hippo and livestock grazing can differentially modify vegetation structure, they may have
contrasting effects on the species richness, abundance and distribution of other
wildlife species, especially in dry seasons when most large herbivores concentrate
within 5 km radius of water in semi-arid savannas (Western 1975; Redfern et al.
2003). Our limited current understanding of these processes support the need for
investigations that encompass both protected and pastoral systems and elucidate
how hippo and livestock grazing modify the structure of riparian-edge habitats and
their utilization by other wild ungulates in savannas.
We investigated the effects of hippopotamus and livestock grazing along a
riparian habitat in the Masai Mara region of Kenya, to address the following two
overarching questions: (1) How does hippo and livestock grazing modify vegetation
structure and cover along distance from rivers in semi-arid savannas? (2) How does
the impact of hippo and livestock grazing on vegetation along distance from water
influence the distribution of biomass and species richness of the other wild ungulates? We expected hippo and livestock grazing activities to have contrasting effects
on vegetation structure and cover based on differences in their grazing strategies:
hippos pluck grasses, create and maintain short grass lawns while livestock are
bulk grazers, and frequently uproot shallow-rooted grasses. We also hypothesized
that if the intensity of grazing declines with increasing distance from water
sources, then vegetation height and basal cover will increase with distance from
water in both hippo and livestock dominated landscapes. Since hippos create and
maintain mosaics of short grass lawns intermixed with medium to tall grasses and
livestock grazing creates uniformly short grasslands, hippo-dominated areas will be
more spatially heterogeneous and attract a more diverse array and abundance of
other wild herbivore species close to water but wild herbivores will tend to avoid
areas near water in livestock-dominated areas. Tests of these hypotheses are essential to predicting the long-term effects of sedentarization of pastoralists and the
associated intensification of land use and competition between livestock and herbivores around water sources due to declining forage resources.
Hippo and livestock grazing, facilitation of other herbivores
71
Methods
Study area
The Mara Region (Mara) is located in south western Kenya, between latitudes
34º45´ E and 36º00´ E and is bounded by the Serengeti National Park (SNP) in
Tanzania to the south and Siria escarpment to the west (Fig. 5.1). This region forms
the northernmost limit of the Serengeti-Mara ecosystem, covering some 25,000 km2
and straddling the Kenya-Tanzania boundary. The ecosystem comprises several
wildlife conservation administrations and conservation-pastoralist multiple land
use zones in each of the two countries (Sinclair and Arcese 1995). The Mara covers
about 5,500 km2, with the Masai Mara National Reserve (MMNR) covering some
1,530 km2 while the adjacent pastoral ranches, including Koyiaki (931 km2),
Olkinyei (787 km2), Siana (1316 km2), Lemek (717 km2) and Ol Chorro Oiroua
(59 km2) cover a combined total of about 4,000 km2. The Mara receives an annual
rainfall of about 600 mm in the south east, rising to about 1300 mm at the north
western edge (Norton-Griffiths et al. 1975; Ogutu et al. in press). Rainfall is
bimodal, with the short rains falling from November to December and the long
rains from January to June, though January and February are often dry. The vegetation is predominantly grassland, with isolated scrublands and woodlands, especially along the drainage lines and on hill tops (Epp and Agatsiva 1980).
Several rivers and numerous streams drain the Mara, with the Mara River being
the only permanent river. Sand, Talek and Olare-Orok Rivers, the main tributaries
of the Mara River, are largely seasonal. The Mara River is about 396 km long and
its flow through the MMNR and SNP sustains a large variety of abundant herbivore
species, 10 of which form the main focus of this study and include the hippopotamus (Hippopotamus amphibious, Linnaeus 1758), wildebeest (Connochaetes
taurinus, Burchell 1823), Burchell's zebra (Equus burchelli, Gray 1824), the African
buffalo (Syncerus caffer, Sparrman 1779), topi (Damaliscus korrigum, Ogilby 1837 ),
Coke’s hartebeest (Alcelaphus buselaphus, Gunther 1884), Grant’s gazelle (Gazella
granti, Brooke 1872), Thomson’s gazelle (Gazella thomsoni, Günther 1884), warthog
(Phacochoerus aethiopicus, Gmelin 1788) and impala (Aepyceros melampus, Lichtenstein 1812). Populations of these herbivore species face water-related constraints
in the Mara in the dry season, including increasing water shortages and declining
water quality linked to expanding irrigated cultivation, unregulated water extractions and deforestation of the Mau Forest catchments of the Mara River (Mati et al.
2008).
Marked declines in herbivore numbers in the Mara have been attributed to their
progressive exclusion from the pastoral ranches by land use changes, including
expanding mechanized and subsistence agriculture and settlements, which have
affected over 8% of the Mara and caused land cover changes in at least 36% of the
pastoral ranches adjoining the MMNR (Homewood et al. 2001; Lamprey and Reid
72
Chapter 5
a
rou
rr
Kenya
O
ho
lC
i
oO
N
Lemek
20
Nairobi
19
Tanzania
18
Olkinyei
15
24
23
14
21
17 9
25
13
7
11
Koyiaki
22
12
4
10
Se
5 km transects
ren
get
iN
atio
5
6
3
1
2
nal
Pa
rk,
MMNR
Tan
z
ani
a
16
Siana
Kenya
20 km
Figure 5.1 Map of Masai Mara National Reserve and the adjoining pastoral ranches showing the
transects (numbered), radiating from rivers, sampled during 2007–2008. The four study sites were
the protected Masai Mara National Reserve (1530 km2) and the Koyiaki (931 km2), Lemek (717 km2)
and Ol Chorro Oiroua (59 km2) pastoral ranches.
2004; Mati et al. 2008). These changes have intensified competition between livestock and wild herbivores in the pastoral ranches of the Mara. Moreover, sedentarization of the formerly semi-nomadic Maasai pastoralists (Kimani and Pickard
1998, Western et al. 2009) and the associated intensification of land use and grazing
by large numbers of livestock in the pastoral ranches accelerate range degradation
and fragmentation, including along riparian habitats. Rising temperatures and
recurrent droughts (Ogutu et al. 2007) have further amplified herbivore mortalities
in the Serengeti-Mara ecosystem.
Sampling design
We selected two landscapes; a protected conservation reserve, the Masai Mara
National Reserve, and the adjoining community pastoral ranches of Koyiaki, Lemek
and Ol Chorro Oiroua (Fig. 5.1). Livestock grazing is prohibited in the reserve except
Hippo and livestock grazing, facilitation of other herbivores
73
for illegal incursions but livestock and wildlife graze together in the pastoral
ranches. We established 25 random transects, each 5 km long and radiating from
the Mara, Talek and Olare Orok Rivers. Sixteen transects were located in areas
grazed by hippopotamus and other wild herbivores, while another nine transects
were placed in areas grazed by livestock, hippos and other wild herbivores (Fig.
5.1). However, along the 5 km riparian strip, hippos and livestock are the main resident grazers in the MMNR and the pastoral ranches, respectively. Topography
increased rather gently away from rivers within the 5 km distance sampled by
transects in both the reserve (range 1668 m to 1718 m) and the pastoral ranches
(1773 m to 1836 m).
Along each transect, we established 13 sampling plots each measuring 10 × 10 m2
at distances of 0, 100, 250, 500, 750, 1000, 1250, 1500, 2000, 2500, 3000, 4000 and
5000 meters from rivers. In each plot, we visually estimated the percent cover of
three growth forms of vegetation (grasses, forbs and shrubs) and bare ground.
Grasses were further subdivided into three height classes: less than 10 cm tall, 1030 cm, and greater than 30 cm. The cover measurements provided a simple, quick
and efficient method for assessing rangeland conditions. To estimate how herbivores other than hippo and livestock utilized the landscape, we counted all herbivore dung or pellet piles in each plot. Dung and pellet counts are likely reliable as
relative measures of habitat use by herbivores because none of the herbivore
species we studied is strictly territorial. Additionally, we enumerated all herbivores
sighted within 200 m on either side of each plot. To indicate how herbivores utilize
the rangelands in space and time, the herbivore counts were converted to biomass
using unit weights in Coe et al. (1976). Further, we assessed herbivore predation
risk by estimating the percentage visibility of a predator concealed in vegetation at
a distance of 30 m from the centre of each plot along 0°, 90°, 180° and 270° bearings
using the method of Hopcraft (2002). Finally, because hippos have been shown to
create and maintain mosaics of short-grass lawns, we counted all hippopotamus
trails within a 50 m radius from the center of each plot. Field samplings were
carried out during the early dry (July-August) and late dry (September-October)
seasons of 2007 and 2008 and in the late wet season (March-April) of 2008. We
were unable to access the study area to obtain samples for the early wet (JanuaryFebruary) season of 2008 as scheduled due to the outbreak of widespread post-election violence in Kenya at the time. Transects were treated as the unit of replication.
The total of 650 samples (n = 325 plots × 2 seasons) dropped to 636 as 14 samples
(n = 8 for the reserve and n = 6 for the ranches) were discarded because the associated plots were either burned in the dry season, or were inaccessible in the wet
season due to heavy rainfall. The 16 transects used in the reserve therefore
produced 406 samples during the wet and dry seasons combined whereas the
9 transects used in the pastoral ranches produced 230 samples over the same
period.
