Download The impacts of elephants on biodiversity in the Eastern

Elephant Conservation
South African Journal of Science 102, September/October 2006
395
The impacts of elephants on biodiversity in the
Eastern Cape Subtropical Thickets
Graham I.H. Kerley and Marietjie Landman
We review available information on the impact of elephants on the
Subtropical Thickets of the Eastern Cape province as a contribution
to the current debate around biodiversity and the need to manage
elephant populations. This ecologically diverse region historically
supported an abundance of elephants that was incrementally
reduced to a single population limited to the Addo Elephant
National Park (AENP). The results of research on elephant impacts
associated with this population has shown that these animals
influence many ecological processes, and patterns, including soil
features, landscape patchiness and plant biomass and diversity.
Furthermore, elephants influence insect, bird and antelope abundances and reduce browse availability for black rhinoceros. We
conclude that elephants affect biodiversity at all levels investigated
but that further research is necessary to identify the mechanisms
responsible. Of specific concern is the observation that the AENP
represents the only current example where elephants may be
driving many endemic plants to extinction. This suggests that
managing elephant impacts in Subtropical Thickets, specifically, is
a matter of urgency.
Introduction
The management of elephants, Loxodonta africana, in conservation areas has faced policy- and decision-makers with burgeoning populations1 and the loss of biodiversity,2,3 giving rise
to the so-called ‘elephant problem’.4 Elephant management is
particularly challenging given the high costs and strong public
emotions elicited by culling, contraception and translocation
(see Whyte et al.5). Furthermore, the information on which
managers base decisions is frequently contradictory, and overviews of elephant impacts on savanna landscapes have only
recently become available.6,7 Understanding the effects of
elephants in South Africa has focused on these savannas, despite
the fact that these animals are habitat generalists and historically
have occurred in all South Africa’s biomes except deserts.8
Elephants play a key role in ecological processes in the Subtropical Thickets of the Eastern Cape.9–11 We review the studies
on elephant impacts in these thickets, in order to contribute to
the larger debate. If we are to manage and conserve Eastern
Cape thicket successfully, we need to develop appropriate
elephant management strategies based on a sound scientific
understanding which is influenced by and reflects the history of
elephant exploitation, conservation, research and management
in the Eastern Cape. We include background information to
contextualize this synthesis.
History of elephants in the Eastern Cape
Historically, elephants occurred at high densities in the Eastern
Cape, particularly in the thicket vegetation east of the Gamtoos
River.8,12 Soon after Europeans settled in the Cape, these elephant
herds attracted hunter/explorers in search of ivory. This resulted
in a rapid eradication of the animals from over 90% of the land*Centre for African Conservation Ecology, Department of Zoology, P.O. Box 77000,
Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa.
†
Author for correspondence. E-mail: [email protected]
scape8 before any real details of their ecology were documented.
The Eastern Cape is the interface between summer and winter
rainfall systems. Together with geologically and topographically
complex landscapes, this results in a complex interdigitation
of all seven of South Africa’s biomes in the province, which
complicates our understanding of elephant ecology in the
region. Boshoff et al.12 postulated that elephants occurred at high
densities on the coastal lowlands and along the river valleys,
where there was abundant forage and water. The interfluves
would have been used relatively ephemerally, given the low
frequency of surface water. Boshoff et al.12 also hypothesized that
elephant movements between valley systems took place on the
coastal lowlands or along the more densely vegetated escarpment. A recent reconstruction of elephant distribution and
densities within the area of the Subtropical Thicket Ecosystem
Planning (STEP) project suggests that elephant abundance
varied considerably between the different vegetation types.13
This would undoubtedly have influenced the relative impact of
elephants across these landscapes, but information on this is not
available.
By the end of the 19th century, the once vast population of
elephants in the Eastern Cape was reduced to a single herd in the
Addo bush. This herd, then estimated at about 140 individuals,
was the largest of the four remaining herds [others being at
Knysna, the Kruger National Park (KNP) and Thembe] in South
Africa.14 Through an active attempt at eradication, the population was further reduced to about 16 animals in 1919.15 Ongoing
attrition through contact with humans reduced the population
to 12 animals in 1931, at which time the Addo Elephant National
Park (AENP) was established.16 Mortalities of the unfenced
population remained high, so that when the AENP was finally
fenced (an area of 23.3 km2) in 1954, the population was just 22
animals.14 Subsequently, the population has grown significantly
(Fig. 1), and now numbers over 440 individuals. In response to
the expanding population, the area available to elephants has
been increased3,17 on six occasions and now exceeds 250 km2.