74
Chapter 5
Data analysis
We used a multivariate generalized linear model to relate the proportions of vegetation cover in different growth forms and bare ground and the number of hippo
trails to distance from water, landscape, season and their interactions, assuming a
binomial error distribution and a logit link function (Ruppert et al. 2003). We used
a multivariate test of significance to evaluate the significance of the relationships
between the number of hippo trails and the proportions of grass cover and bare
ground. Further, we used a generalized linear model with a log-normal error distribution and the identity link function to relate aggregate herbivore biomass, dung
piles and species richness to distance from water, landscapes, seasons and their
interactions. Finally, we used a multiple linear regression to relate vegetation structure to predation risk and herbivore biomass. Herbivore counts were converted to
biomass, aggregated over all species and log-transformed whereas the percentage
cover of vegetation was arcsine square-root transformed prior to analyses. We
performed residual and influence diagnostics to evaluate the goodness-of-fit of the
selected models and examined plots of distributions of residuals against the linear
predictors, Q-Q plots of the normal distribution, box-whisker plots of residuals and
frequency histograms of residuals to detect outliers or departure from normality.
Preliminary analyses showed no significant differences in the distribution patterns
away from rivers for the early dry (July-August) and late dry (September-October)
season samples within either 2007 or 2008, or between both years. Therefore, we
averaged (pooled) the early and late dry season samples of 2007 and 2008 to obtain
one dry season sample for both years, which we compared with the late wet season
(April-May) sample of 2008 in the analyses. All models were fitted in Statistica
version-8 (StatSoft 2007) and in the SAS GLIMMIX procedure (SAS Institute 2009).
Results
Distribution of hippo trails from water
The mean number of hippo trails was significantly higher (F1, 584 = 36.9, P < 0.001)
in the MMNR (0.31±0.02, n = 406 samples) than in the pastoral ranches
(0.13±0.02, n = 230). Hippo trails declined significantly with distance from rivers
(F12, 584 = 10.8, P < 0.001) and this pattern was similar in both landscapes (F12, 584
= 1.5, P = 0.147; Fig. 5.2). During the wet season, hippos actively utilized a strip
within 2.5 km on either side of the rivers in both landscapes but extended this to
3 km in the pastoral ranches and 4 km in the MMNR during the dry season.
Distributions of vegetation cover from water
The mean percentage cover of the five components of vegetation differed significantly across seasons (F6, 589 = 12.6, P < 0.001), landscapes (F6, 589 = 19.5, P < 0.001)
Hippo and livestock grazing, facilitation of other herbivores
75
mean number of hippo trails
1.0
Masai Mara National Reserve
pastoral ranches
0.8
Figure 5.2 Mean number of
hippopotamus trails as a function of distance from water in
the Mara Region of Kenya
during 2007–2008. Solid and
dashed lines denote the Masai
Mara National Reserve and
pastoral ranches, respectively.
0.6
0.4
0.2
0.0
0
0.1 0.25 0.5 0.75 1 1.25 1.5
2
2.5
3
4
5
distance from water (km)
and along distance from water (F72, 3210 = 3.9, P < 0.001, Fig. 5.3, Table 5.1). The
mean percentage cover of bare ground was similar in both seasons and landscapes,
but declined significantly with increasing distance from water (P < 0.001), and the
pattern of this decline varied between landscapes (P < 0.001), such that the percent
cover of bare ground was lower in the pastoral ranches than in the MMNR within
500 m from rivers but became higher in the ranches than the reserve at greater
distances from rivers (Fig. 5.3A). The mean percentage cover for grasses shorter
than 10 cm was higher in the dry (0.85±0.03, n = 313) than the wet (0.64±0.03, n
= 323, P < 0.001) season, in the pastoral ranches (0.92±0.03, n= 230) than in the
MMNR (0.57±0.02, n = 406, P < 0.001) and declined significantly with increasing
distance from rivers (P = 0.009). The declines with distance was significant for the
MMNR but not for the pastoral ranches (Fig. 5.3B). The mean percentage cover of
grasses 10–30 cm tall was similar in both seasons, but was marginally higher in the
MMNR (0.28±0.02, n =406) than in the pastoral ranches (0.23±0.02, n = 230, P =
0.072). For the reserve, the percent cover of grasses in the 10–30 cm height class
first increased up to 1.25 km from water and then declined with further increase in
distance from rivers. For the ranches, the corresponding percent cover increased
from the river up to 0.75 km, declined between 0.75 km and 2 km and increased
thereafter (Fig. 5.3C).
The mean percentage cover for grasses taller than 30 cm was higher in the wet
(0.54±0.03, n = 323) than the dry (0.28±0.03, n = 313, P < 0.001) season, in the
MMNR (0.58±0.03, n = 406) than in the pastoral ranches (0.25±0.03, n = 230, P <
0.001) and increased significantly away from rivers. The increase was steeper in the
MMNR than the pastoral ranches after 0.5 km from water (P = 0.001, Fig. 5.3D).
The mean percentage cover of forbs was higher in the wet (0.14±0.01, n = 323)
than the dry (0.11±0.01, n = 313, P = 0.007) season, similar in both landscapes but
first increased with increasing distance from water and then declined steadily
thereafter in both landscapes (P < 0.001, Fig. 5.3E). For shrubs, the mean percentage
76
Chapter 5
cover was similar in the dry and wet seasons but higher in the pastoral ranches
(0.06±0.01, n = 230) than the MMNR (0.04±0.00, n = 406, P = 0.009) but declined
similarly with distance from water in both landscapes (P < 0.001, Table 5.1, Fig.
5.3F). A multivariate test of significance showed that there were significant relationships between hippo trails and percent cover of bare ground and grasses (Wilks’
Lambda = 0.81; F6, 627 = 12.4, P < 0.001), such that the cover of bare ground (P <
0.6
bare ground
Masai Mara National Reserve
pastoral ranches
0.5
mean cover
A
0.4
D
forbs
E
shrubs
F
0.8
0.6
0.2
0.4
0.1
0.2
0.0
0.0
grass <10 cm
B
1.0
mean cover
grass >30 cm
1.0
0.3
1.2
0.20
0.16
0.8
0.12
0.6
0.08
0.4
0.04
0.2
0.0
0.5
mean cover
1.2
0.00
grass 10–30 cm
C
1.0
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.02
0.0
0.00
0 0.1 0.25 0.5 0.75 1 1.25 1.5 2 2.5 3
distance from water (km)
4
5
0 0.1 0.25 0.5 0.75 1 1.25 1.5 2 2.5 3
4
5
distance from water (km)
Figure 5.3 Mean percent cover of vegetation and bare soil and interactions between landscape and
distance from water in the Mara Region of Kenya during 2007–2008. Solid and dashed lines denote
the Masai Mara National Reserve and pastoral ranches, respectively.
Hippo and livestock grazing, facilitation of other herbivores
77
Table 5.1 Results of statistical tests of the effects of season, landscape, distance from water and
their interactions on the mean percentage cover of bare ground, grass and shrubs in the Mara
Region of Kenya. NDF is the numerator and DDF the denominator degrees of freedom, respectively.
Variable
Effects
NDF
DDF
F
P>F
FBare ground
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
805.5
1.6
2.0
11.9
0.9
0.4
5.3
<0.001
0.209
0.154
<0.001
0.345
0.968
<0.001
Grass <10 cm
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
1589.8
29.5
87.1
4.6
0.3
0.5
3.0
<0.001
<0.001
<0.001
<0.001
0.617
0.886
0.009
Grass 10–30 cm
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
295.9
1.1
3.2
2.4
0.1
0.8
1.2
<0.001
0.306
0.072
0.005
0.775
0.644
0.315
Grass >30 cm
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
378.4
37.5
60.9
3.6
0.1
0.6
2.7
<0.001
<0.001
<0.001
0.002
0.782
0.862
0.001
Forbs
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
583.6
7.4
0.5
5.0
0.1
0.5
0.3
<0.001
0.007
0.475
<0.001
0.755
0.897
0.988
Shrubs
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
1
1
1
12
1
12
12
594
594
594
594
594
594
594
195.3
0.2
7.0
5.4
0.0
0.3
1.0
<0.001
0.639
0.009
<0.001
0.884
0.992
0.469
78
Chapter 5
0.001), forbs (P < 0.001) and grasses shorter than 10 cm (P = 0.002) increased significantly with increasing number of hippopotamus trails, whereas the cover of
grasses taller than 30 cm (P = 0.001) declined with increasing number of trails.
Herbivore dung piles, biomass and species richness
The mean number of dung piles per plot was significantly higher in the dry
(1.28±0.12, n = 313) than the wet (0.39±0.11, n = 323, Table 5.2) season, reflecting
the influx of the migratory herbivores in the dry season. There were more dung
piles per plot in the MMNR (1.07±0.10, n = 406) than the pastoral ranches
(0.60±0.13, n = 230) but this pattern varied seasonally such that in the dry season
the MMNR had more dung piles per plot (1.69±0.22, n = 199) than the pastoral
ranches (0.84±0.19, n = 114) whereas in the wet season the numbers of dung piles
were similar between the MMNR (0.42±0.06, n = 207) and the pastoral ranches
(0.37±0.08, n = 116).
Table 5.2 Results of statistical tests of the effects of season, landscape, distance from water and
their interactions on the density of herbivore dung piles in the Mara Region of Kenya. NDF is the
numerator and DDF the denominator degrees of freedom, respectively.
Effects
IIntercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
NDF
DDF
MS
1
1
1
12
1
12
12
596
596
596
596
596
596
596
71.84
10.54
2.93
0.33
1.53
0.31
0.13
F
P
200.90
29.49
8.18
0.92
4.29
0.87
0.36
<0.001
<0.001
0.004
0.523
0.039
0.582
0.977
The total herbivore biomass was significantly higher in the dry (3.79±0.25, n =
313) than the wet (2.04±0.24, n = 323; Table 5.3) season. The MMNR had more
herbivore biomass (3.46±0.21, n = 406) than the pastoral ranches (2.38±0.28, n =
230), and biomass increased significantly linearly with distance from water in the
ranches (Fig. 5.4). In the MMNR, by contrast, the total herbivore biomass increased
with distance from water up to 0.5 km, declined between 0.5 and 2 km from water
and then increased thereafter (Fig. 5.4).
Herbivore species richness was significantly higher during the dry (1.05±0.02,
n = 313) than the wet (0.87±0.23, n = 323) season and was higher in the MMNR
(1.00±0.02, n = 406) than in the pastoral ranches (0.93±0.02, n = 230, Table 5.4).
The number of herbivore species increased significantly with distance from water
in the dry season, but in the wet season the number of species did not show a
Hippo and livestock grazing, facilitation of other herbivores
79
consistent pattern of variation with distance from water. The number of species
increased significantly linearly with distance from water in the ranches but in the
reserve it increased between 0 and 0.5 km, declined between 0.5 and 1.5 km and
then increased thereafter (Fig. 5.5).
Relationship between vegetation structure, predation risk and herbivore
biomass
Multiple linear regression analysis showed that predation risk was negatively
correlated with the percentage cover of grasses shorter than 10 cm (t632 = –3.18;
P = 0.001) and between 10 and 30 cm tall (t632 = –3.22; P = 0.001) but positively
correlated with the percentage cover of grasses taller than 30 cm (t632 = 2.52;
P = 0.011). Multiple linear regression analysis also showed that herbivore biomass
was negatively correlated with predation risk (t634 = –6.09; P < 0.001), such that
herbivore biomass declined as predation risk increased, implying that herbivores
avoided areas dominated by grasses taller than 30 cm.
LN (herbivore biomass)
7
Masai Mara National Reserve
pastoral ranches
6
Figure 5.4 The distribution
of herbivore biomass along
distance from water in the
Mara Region of Kenya during
2007–2008. Solid and dashed
lines denote the Masai Mara
National Reserve and pastoral
ranches, respectively.
5
4
3
2
1
0
0
0.1 0.25 0.5 0.75 1 1.25 1.5
2
2.5
3
4
5
distance from water (km)
Table 5.3 Results of statistical tests of the effects of season, landscape, distance from water and
their interactions on aggregate herbivore biomass in the Mara Region of Kenya. NDF is the numerator and DDF the denominator degrees of freedom, respectively.
Effects
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
80
Chapter 5
NDF
DDF
MS
F
P
1
1
1
12
1
12
12
596
596
596
596
596
596
596
4998.00
451.30
172.69
66.45
34.56
26.49
50.66
270.81
24.45
9.36
3.60
1.87
1.44
2.74
<0.001
<0.001
0.002
<0.001
0.172
0.145
0.001
Discussion
Hippopotamus and livestock grazing in the Mara influence the structural patterns
of vegetation along distance from rivers. Other herbivores attracted to areas close
to water and to the short and nutritious grass swards maintained by hippos, exert
additional impacts on vegetation structure and basal cover in the piosphere gradients originating from rivers (Butt et al. 2009; Ogutu et al. 2010). Piosphere gradients
were not clearly discernible in the pastoral ranches possibly due to the intense and
homogenizing effects of livestock grazing (Fig. 5.3; Adler et al. 2001). Hippos
extended their grazing range further from water points in dry seasons, likely due to
forage depletion near water as the dry season progresses (O’Connor and Campbell
1986), but this seasonal range expansion was more constrained in the pastoral areas
where herders graze livestock along rivers in dry seasons, thus depleting vegetation
and interfering with hippo ranging pattern from water (Belsky 1999; Thrash 2000).
An earlier record of hippo grazing range along the Mara river, north of the SNP in the
species richness (no. species)
1.3
Masai Mara National Reserve
pastoral ranches
Figure 5.5 The distribution
of large herbivore species
richness (number of different
species) along distance from
water in the Mara Region of
Kenya during 2007–2008.
Solid and dashed lines denote
the Masai Mara National
Reserve and pastoral ranches,
respectively.
1.2
1.1
1.0
0.9
0.8
0.7
0
0.1 0.25 0.5 0.75 1 1.25 1.5
2
2.5
3
4
5
distance from water (km)
Table 5.4 Results of statistical tests of the effects of season, landscape, distance from water and
their interactions on large herbivore species richness in the Mara Region of Kenya. NDF is the
numerator and DDF the denominator degrees of freedom, respectively.
Effects
Intercept
Season
Landscape
Distance
Season × Landscape
Season × Distance
Landscape × Distance
NDF
DDF
MS
1
1
1
12
1
12
12
596
596
596
596
596
596
596
544.39
4.67
0.66
0.57
0.06
0.27
0.49
F
3347.31
28.7
4.04
3.53
0.35
1.68
3.01
P
<0.001
<0.001
0.044
<0.001
0.553
0.068
<0.001
Hippo and livestock grazing, facilitation of other herbivores
81
1970s, of 1.5 km (Olivier and Laurie 1974), was smaller than the present estimate of
4 km, suggesting that the recent dramatic increase in the population of Mara hippos
(Kanga et al. 2011) or the progressive compression of hippo distribution by changing
land use over the last three decades and competition with livestock and other
herbivores along the riparian-edge habitats (Reid et al. 2003), probably compel
hippos to travel further from water to satisfy their forage requirements.
Unlike in the pastoral ranches, a sacrificial zone with heavily depleted grass
cover due to repeated grazing and trampling by hippos leaving and returning to
water (Thrash and Derry 1999) was well established in the MMNR and extended
for about 250 m from river banks. This area was characteristically denuded, had
the highest number of hippo trails and the highest percent cover of short grasses
and forbs. However, the pastoral ranches had a higher percent cover of bare ground
than the MMNR beyond 750 m from water, which can be attributed to impacts of
heavy grazing and trampling by large numbers of livestock (Fig. 5.3A). Therefore,
differences in their grazing strategies may explain the contrasting impacts of hippo
and livestock grazing on patterns of variation in vegetation structure and cover
along gradients extending away from riparian habitats in the Mara. We found that
close to water, grass was very short in both landscapes, and that this short grass
cover declined progressively with increasing distance from water in the MMNR but
remained high in the pastoral areas up to 5 km from water, thus signifying the
effects of heavy livestock grazing in the pastoral ranches. The shifting mosaics of
short grass lawns interspersed with patches of medium and tall grasses were characteristically evident within 2.5 km from water in MMNR and can be attributed to
hippo grazing, as this distance corresponds to the active grazing range of hippos
from water. These mosaics of short grass lawns are well recognized for their high
quality-forage (McNaughton 1983; Fryxell 1991; Adler et al. 2001; Olff et al. 2002).
In contrast, vegetation cover in the pastoral ranches was dominated by homogenous short grasses, often shorter than 10 cm, associated with intense and sustained
livestock grazing (Fig. 5.3B). Although grazing kept grass height relatively low,
grasses still constituted the main fraction of herbaceous cover.
Spatial heterogeneity of vegetation increases with patch grazing and decreases
with homogeneous grazing (Adler et al. 2001), and influences how herbivores utilize
landscapes, especially in areas where forage and water availability are major
limiting factors, such as the Masai Mara. Our results show that herbivore dung,
biomass and species richness were significantly higher during the dry than the wet
season, implying that forage and water are more heavily utilized during dry
seasons in Mara. Furthermore, herbivores utilized the MMNR more during the dry
season, because they are excluded from the pastoral areas by heavy livestock
grazing at this time, and because of the influx of enormous herds of migratory
wildebeest, zebra and Thomson’s gazelles. Herbivore biomass and species richness
were higher in the MMNR than the pastoral areas, with quadratic distribution
82
Chapter 5
patterns from water apparent in the MMNR and linear patterns evident in the
ranches (Fig. 5.4 and 5.5), implying that herbivores were more repelled from water
points in the pastoral ranches. We postulate that the effects of shifting mosaics of
grazing lawns maintained by hippos improve quality of available forage close to
water that attract herbivores in the MMNR riparian-edge habitats (McNaughton
1983; Owen-Smith 1988; Fryxell 1991; Eltringham 1999; Adler et al. 2001; Olff et
al. 2002; Arsenault and Owen-Smith 2002; Verweij et al. 2006; Van Wieren and
Bakker 2008). In contrast, the intense and homogenous livestock grazing in the
pastoral ranches limit forage intake by herbivores (Arsenault and Owen-Smith
2002; Verweij et al. 2006), and repel herbivores from water points. Ultimately,
continued sedentarization of pastoralists in the Mara region will progressively
exclude herbivores and other wildlife from the pastoral areas of the Mara, similar
to patterns reported for other parts of Masailand (Western et al. 2009; Msoffe et al.
2011).