These expansions form part of a programme aimed at establishing a mega-reserve of over 3000 km2, which includes five of
South Africa’s seven biomes.18 Although historically focused on
the conservation of elephants, current policy for the AENP treats
the elephants together with other forms of biodiversity and
recognizes the need to maintain elephant populations as an
integral part of the patterns and processes required to minimize
the loss of biodiversity.19
In the last decade, elephants have been re-introduced into 11
private nature reserves — Amakhala, Asante Sana, Blaauwbosch,
Hopewell, Kariega, Kuzuko, Kwandwe, Kwantu, Lalibela,
Pumba and Shamwari — in the Eastern Cape in response to the
burgeoning ecotourism industry.20 The process has been facilitated by the ready availability of elephants.
Subtropical Thickets of the Eastern Cape
Subtropical Thicket has recently been recognized as a biome in
its own right.21 These thickets are characteristic of the hot, semiarid river valleys of the Eastern Cape, where annual rainfall
varies from 200 mm for arid thicket types to 1050 mm for coastal
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South African Journal of Science 102, September/October 2006
Elephant Conservation
forest thicket mosaics,22 and are typically dense,
low-growing (3–5 m), spinescent, evergreen and
dominated by woody shrubs. The importance of
succulent plants varies among the 112 thicket
types in response to variations in altitude and
rainfall.22 Thickets are characterized by a high
degree of endemism,22 particularly among the
geophytes and low-growing succulents.23
Historically, Subtropical Thicket supported a
high diversity of vertebrate browsers,8 ranging in
body size from the blue duiker (Philantomba
monticola) to elephant. The nutritious, evergreen
and spinescent nature of the vegetation has led
to hypotheses of co-evolution between the
vegetation and associated browsers.24 These
landscapes are, however, vulnerable to desertification24,25 in response to inappropriate herbivore
management.26
Elephant research in the Eastern Cape
Fig. 1. Elephant population size (solid line)29 and density (thin, dashed line) in relation to the maximum
Other than for research conducted in the estimated ecological carrying capacity (thick, hatched line)35 for the main elephant enclosure of the
Park (AENP). The declines in density due to park enlargement in 1977, 1982,
AENP and records of re-introduction,20 there are Addo Elephant National
3,17
no published studies on elephants in the Eastern 1984, 1990 and 1997 and the translocation in 2003 of elephants to the Nyathi Concession Area, are
shown in the figure. Together with the Nyathi population and the recently established Kuzuko population
Cape. Consequently, our understanding of the in the AENP, the total population now exceeds 440.
role of elephants in the province is limited to a
single population in a few vegetation types. Given the 112 vege- vore densities at which vegetation communities will not be
tation types described for Subtropical Thicket,22 considerable negatively impacted by herbivores (but see Owen-Smith et al.36
caution should be used when extrapolating these findings.
for the limitations inherent in this concept). Observations on the
Elephant research in the AENP has focused on either the impact of elephants on thicket vegetation should, therefore,
elephants or their impacts. Elephant-focused work has be interpreted bearing in mind the fact that these elephant
34
described population demographics,27–29 social interactions,14,30 densities are ‘exceptionally and unusually high’.
31
32
ranging behaviour and genetics. This research is crucial to
A further assumption underlying research in the AENP is
understanding the background to the elephant impacts, but is that the changes observed within the elephant enclosure can be
frequently not couched in terms of these effects. It is significant directly attributed to the elephants (cf. Stuart-Hill,10 Penzhorn
that the AENP elephant population has been growing at an et al.33). Stuart-Hill10 justified this on the basis that elephants
average annual rate of 5.8% (Fig. 1), close to the theoretical comprised 78% of the large mammal herbivore biomass in the
maximum.14,29
AENP during his study. However, the presence of a range of
Research on elephant impacts in the AENP was pioneered by other herbivores [e.g. black rhinoceros, kudu (Tragelaphus
Penzhorn et al.33 and has followed a tradition of comparing strepsiceros), bushbuck (Tragelaphus scriptus), buffalo (Syncerus
elephant-occupied areas (the elephant enclosure) with a suite of caffer)], frequently at high densities, calls this assumption into
sites within the AENP protected from elephant browsing question.34 Furthermore, elephant impacts may have knock-on
(so-called ‘botanical reserves’ — reviewed below). This approach effects. Thus, Kerley et al.37 postulated that path creation by
assumes that the botanical reserves represent a ‘control treat- elephants might facilitate access to geophytes and small succument’, which can be used as a baseline to assess the impacts of lent plants for tortoises in otherwise impenetrable thicket.