High vegetation cover limits the ability of herbivores to scan their surroundings,
but also provide good concealment cover for ambush predators (Hopcraft 2002;
Verdolin 2006; Hopcraft et al. 2010). Our results demonstrate that herbivores were
more abundant in areas of short to medium grass swards, than in areas dominated
by tall grasses and hence associated with higher predation risks. This may imply
that herbivores were avoiding areas of tall grasses, not only because these areas are
of lower forage quality but also because of the increased risks of predation. Specifically, predation risk was lower in areas dominated by grasses shorter than 30 cm
but higher in areas dominated by tall grasses, implying that areas with mosaics of
short grass lawns maintained by hippo grazing likely reduced predation risk. Therefore, loss of keystone species like hippopotamus may adversely impact the integrity
of ecosystems and their services (Coppollilo et al. 2004)
Hippopotamus and other herbivores are apparently able to spread impacts of
their grazing in the MMNR and sustain characteristic patterns of distribution of
vegetation structure and cover, enabling them to access more forage resources
through the dry season (Arsenault and Owen-Smith 2002). This could explain the
higher herbivore biomass and species richness we recorded in the MMNR. In
contrast, the pastoral ranches experience year-round intense livestock grazing,
resulting in homogenous short grasslands, thus amplifying competition for forage
and water in areas accessed by pastoralists, including parts of the MMNR, especially during dry periods.
The grazing gradients from riparian-edge habitats in the MMNR revealed by this
study are consistent with findings of other studies conducted elsewhere in piospheres (Andrew 1988; Perkins and Thomas 1993; Thrash and Derry 1999), but were
hardly evident in the pastoral ranches. Our results thus demonstrate conspicuous
differences in the effects of hippopotamus and livestock grazing, with hippo grazing
enhancing spatial heterogeneity of vegetation which, in turn, attracts a rich herbi-
Hippo and livestock grazing, facilitation of other herbivores
83
vore assemblage, whereas livestock grazing homogenizes landscapes and repel wild
herbivores, especially from water sources.
Acknowledgements
We thank Charles Matankory and Sospeter Kiambi for assistance with field work and for
arranging field logistics. We also thank the Kenya Wildlife Service rangers for providing
security during field work, wardens of the Masai Mara Reserve and the management of the
Koyiaki and Lemek pastoral ranches for allowing us unlimited access to the study area. EK
was supported by the Netherlands Fellowship Program (NFP) and the University of
Groningen through the Government of Kenya and by the Frankfurt Zoological Society (FZS).
84
Chapter 5
Hippo and livestock grazing, facilitation of other herbivores
85
Chapter six
6
Hippopotamus as agents of change on
riparian-edge communities: A synthesis
Introduction
Hippopotamus are semi-aquatic herbivores, and water, in which mating, playing,
fighting and defecation all takes place, is at the centre of their social life. That is
probably why hippopotamus means “river horse” in the ancient Greek language.
Hippos also require an open terrestrial grazing land, where their grazing range is
limited to grassland mosaics within immediate reach of water. This is a unique
lifestyle compared to other grazing herbivores. Due to their large size, habitat and
food requirements, hippos tend to have substantial impacts on riparian communities. Their grazing strategies and physical alterations to the environment affect
plant and wildlife community compositions in areas they frequently graze
(Thornton 1971; Lock 1972; Eltringham 1999). Due to their high dependence on
water, their fortunes fluctuate along with those of the water and the wetlands they
depend on (Klingel 1995). Despite our knowledge that hippos require water and
wetlands for survival, and that wetlands are severely threatened, research on
hippopotamus is rare in Kenya. Therefore, this thesis sought to address an important knowledge gap, by studying the nature and dynamics of human-hippo conflicts
throughout Kenyan wetlands to understand the challenges facing the conservation
of this unique herbivore, under changing land-use and increasing anthropogenic
impacts on wetlands; long-term hippo population dynamics in a premier conservation estate in Kenya and East Africa, the Masai Mara. Furthermore, we analyzed
the impact and consequences of hippo grazing on vegetation and other herbivores
in riparian-edge habitats of the Mara and how these are modified by land use and
seasonality in rainfall.
Insights into the ecology of hippopotamus
Wildlife population size estimates are central to conservation and management of
biological diversity. Counting animals, however, can be difficult as some animals
are on the move and many others actively avoid human counters, complicating
detection and accurate counting. Consequently, we are rarely in a position to obtain
absolute total counts in the wild. In studying the hippopotamus population in the
Mara Region of Kenya, we established that our 2006 census is the first detailed and
complete count for hippopotamus along the Kenyan section of the Mara River
system (Chapter 3), although other counts provide good general references. Results
from our count showed that there are over 4,000 hippos in Mara and this may be
one of the largest single hippo population in Kenya. Thus the Mara is an important
hippo conservation stronghold in Kenya and currently holds about 3% of the
African hippopotamus population. Our experience from the census is that hippos
are usually submerged and do not synchronise their surfacing, thus there are high
88
Chapter 6
density (no./km river)
40
Masai Mara National Reserve
pastoral ranches
30
R2 = 0.980
20
10
Figure 6.1 Dramatic increases
of hippopotamus densities in
the Mara Region of Kenya
during 1958 to 2006. Diamond
and pyramid marks denote
MMNR and pastoral ranches
respectively.
0
1960
1970
1980
1990
2000
2010
chances of undercounting during ground censuses as hippos actively avoid counters. However, more accurate numbers can be obtained if individuals and groups
are in shallow water and pools. Our results demonstrate that the Mara hippo population density dramatically increased during 1958 to 2006 (Fig. 6.1) and that hippos
spatially expanded their range into the pastoral ranches, suggesting that the
community pastoral ranches provide important habitats for hippos in the Mara.
Smuts and Whyte (1981) describe the reproductive strategy of the hippo as one
well adapted to the semi-arid environments, such that when resources are limiting,
populations are able to maintain stable populations by delayed sexual maturity and
fecundity and so adjust to the carrying capacity of the environment; equally, populations are capable of rapid increase when resources become abundant. Surprisingly, our results reveal considerable increases in hippopotamus numbers and range
expansions, while hydrological investigations showed that between 1973 and 2000,
the Mara River flow regimes changed drastically, with sharp increases in peaks,
attenuated hydrographs and reduced base flows (Mutie et al. 2005), that unfavorably affected shelter and day living space for hippos (Mati et al. 2008). In addition,
Oliver and Laurie (1974) and Tembo (1987) suggest that shelter and day-living space
generally regulate hippopotamus populations. Therefore, our census results have
important implications for hippopotamus ecology, because, contrary to these
suggestions, the Mara hippo population dramatically increased during 1970 to 2006
when their shelter and day-living spaces were reasonably unstable. The only plausible explanation that we can put forward for this situation is that the Mara has
relatively abundant and sufficient forage (O’Connor and Campbell 1986; Boutton
et al. 1988; Onyeanusi 1988), which may outweigh the limitation imposed by deteriorating shelter and day-living space. However, it is evident that the massive
destruction of the Mau Forests which forms the catchment of the Mara River and
the spiralling extraction of water for irrigations and expansion of settlements, all of
which combine to reduce the quantity and quality of water in the Mara River will,
Synthesis
89
unless regulated, certainly reduce hippo range in the Mara and probably also their
abundance.
The spatial range expansion for hippos recorded in Chapter-3 further reveals
that the occupation of Keekorok pool and the Mara River outside MMNR are new
colonisation events which occurred after 1982. The nearest hippo groups to
Keekorok pool are at Mara Bridge, a distance of over 25 km, while the colonised
range along Mara River is over 60 km upstream into the pastoral ranches. This
demonstrates a remarkable capacity of the common hippopotamus to disperse to
new localities. The length of the Mara River that was covered during the count is
99 km (53.3 km in MMNR and 45.7 km in the pastoral ranches). Using the identified
hippo grazing range lengths (Chapter 4 and 5), we estimate the hippo grazing area
at 426.4 km2 and 274.2 km2, for the MMNR and pastoral ranches, respectively,
resulting in a nocturnal terrestrial feeding density of 4.51 hippos/km2 in the MMNR
and 5.73 hippos/km2 in the pastoral ranches during dry seasons. However, during
the wet seasons, hippos ranged shorter distances, shifting their nocturnal terrestrial feeding density upwards to 7.22 hippos/km2 in the MMNR and 6.87 hippos/
km2 in the pastoral ranches. An interesting question then is whether the Mara is
overpopulated with hippos? Interestingly, the present hippo densities have not
changed much from the 5 hippos/km2 recorded in the MMNR in 1970 (Olivier and
Laurie 1974), and are similar to 4.5 hippos/km2 recorded in Liwonde National Park,
Malawi in 2002/2003 (Harisson et al. 2007), 4 hippos/km2 reported for Luangwa
River, Zambia in 1982/1983 (Tembo 1987) but are lower than the 28 hippos/km2
reported for Queen Elizabeth National Park, Uganda between 1963 to 1967 (Field
and Laws 1970). It is therefore well possible that Mara can hold more hippos than it
supports at present.
Ecological influences of hippopotamus within Mara riparian-edge
community
Hippopotamus grazing pressure transforms tall grasslands into a shifting mosaic of
short, medium and tall grass patches, and they maintain the short grass swards
through repeated and continuous grazing. This is demonstrated by our results in
Chapter 4 and 5, in which areas within the MMNR where hippos are the main resident grazers, the spatial distribution of their grazing lawns contributes significantly to structural diversity of vegetation within the strip of 0-3 km from rivers
(Fig. 6.2, O’Connor and Campbell 1984). Although McNaughton (1984, 1985) notes
that large herds of migratory mesoherbivores create and maintain grazing lawns,
our results suggest that hippos have superior effects on lawn formation and maintenance within the riparian-edge habitats. Our suggestion is supported by the fact
that even with over one million immigrant mixed herds of wildebeest, zebra and
90
Chapter 6
river/water point
;
ne
zo
e g, s
ific lin re
cr mp ivo
,
Sa tra rb
ne
zo
he
all
ion
a t of d
iliz cs n
ut sai m a es
po mo diu tch
hip ith me pa
ne
gh w rt, ass
zo
o
r
Hi
sh ll g
ion ed s
at ix ore
ta
iliz m iv
ut by erb
po zed oh
hip utili mes
w
t
Lo bu s of
rd
he
Vegetation is destroyed'
forbs and bare ground
cover dominate, with
noticeable invasive/
weedy/annual species
250 m
Lawns and heavily grazed
grass/forbs mixture, with
noticeable woody species
recruit and establish
3500 m
Tall grasses, accumulated
biomass, minimal forbs
and woody species gets rare
5 0 00 m
Figure 6.2 Hippopotamus grazing in the MMNR, diversify vegetation structure and enhance
spatial heterogeneity by creating and maintaining mosaics of short and medium grass patches.
Thomson gazelle in the Mara during the dry season, evidence of grazing lawns was
only clear within the 3 km strip zone from rivers, a zone that correspond with
hippo grazing range (Chapter 4 and 5), notwithstanding the fact that the mixed
herds of mesoherbivores freely grazed beyond the 3 km hippo range from rivers.
There are many other factors influencing vegetation structures in the Mara, but our
results show that hippopotamus grazing has distinct impacts at the riparian-edge
habitat patch level. This finding guides us to conclude that hippopotamus is a
keystone ecosystem engineer able to profoundly modify ecosystems and facilitate
other herbivores in the Mara (Owen-Smith 1987) in ways that even large herds of
mesoherbivores cannot.
Hippos establish a well-developed pathway network for exit and entry into
water and for accessing their grazing range. Their trampling effects at riverbank
exits cause considerable increase in bare ground, which, in addition to the denuded
trails, accelerate soil erosion (Thornton 1971; Lock 1972). Furthermore, the shortgrass grazed areas decrease available combustible biomass, promoting their invasion by woody species (Fig. 6.2, Fig. 6.3, and Chapter 4 and 5). The effects of hippopotamus grazing on soils, plants and other herbivores was most pronounced immediately at the riverbanks due to their trampling effects (the ‘piosphere’ effect, Lange
1969) and at intermediate distances from rivers (Fig. 6.3). Hippo-grazing effects
enhanced vegetation structural diversity and improved spatial heterogeneity associated with the riparian zone, and thus enhanced species diversity and caused
Synthesis
91
mosaic of short,
medium and tall
grass patches zone
grass height/biomass
relative measure
predation risk
grass quality
Figure 6.3 Effects of hippopotamus grazing on the quality and
quantity of grass with distance
from rivers in Mara and its associated influence predations risk to
mesoherbivores.
edge of hippo grazing range
distance from rivers
compositional shifts in both plants and herbivores at intermediate distances from
rivers within the MMNR (Chapter 4 and 5). However, grazing effects of mesoherbivores and livestock did not achieve similar outcomes. Heavy grazing by livestock in
the pastoral ranches reduced the quality and quantity of forage and vegetation
cover, and further adversely affected plants and herbivore species richness, abundance and composition. Sedentarization of the pastoral Maasai in the Mara has
resulted in year-round congregation of livestock around water sources, which
clearly reduces standing biomass and herbaceous species diversity, thus reducing
the ability of the pastoral ranches to support diverse wildlife assemblages.
Threats to hippopotamus
The common hippopotamus’ population in Africa declined due to habitat loss,
exploitation and conflicts with people by 7-20% during 1996 to 2004, and the
current population is likely between 125,000 and 148,000 hippos in the wild
(Lewison and Oliver 2008). Unfortunately, detailed information on hippopotamus
population trends and associated threats are lacking in Kenya. However, results in
Chapter 2 and 3 show numerous threats to hippopotamus conservation across
Kenya and the major threats include declining habitats and range, incompatible
land uses, land-use changes and conflicts with people. In the Mara region, deforestation, agricultural expansion and intensification (including irrigation) and
expansion of human settlements, have negatively affected the hydrologic, physical
and social aspects of the Mara River Basin (IUCN 2000; Gereta et. al. 2002; Mati et
al. 2008), with potentially adverse consequences for the Mara River hippopotamus
population. Although this thesis did not directly quantify threats to hippos in the
Mara, it is probable that changes in the hydrological cycles of the Mara River could
reduce the hippo range and hence also their abundance. This bleak scenario is repli-
92
Chapter 6
cated in many other Kenyan wetlands (Crafter et al. 1992), which are important for
hippopotamus conservation.
Human-hippo conflicts pose an important threats and challenges to hippo
conservation endeavors in Kenya, especially with increasing pressures on wetlands
and a realization that a substantial number of hippos inhabit wetlands that often
extend outside of protected areas, into the agricultural landscapes, thus increasing
interactions between people and hippos. Consistent with these increased interactions, human-hippo conflicts have increased in space and time in Kenya (Chapter 2).
These conflicts are not likely to decline and may in fact be intensifying as
increasing human population and land-use changes impact more adversely on
wetlands. However, historical records show that hippos have long been a problem
to rural farmers in various parts of Kenya and were accorded official protection in
the 1920s to reduce their persecution (Kenya Game Department 1953).
We analyzed the nature, intensity, seasonality, spatial and temporal patterns in
human-hippo conflict incidences reported from wildlife stations Kenya-wide, over a
12-year period spanning 1997-2008, and established that human-hippo conflicts
indeed increased 26-fold. Hippos killed 67 and injured 88 people, in addition to
being involved in 47 incidences of livestock attacks. In retaliation, 522 hippopotamus were killed as a problem animal control strategy by wildlife mangers. Based
on the Kenya Wildlife Service 8-Conservation Regions across Kenya then, our
results show that 50% of these regions experienced considerable increases in
conflicts over time. This dramatic rise in human-hippo conflicts in Kenya is a
consequence of both natural and human-mediated disturbances on hippo habitats
including wetlands (Crafter et al. 1992). Hippopotamus habitat losses have accelerated through conversion of hippo grazing ranges into agricultural use, while
aquatic refugia are diminished by irrigation and other human activities along
riverine systems (Eltringham 1999; Smuts and Whyte 1981). The high frequency of
crop raiding behavior shown by hippos is construed to indicate increasing agricultural expansion on wetland habitats in Kenya. However, the high lose of hippos
through unregulated problem animal control measures, is of conservation concerns.
Illegal hunting (poaching) of hippopotamus is rare in Kenya, but in the rest of
Africa, poaching of hippos is mostly done for meat though there is increasing
demand for hippo canine teeth (Weiler et al. 1994), the ivory of which is considered
better quality than that of the elephant. If trade in hippo teeth develop and thrive,
then hippopotamus will be at serious risks (Eltringham 1993).
Concluding Remarks
In this thesis, I show that hippos increased dramatically in numbers and extended
their range in the Mara against a background of deteriorating habitat conditions
Synthesis
93
related to recurrent droughts, rising temperatures and progressive habitat desiccation and fundamental land use changes, implying that hippos can increase rapidly
even in a context of considerable climatic variability. However, it is certain that
anthropogenic land-use changes affecting the Mara River and its hydrological
cycles may constrict the Mara hippo population range and possibly reduce their
abundance. The hippo census was very challenging and further investigations
should focus on how to improve census techniques for hippopotamus.
Our results indicate that ecological processes or habitat characteristics associated with hippo grazing structure plant species richness and composition and the
distribution of other herbivore along riparian-edge habitats. Plant species richness
and composition varied along the hippo-grazing gradient from rivers, partially in
response to spatial heterogeneity of vegetation structure resulting from hippo
grazing activities. Consequently, hippos facilitated grazing by mesoherbivores and
enhanced co-existence of plant species, thereby promoting species diversity. Therefore, along the riparian-edge habitats the hippopotamus is an important ecosystem
engineer and a keystone herbivore, creating and maintaining shifting mosaics of
grazing lawns, an ecosystem function that was not effectively performed by large
herds of grazing mesoherbivores. Furthermore, large herds of livestock in the
pastoral ranches homogenized vegetation and shifted plant species composition
towards invasive and unpalatable annuals; in addition, presence of livestock
repelled other herbivores away from water sources and disrupted their distribution
patterns.
94
Chapter 6
Synthesis
95
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Summary
Samenvatting
Summary
Samenvatting
Executive summary
African savannahs are endowed with a high diversity and abundance of large
mammalian herbivores but increasing degradation, fragmentation and loss of
wildlife habitats particularly in rangelands primarily to cultivation and settlements
due to human population growth and development is accelerating wildlife population declines. The expansion of settlements and cultivation has had a major adverse
effect on the distribution and abundance of wild herbivores, both at the population
and community levels. One such herbivore species is the common hippopotamus,
commonly referred to as “hippo”. Hippos have featured in human affairs since at
least the time of the Egyptian Pharaohs, where they were venerated as gods and
have been portrayed in art down the ages. Primary threats to hippopotamus
conservation include the loss of essential grazing lands to cultivation, settlements,
illegal harvests for bushmeat and canine teeth ivory trade, retributive killings for
destroying crops and growing pressure on fresh water resources, leading to the loss
of their prime habitats. Ultimately, the reliance of hippos on fresh water habitats
puts them at odds with increasing human populations, exacerbating their vulnerability. Due to increasing human population and agricultural expansion and development in and around wetlands, hippos often run into frequent conflicts with people.
The protection of riparian and wetland habitats and control of poaching of hippos
are therefore two pressing challenges facing contemporary hippo conservation
efforts, as are measures to prevent the drying-up of water courses due to deforestation, spiraling water extraction and global warming and loss of riparian-edge
grazing grounds.