elephants on thicket vegetation.34 This assumption may not be Moreover, tortoises selectively feed on many geophytes,38 which
realistic, however, since areas protected from megaherbivore are preferentially not eaten by elephants,39 but have been
[elephant and black rhinoceros (Diceros bicornis)] browsing may identified as declining in the presence of elephants.3,40
experience ‘megaherbivore release’,34 whereby plant and animal
communities may develop differently in the absence of mega- Role of elephants in Subtropical Thicket
Despite their bulk and status as a keystone species,41 elephants
herbivores (cf. Stuart-Hill,10 Kerley et al.11).
do
not influence all aspects of ecosystem functioning: for
Since the AENP population was fenced in 1954, elephant
densities, ranging between 1.0 and 4.1 km–2 (Fig. 1), have consis- example, they do not act as predators. An understanding of their
tently been above the recommended carrying capacity. Two effects depends on appreciating their roles in the ecosystem and
42
estimates of carrying capacity appear in the scientific literature. the associated mechanisms involved. Boshoff et al. showed that
elephants
played
a
role
in
14
of
19
broad
ecological
processes
Based on a comparison of elephant densities in areas of Africa
33
important
in
thicket
(Table
1).
This
contrasts
with
a smaller
with similar rainfall, Penzhorn et al. recommended that densities
in the AENP should not exceed 0.4 km–2. More recently, Boshoff species such as blue duiker, which contributed to only five such
42
et al.,35 using a forage production approach with modelled processes.
Elephants are mixed feeders, consuming a range of plants and
potential herbivore communities, estimated an ecological carrying capacity of between 0.25 and 0.52 km–2 for the AENP. Thus, plant parts from grasses to browse, bark, fruit and bulbs.41 Their
even the lowest density of elephants since fencing of the AENP large body size and robust feeding allow them to have a broad
has been more than double than published estimates of carrying diet — 146 plant species in the AENP.43 This is nearly five times
capacity (Fig. 1). These estimates are attempts to identify herbi- the species consumed by coexisting ruminant browsers, like
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South African Journal of Science 102, September/October 2006
397
42
Table 1. Broad ecological processes (n = 19), modified from Boshoff et al., operating in thicket vegetation mediated by
megaherbivores, mesoherbivores (19 species), omnivores (3 species) and carnivores (18 species).
Broad ecological process
No. of species in category
Trophic processes
Bulk grazing
Concentrate grazing
Browsing
Frugivory
Predation
Scavenging
Transport processes
Seed dispersal
Nutrient dispersal
Habitat architecture processes
Plant form
Grazing lawns
Path opening
Bipedturbation processes
Wallowing
Dust bathing
Digging
Hoof action
Geophagy
River beds
Other processes
Litter production
Germination facilitation
Total no. of processes affected
Megaherbivores
Mesoherbivores
Omnivores
Carnivores
2
19
3
18
1
3
9
7
17
6
18
2
3
3
2
9
6
18
Elephant
Black rhinoceros &
hippopotamus
1
1
1
1
1
1
1
1
1
2
2
19
19
3
3
1
2
1
2
7
5
5
1
1
1
5
1
19
1
1
1
1
1
2
1
1
6
1
1
1
1
1
2
19
2
2
6
14
12
14
12
8
kudu, bushbuck, or eland (Taurotragus oryx), and is approached,
at 120 species, only by the black rhinoceros.11 Elephant foraging
will, therefore, influence the fate of more plant species in the
Eastern Cape thickets than any other mammal herbivore.
Impacts of elephants in the AENP
If we are to understand the impacts of elephants, the connections between the animals and their consequences need to be
clearly understood and demonstrated. For example, for over 30
years the observation that Aloe species were disappearing in the
elephant enclosure was assumed to be a response to elephant
herbivory33 even though aloes are generally recognized to be
unpalatable.45 Only in 2004 did Davis46 show that elephants
actually consume aloes in limited quantities in some seasons.