Hippopotamus are classified together with elephant, rhinoceros and giraffe as
“megaherbivores”, a select group of terrestrial plant-feeding mammals with a body
mass in excess of 1,000 kg. The very large body size has fundamental consequences
for the ecology of this group that sets them apart from other herbivores. Megaherbivores are generally threatened throughout their historic distributional ranges,
and their continued survival is precarious as their expansive habitat requirements
often brings them into direct conflicts with rising human populations. Nonetheless,
hippopotamus differ from the other megaherbivores in having a dual requirement
of a day-living space in water and an open grazing range often visited at night. This
requirement greatly affects the manner in which hippos utilize resources and
survive in landscapes dominated by high human population densities and continuous land use changes. Recent estimates covering 1996 to 2004, suggest that about
125,000 -148,000 hippos occur in fragmented populations in rivers, lakes and other
wetlands of 36 African countries, of which 20 countries have confirmed declining
populations, seven have populations of unknown status, nine have stable populations and three (Algeria, Egypt and Mauritania) have experienced recent extinctions. Based on this estimated global population, coupled with intensifying
112
Summary
conservation threats, the hippopotamus was categorized as Vulnerable on the IUCN
Red List in 2006.
Despite its imposing size, Vulnerable status and grazing impacts on riparian
habitats, and rising conflicts with people, the hippopotamus has not been well
studied or monitored in many parts of its range, including Kenya where the species
has been officially protected since the 1920s. Yet, monitoring of hippopotamus is
necessary to understand the factors under-pinning its population dynamics and
hence to develop an understanding of how the hippos influences, and are influenced by changes in riparian habitats and how they respond to land-use changes,
climate change and variability, and conflicts with humans. Thus, in this thesis, I
sought to fill these glaring gaps in our knowledge, to inform efforts aimed at
addressing the ongoing shifts in human disturbance regimes on riparian-edge habitats and their consequences for hippopotamus conservation. Consequently, the
prime aim of this thesis was to analyze (1) human-hippo conflicts in Kenya over the
12-year period covering 1997–2008, (2) hippo population dynamics in the Mara
region of Kenya during 1958–2006 and (3) the patterns and consequences of hippo
and livestock grazing in the riparian habitats of the Mara on vegetation structure,
species richness and composition and herbivore abundance and species diversity
under protection and pastoralism. The riparian habitats of the Mara River Basin
that cover some 13,750 km2, situated in southwestern Kenya and in northern
Tanzania formed the centerpiece of this thesis.
In the thesis, I start by presenting a detailed review of contemporary hippo
conservation and management efforts in Kenya. I then examine key management
issues on human-hippo conflicts, identifying the extent, severity and distribution
(spatial and temporal) of hippo-related damages to crops and how retaliatory
killings of hippos are undermining conservation efforts in Kenya (chapter 2). I
assembled and analyzed a national human-hippo conflict data collected by the
Kenya Wildlife Service, covering a12-year period spanning 1997-2008, and established that human-hippo conflicts increased 26-fold Kenya-wide in this period. This
dramatic rise in human-hippo conflicts is a consequence of both natural and
human-mediated disturbances on hippo habitats, including wetlands. These
conflicts are not likely to decline any time soon, and are likely intensifying, as
increasing human population and land-use changes impact more adversely on
wetlands, portending a precarious future for hippos outside Kenyan protected
areas. I further explored the population status of hippopotamus at one of Kenya’s
premier conservation estates, the Masai Mara, where I surveyed and counted
hippopotamus along 155.3 km of the Mara river system (chapter 3). I compared the
survey results with those of previous surveys and revealed how hippopotamus
populations surprisingly increased rapidly and expanded their range in a context of
considerable climatic variability and against a background of deteriorating habitat
conditions over a period of 35 years covering 1970 to 2006. I concluded that
Summary
113
increased anthropogenic activities, especially land-use changes on the group
ranches surrounding the world-famous Masai Mara National Reserve and destruction of the Mau catchments of the Mara River, unless regulated, will continue to
adversely affect the hippopotamus population, with significant spill-over effects on
other mammalian grazers dependent on grazing lawns created and maintained by
hippopotamus.
When occurring at high densities, hippos can extensively damage rangelands
through sustained grazing and trampling. However, there is a great deal of debate
and controversy as to whether hippopotamus impacts on riparian vegetation structure and other ecosystem processes should be regarded as undesirable, or whether
they are reversible states whose trajectories are governed by fluctuations in
hippopotamus population sizes. Central to this debate and resolving this controversy is the identification of appropriate benchmark states characterizing the vegetation states prior to local colonization by hippos or extirpation of hippo
populations (chapter 4). In a field experiment, I investigated the interactive effects
of hippopotamus and livestock grazing on vegetation structure, plant species
composition and richness and herbivore abundance and species diversity in the
protected Masai Mara National Reserve and its adjoining pastoral lands. I demonstrated the important engineering impacts of hippopotamus on vegetation that
facilitates the establishment and co-existence of myriads of plant species, culminating in enhanced species richness in areas experiencing intermediate grazing
levels, especially within the protected landscape. Hippopotamus grazing activities
created and maintained a shifting mosaic of patches of differentiated vegetation
structure, enhancing structural heterogeneity of vegetation and attracting a diverse
and abundant herbivore assemblage at intermediate distances from rivers, thus
facilitating riparian-edge habitat use by other wild herbivores (chapter 5).
In conclusion, I synthesize the key results of this thesis and discuss the conservation status of hippos in Kenya and the ecological role of hippos along riparianedge habitats (chapter 6). The results of this thesis on the effects of hippopotamus
on the vegetation structure, plant species composition and richness, clearly points
to a keystone role for hippos. Further, these results confirm that hippopotamus
affect a number of ecosystem-level functions and that their influence on other
herbivores may be much more important than has been appreciated previously.
Plant species composition and richness patterns were variable between hippopotamus and livestock-dominated landscapes but different factors were driving
species composition between the two landscapes. The abundance and species richness of the other herbivores were consistently higher on the hippopotamus dominated landscape. I suggest that hippopotamus provide some unique ecosystem
engineering functions not duplicated by mesoherbivores along riparian-edge habitats. Consequently, any significant decline of hippopotamus populations in the
Serengeti-Mara ecosystem and elsewhere in African savannas may precipitate
114
Summary
considerable erosion of habitat heterogeneity within riparian-edges and the unique
biological diversity they support. From a practical hippopotamus conservation
perspective, I recommend the urgent preparation and implementation of a management plan for Kenyan hippos, establishment of effective Kenya-wide programs to
protect hippos from the bushmeat and canine teeth ivory trade and retaliatory
killings and effective monitoring and reporting of hippo-human conflicts for timely
identification of hotspots of conflicts and targeted remedial interventions
throughout Kenya. I further recommend that urgent steps be taken to alleviate the
degradation, fragmentation and loss of hippo habitats in Kenya, especially in
regions experiencing increasing human and livestock numbers and expanding
settlements, cultivation and other land use changes, yet supporting substantial
hippo populations.
Summary
115
Samenvatting
Afrikaanse savannes worden gekenmerkt door een hoge diversiteit en abundantie
van grote herbivore zoogdieren, maar leiden in toenemende mate aan degradatie,
fragmentatie en verlies van habitats met name in en rond natuurgebieden. De
uitbreiding van nederzettingen en de landbouw heeft een grote negatieve invloed
op de verspreiding en de abundantie van wilde grazers, zowel op het niveau van de
populatie als op het niveau van de levensgemeenschap. Een van deze grazende
soorten is het nijlpaard. Nijlpaarden zijn in interactie met mensen tenminste sinds
de tijd van de Egyptische farao's, waar ze werden vereerd als goden en zijn geportretteerd in de kunst door de eeuwen heen. Primaire bedreigingen voor de instandhouding het nijlpaard zijn onder andere het verlies van essentiële weidegronden
door uitbreiding van de landbouw, nederzettingen, illegale oogsten voor bushmeat
en ivoor, ‘wraak’ moorden voor de vernietiging van gewassen en toenemende druk
op de zoetwatervoorraden, wat leidt tot het verlies van hun primaire habitat. Ten
slotte zet de reputatie van nijlpaarden ze op gespannen voet met de toenemende
dichtheden van mensen, wat hun kwetsbaarheid verergert. Door de toenemende
menselijke bevolkingsdichtheid en uitbreiding van de landbouw en de ontwikkeling
in en rond waterrijke gebieden, komen nijlpaarden vaak ook in conflicten met
mensen. De bescherming van de oevers van rivieren en wetland-habitats en de
controle op het stropen van nijlpaarden zijn dus twee hedendaagse uitdagingen
belangrijk voor instandhouding van de nijlpaarden populaties, evenals maatregelen
ter preventie van het opdrogen van waterlopen als gevolg van ontbossing, de stijgende waterwinning en opwarming van de aarde, en degradatie van habitats langs
rivieren.
Nijlpaard worden samen met olifanten, neushoorn en giraf gezien als "megaherbivoren", een selecte groep van terrestrische herbivore zoogdieren met een
lichaamsgewicht van meer dan 1.000 kg. De zeer grote lichaamsgrootte heeft fundamentele gevolgen voor de ecologie van deze groep die hen onderscheidt van andere
planteneters. Megaherbivoren zijn over het algemeen bedreigd, en hun voortbestaan is onzeker sinds hun specifieke habitat eisen hen vaak in direct conflict
brengen met de groeiende menselijke bevolking. Niettemin, verschilt het nijlpaard
van de andere megaherbivoren in de dubbele beslag op zowel een dag-leefruimte in
water en een open begrazingsgebied dat vaak ’s nachts bezocht wordt. Dit dubbele
vereiste is van grote invloed op de wijze waarop de nijlpaarden overleven in landschappen welke wordengekenmerkt door een hoge menselijke bevolkingsdichtheid
en continue veranderingen in landgebruik. Uit recente schattingen over 1996 tot
2004 blijkt dat ongeveer 125.000 –148.000 nijlpaarden voorkomen in versnipperde
populaties in rivieren, meren en andere watergebieden van 36 Afrikaanse landen,
waarvan 20 landen een dalende aantallen nijlpaarden laten zien, zeven landen
hebben populaties van een onbekende status, negen landen hebben stabiele popula-
116
Samenvatting
ties en in drie landen (Algerije, Egypte en Mauritanië) zijn nijlpaarden uitgestorven.
Op basis van deze geschatte wereldpopulatie, in combinatie met intensivering van
het behoud bedreigingen, is het nijlpaard gecategoriseerd als ’kwetsbaar’ op de
IUCN Rode Lijst in 2006.
Ondanks zijn imposante afmetingen, zijn kwetsbare status en zijn begrazingseffecten op oeverhabitats en de groeiende conflicten met mensen, is het nijlpaard niet
goed onderzocht in veel delen van zijn leefomgeving, waaronder Kenia, waar de
soort officieel is beschermd sinds 1920. Daarom is onderzoek van de nijlpaard
noodzakelijk om de oorzaken te begrijpen van populatie fluctuaties en daarmee ook
tot inzicht te komen hoe nijlpaarden veranderingen in de oeverstaten habitats beïnvloeden en hoe ze daar zelf door beïnvloed worden en hoe ze reageren op ontwikkelingen en veranderingen in landgebruik, klimaatverandering en variabiliteit, en
conflicten met mensen.
In dit proefschrift heb ik geprobeerd om deze grote gaten in onze kennis te
vullen, om inspanningen gericht op het aanpakken van veranderingen in de menselijke verstoring regimes op oeverhabitats en de gevolgen daarvan voor de instandhouding nijlpaard te faciliteren. Dus voornaamste doel van dit proefschrift was om
het volgende te onderzoeken: (1) conflicten tussen mensen en nijlpaarden in Kenia
over een periode van 12 jaar (1997–2008), (2) de populatiedynamiek van nijlpaarden
in de Mara regio van Kenya tijdens 1958-2006 en (3 ) de patronen en de gevolgen
van nijlpaard en vee begrazing in de oeverhabitats van de Mara rivierin Kenia op
de vegetatie structuur, soortenrijkdom en de samenstelling, en abundantie en soortenrijkdom van herbivoren, waarbij volledige bescherming is vergeleken met
pastoralisch gebruik door Masai herders. De overhabitats van de Mara River Basin ,
die wel 13.750 km2 bestrijken, en gelegen zijn in het zuidwesten van Kenia en in het
noorden van Tanzania, vormen de kern van dit proefschrift.
In het proefschrift, begin ik met het presenteren van een gedetailleerd overzicht
van de hedendaagse nijlpaard bescherming en de beheersinspanningen ten aanzien
van deze soort in Kenia. Vervolgens heb ik belangrijke vraagstukken over menshippo conflicten onderzocht, en de omvang, de ernst en de verspreiding (ruimtelijke
en temporeel) van nijlpaard-gerelateerde schade aan gewassen onderzocht en
onderzoek gedaan naar hoe ’wraak-moorden’ op nijlpaarden pogingen tot zijn de
instandhouding van de soort in Kenia ondermijnen (hoofdstuk 2). Ik analyseer een
nationale mens-nijlpaard conflict dataset verzameld door de Kenya Wildlife Service,
12 jaar lang verspreid over 1997–2008, en stel vast dat de mens-nijlpaard conflicten
26-voudig stegen in heel Kenia in deze periode. Deze dramatische toename van de
mens-nijlpaard conflicten is een gevolg van zowel natuurlijke als door de mens
veroorzaakte verstoring van nijlpaarden habitats, met inbegrip van wetlands. Deze
conflicten zullen waarschijnlijk niet op korte termijn afnemen, en zelfs intensiveren, door toename van de menselijke bevolking en veranderingen in landgebruik
die vooral negatief uitpakken voor wetlands, wat zorgt voor een onzekere toekomst
Samenvatting
117
voor nijlpaarden buiten de Keniaanse beschermde gebieden. Verder heb ik gekeken
naar de populatie status van nijlpaarden in één van Kenia’s belangrijkse natuurgebieden, de Masai Mara, waar ik nijlpaarden zocht en telde langs 155.3 km van de
Mara rivier (hoofdstuk 3). Ik vergeleek de resultaten van de telling met die van
eerdere onderzoeken en heb laten zien hoe nijlpaard populaties verrassend snel
stegen tijdens grote klimaatsveranderingen en tegen een achtergrond van verslechterende habitats omstandigheden over een periode van 35 jaar van 1970 tot 2006.
Ik concludeerde dat meer menselijke activiteiten, vooral veranderingen in landgebruik op de groep ranches rond de wereld-beroemde Masai Mara National Reserve
en de vernietiging van de Mau stroomgebieden van de Mara rivier, tenzij gereguleerd, negatieve invloed zal blijven hebben op de nijlpaarden populatie, met
aanzienlijke spill-over effecten op andere grazende zoogdieren die afhankelijk zijn
van de ‘grazing lawns’, gemaakt en onderhouden door het nijlpaard.
Bij hoge dichtheden kunnen nijlpaarden grote effecten aanrichten in natuurgebieden door aanhoudende begrazing en vertrapping. Er is een controverse over de
vraag of nijlpaard effecten op de oevervegetatie structuur en andere ecosysteem
processen moeten worden aangemerkt als ongewenst, of dat ze reversibel zijn,
waarvan de trajecten worden beheerst door schommelingen in de nijlpaarden populatie. Centraal in dit debat en het oplossen van deze controverse is de goed meting
van de uitgangssituatie van de vegetatie voorafgaand aan de lokale kolonisatie door
nijlpaarden of voorafgaand aan de uitroeiing van nijlpaard populaties (hoofdstuk 4).
In een veldexperiment onderzocht ik de interactieve effecten van nijlpaarden en
begrazing door vee op de vegetatiestructuur, de samenstelling van plantensoorten
en de soortenrijkdom van planten, en herbivoor abundantie en soortendiversiteit in
de beschermde Masai Mara National Reserve en de aangrenzende pastorale
gronden. Ik heb aangetoond dat belangrijke ’ecosystem engineering’ effecten van
nijlpaarden er voor zorgen dat myriade plantensoorten makkelijker vestigen en coexisteren, zorgend voor een hogere rijkdom aan soorten in gebieden met gemiddelde
begrazings niveaus, met name binnen het beschermde reservaat landschap. Nijlpaard begrazings activiteiten creëren en onderhouden een schuivende mozaïeken
van patches van gedifferentieerde vegetatie structuur, versterken de structurele
heterogeniteit van de vegetatie en ze trekken een gevarieerde en grote herbivoor
gemeenschap aan op middelmatige afstanden van rivieren, door het faciliteren van
het gebruik van oeverhabitats voor andere wilde herbivoren (hoofdstuk 5).
In de synthese bespreek ik de belangrijkste resultaten van dit proefschrift en de
beschermings-staat van nijlpaarden in Kenia en de ecologische rol van nijlpaarden
langs de oeverhabitats (hoofdstuk 6). De resultaten van dit proefschrift over de
effecten van nijlpaarden op de vegetatie structuur, wijst duidelijk op een sleutelrol
voor nijlpaarden. Verder bevestigen deze resultaten dat nijlpaarden een aantal
ecosysteemfuncties beïnvloeden en dat hun invloed op andere herbivoren veel
belangrijker zijn dan eerder werd aangenomen. Samenstelling van plantensoorten
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Samenvatting
en soortenrijkdom patronen verschilden tussen door nijlpaarden en vee begraasde
landschappen, maar de abundantie en rijkdom van de andere herbivore soorten
werden steeds hoger gevonden in door het nijlpaard gedomineerde landschappen. Ik
stel voor dat nijlpaarden een aantal unieke ‘ecosystem engineering’ functies hebben
die mesoherbivores langs de oeverstaten-edge habitats niet kunnen bieden. Bijgevolg kan een grote daling van de nijlpaard populatie in het Serengeti-Mara ecosysteem en elders in de Afrikaanse savannes zorgen voor een aanzienlijke verlies van
de habitat heterogeniteit binnen riviergebieden-randen en voor een afname van de
unieke biologische diversiteit die ze ondersteunen. Vanuit een praktisch oogpunt
van nijlpaard bescherming, adviseer ik de dringende voorbereiding en uitvoering
van een beheersplan voor Keniaanse nijlpaarden, het opstellen van effectieve Keniabrede programma's om nijlpaarden te beschermen tegen de bushmeat en ivoor
handel en de ‘wraakmoorden’ en ik adviseer doeltreffende monitoring en rapportage van nijlpaard-mens conflicten voor de tijdige identificatie van hotspots van
conflicten en gerichte corrigerende interventies in heel Kenia. Ik raad verder aan
dat er dringend maatregelen worden genomen om de degradatie, fragmentatie en
verlies van nijlpaard habitats in Kenia te verlichten, vooral in regio’s waar veel nijlpaarden voorkomen en waar menselijke populaties veestapels en nederzettingen
toenemen, en teelt en andere veranderingen in landgebruik momenteel worden
uitgebreid.
Samenvatting
119
Acknowledgemennts
Acknowledgements
I am delighted that I have completed my PhD dissertation, but I also want to point
out one of the greatest truths of life; that I had munificent goodwill and support
along the way. Competent, committed and dedicated individuals gave me their best
considerations without reservation. I sincerely thank Prof. Han Olff, my promoter
and head of the COCON Research Group who charitably allowed me unlimited
freedom in pursuing divergent hippopotamus research ideas but also ensured that I
received guidance and supervision. I highly value the supervision, guidance and
mentorship that I received from Dr. Joseph Ogutu, whose encouragement and
support from the preliminary to the concluding level enabled me to develop philosophical understanding of this dissertation. My field assistant Charles Matankory,
is a rare and gifted gentleman who made my field achievements possible and
memorable by giving his best enterprises. Anonymous reviewers and contemporaries pleasantly engaged me in lengthy and productive exchanges on hippopotamus ecology, conservation and management, further advancing my philosophy. I
humbly shine this dissertation spotlight on you all!
Over the 5-years that I was engaged on this dissertation, many friends and
colleagues refocused my bearings amid sopping logistical necessities. I am grateful
to Joyce Rietveld for happily providing me with detailed RuG orientation procedures from as early as January 2006. Jacob Hogendorf availed to me a much
needed bicycle during my RuG visits while Jan van den Burg regularly sorted my
RuG IT requirements; these gestures are truly long lasting beyond this dissertation.
Inspiring colleagues from the COCON Research Group included Grant Hopcraft,
Ciska Veen, Irma Knevel, Roos Veeneklas, Sandra van Graaf and Moses, Nina Bhola,
Sanne de Visser, Bernd Freymann, Esther Chang, Cleo Gosling, Fons van der Plas
and Jasper Ruifrok among many others. The COCON secretary, Ingeborg Jansen,
cheerfully guided me through the concluding logistics for this dissertation’s examination, while Dick Visser expertly prepared the book lay-out and facilitated its
printing. Anke Kramer arranged my accommodation at Haren; I greatly appreciate
her efforts of making me feel at home away from Nairobi! At the International
Livestock Research Institute (ILRI) Nairobi, I was privileged to interact and tap the
expertise of Shem Kifugo and Andrew Muchiru. Joseph Mukeka, a colleague at
Kenya Wildlife Service (KWS), gracefully availed the much needed ArcGIS tutorials.
Many thanks to the numerous vehicle technicians/mechanics at the Sekekani (Masai
Mara) garages; they ensured that our revamped landcruiser stayed on the sampling
transects even after endless visits to the garages. Truthfully, all these folks form an
inspiring trail of friends who in their own deeds, contributed towards this dissertation “Harambee” and I collectively thank them all.
Four key institutions supported my undertakings towards this dissertation; the
Netherland Fellowship Program (NFP) provided the main financial grant, while
additional field support came in hardy from the Frankfurt Zoological Society (FZS).
The University of Groningen was my host institution during this dissertation period
122
Acknowledgements
and further supplemented my field grant. Kenya Wildlife Service (KWS) gladly
released me from official duties to concentrate on this dissertation. Thus, the
commitment of these institutions towards my studies truly demonstrates their
truthfulness to the famous Chinese saying that “for one year of prosperity, grows
grains; for a decade of prosperity, grow trees; but for a future of prosperity, grow
people”.
While working on this dissertation, I spent long and extended periods away
from my family. I highly appreciate my wife Geldine for her patience, tolerance and
unwavering support to our family during my absence; her moral support was
steamy and she is my best friend ever! My lovely daughters Renne and Krista and
son Robin, missed enormous fatherly attention without an option to protest and I
thank them too for their patience. Lastly, the completion of this dissertation rekindled memories of the love, support and encouragement that I have endlessly
received from my parents, to always be focused, dedicated and disciplined in whatever I do; the concluding moment for this dissertation testifies that I have once
again kept that promise!
Acknowledgements
123
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Publication list
Kanga, E.M., Ogutu, J., Hans-Peter, P. and Olff, H. (2011). Hippopotamus and
livestock grazing: influences on riparian vegetation and facilitation of other
herbivores in the Mara Region of Kenya. Journal of Landscape and Ecological
Engineering, DOI: 10.1007/s11355-011-0175-y
Kanga, E.M., Ogutu, J., Hans-Peter, P. and Olff, H. (2011). Human-hippo conflicts in
Kenya during 1997-2008: Vulnerability of a megaherbivore to anthropogenic
land use changes. Journal of Land Use Science, DOI: 10.1080/1747423X.2011.
590235
Kanga, E.M., Ogutu, J.O., Olff, H. and Santema, P. (2011). Population trend and
distribution of the Vulnerable common hippopotamus Hippopotamus amphibius
in the Mara Region of Kenya. Oryx, 45: 20–27.
Reed, D., Kanga, E.M. and Behrensmeyer, A.K. (2006). Plio-Pleistocene paleoenvironments at Olduvai based on modern small mammals from Serengeti, Tanzania
and Amboseli, Kenya. Journal of Vertebrate Paleontology, 26(3):114A.
Kanga, E.M. (2003). Ecology and Conservation of duikers in Arabuko-Sokoke Forest
Reserve, Kenya. In: Ecology and Conservation of small antelopes (eds A.B.
Plowman) pp 155-156. Furth: Filander Verlag.
Kanga, E.M. (2001). Survey of Black and white colobus monkeys Colobus angolensis
palliatus in Shimba Hills National Reserve and Maluganji Sanctuary, Kenya.
American Society of Primatology Bulletin, 25: 8–9.
Kanga, E.M. and Heidi C.M. (2000). The status of Angolan black and white colobus
monkeys Colobus angolensis palliatus in Diani forests, Kenya. African Primate,
4: 50–54.
Kanga, E.M. (2000). Survey of Aders’ duiker Cephalophus adersi in Zanzibar,
Tanzania. Gnusletter, 19: 6–9.
P.le F.N. Mouton, A.F. Flemming and E.M. Kanga (1999). Grouping behavior, tailbiting behaviour and sexual dimorphism in the armadillo lizard Cordylus
cataphractus from South Africa. Journal of Zoology, 249: 1–10.
Kanga, E.M. (1995). Preliminary survey of Aders’ duiker Cephalophus adersi in
Arabuko-Sokoke Forest Reserve, Kenya. Gnusletter, 14: 7–10.
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Affiliation of Co-Authors
drs. Peter Santema, Department of Zoology, University of Cambridge, Downing
Street, Cambridge CB2 3EJ, UK.
Dr. Joseph O. Ogutu, Universitaet Hohenheim, Institute of Crop Science, Bioinformatics Unit, Fruwirthstrasse 23, 70599 Stuttgart, Germany.
Prof. Hans-Peter Piepho, Universitaet Hohenheim, Institute of Crop Science, Bioinformatics Unit, Fruwirthstrasse 23, 70599 Stuttgart, Germany.
Prof. Han Olff, COCON-CEES, University of Groningen, Centre for Life Sciences,
Postbox 11103, 9700 CC Groningen, The Netherlands.
126
Curriculum Vitae Erustus M. Kanga
Key Qualifications: Natural Resources Conservation and Management; Ecology and
conservation of tropical forests, savannahs, wetlands and threatened species therein,
with emphasis on research addressing links between species and ecosystem, focusing
on anthropogenic interactions; Climate change vulnerability assessment and mapping.
Education background
2006 – 2011 PhD degree, University of Groningen, Netherlands
Thesis title: “The Kenyan Hippo: Population dynamics, impacts on
riparian vegetation and conflicts with humans”
1997 – 2000 MSc degree in Biology of Conservation, University of Nairobi,
Kenya. Thesis title: “Some Ecological aspects and Conservation of
duikers in Arabuko-Sokoke Forest, Kenya”
1990 – 1994 BSc degree (hon.) in Wildlife Management, Moi University, Kenya
Specialized skills
• Certified and Registered by the National Environment Management Authority
(NEMA) Kenya, as a Lead Expert on matters of Environmental Impact Assessments (EIA) and Environmental Audits (EA).
Professional Experience
1. Natural Resources Conservation - Kenya Wildlife Service (KWS)
i. 2011 – Present: Assistant Director for Ecosystems and Landscape Conservation Department.
ii. 2006 – 2011: Assistant Director for Ecological Monitoring & Biodiversity
Information Management Department.
iii. 2005 – Personal Assistance to Director (PAD); played a key-supporting role
to the Director, as he discharged his corporate mandate.
iv. 2004 – Senior Scientist heading Mweiga Research Station; responsible for
provision of leadership to a multi-disciplinary team in the formulation of
biodiversity conservation strategies and the development of integrated
management plans for Aberdare and Mt. Kenya Forest ecosystems and
the Mwea National Reserve.
v. 2002 – 2003: Research Scientist heading the Ecological Monitoring Unit;
responsible for executing and compiling information on ecological monitoring programs and biodiversity inventories for Kenya’s protected areas.
vi. 2000 – 2001: Assistant Research Scientist for Ecological Monitoring.
2. Assistant Research Scientist - National Museums of Kenya (1997 – 2000).
Centre for Biodiversity (CBD).
3. Research Assistant - Zoo Atlanta's African Biodiversity Conservation
Program (1994 –1996).
127