Elephant impacts are observed at a range of levels, from soils to
coexisting mammals (reviewed below), and in all instances of
such effects, the mechanisms need to be clearly identified.
Soil resources
Using Landscape Functional Analysis, Kerley et al.47 measured
micro-topography and soil nutrients in the AENP elephant
enclosure and botanical reserves. They showed that in the
elephant enclosure there was a decline in the proportion of the
landscape that represented run-on zones, where resources such
as water, litter, soil and nutrients are trapped during overland flow, while the proportion of run-off zones where these
resources are lost, increased. The consequences were a decline in
soil nutrients. Given the lack of replication in this study, it is not
possible to demonstrate that these effects were significant or due
to the elephants. Kerley et al.,47 however, suggested that the
effects of elephants were less deleterious than those of goats.
These studies must be repeated in order to assess whether
elephant impacts do in fact cause dysfunctional landscapes, and
at what elephant densities this occurs.
Litter production
One of the consequences of their feeding style is that elephants
tend to discard large amounts of plant material without ingesting it. Paley48 estimated the amount of this material within
the AENP (27 000 kg/day) as comprising two-thirds of the then
estimated daily food requirements of the elephants. This, in
turn, represents approximately 20% of the estimated total litter
production (excluding faecal material) in Succulent Thicket in
the absence of elephants.49 The elephant-discarded material is
typically much coarser than the normal litter, and differs in terms
of nutrients. Thus, elephants appear to significantly alter the
size, nutrient levels and dynamics of litter in thicket vegetation.
Landscape patchiness
Kerley et al.24 postulated that thicket vegetation is adapted to
small-scale disturbance patches, such as those brought about by
elephant feeding and trampling. Elephants have been observed
to create open patches where they excavate soils to practise
geophagy in Addo, or where they trample the vegetation
around small ephemeral water bodies (G.H.I.K., pers. obs.), but
this has not been quantified.
Using population data and period of elephant occupation
associated with the expansion of the elephant enclosure,
Landman assessed the accumulated elephant presence
(expressed as ‘mean elephant density/yr ’) on the development
of open habitat patches as opposed to dense thicket. She showed
a significant increase in the proportion of the landscape transformed into path or open habitat with increasing elephant
presence. The number of paths initially grows with increasing
elephant occupation, but ultimately declines as the paths
coalesce to form larger open patches. These changes had significant consequences for browse availability (see later), but have
not been further investigated. It may be postulated that path
formation has important knock-on-effects by virtue of changes
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South African Journal of Science 102, September/October 2006
in microclimate associated with the change in vegetation
density. In accordance with this suggestion, Henley50 showed
that the air and soil temperature ranges of such open habitat are
more extreme than that of intact thicket. The change in microclimate has unknown implications for ecosystem processes ranging
from soil litter processes (cf. Lechmere-Oertel49) to plant and
animal physiology, seedling germination and survival.
Impacts on plants
The impact of elephants on the vegetation of the AENP was
first recognized by Penzhorn et al.26 They assessed the richness
and biomass of plants inside and outside the elephant enclosure
and assumed that any difference in vegetation was due to
elephants. Penzhorn et al.33 showed that total plant biomass was
significantly reduced (by 55%) within the enclosure. The mean
mass of Portulacaria afra, Schotia afra, Azima tetracantha and Euclea
undulata was less than half of the mass outside, while that of
Capparis sepiaria increased by 50% in the enclosure. Although
species richness did not differ significantly between treatments,
at least two species, a tree aloe, Aloe africana, and the epiphyte,
Viscum rotundifolium, were absent from the enclosure.
Following the findings of Penzhorn et al.,33 the elephant enclosure was enlarged on three occasions from 1977 to 1984, to
reduce elephant density and thus their influence on the vegetation. Using these variations in elephant density and the time
elapsed since exposure to the animals, Barratt and Hall-Martin17
compared the canopy height and crown diameter of plants and
elephant damage, expressed as a function of canopy volume, in
the elephant enclosure and botanical reserves. There was a
significant reduction in canopy height and volume inside the
elephant enclosure during 1977, while in the botanical reserve
the canopy volume increased. Canopy height and volume
decreased in newly incorporated areas, tending towards the
1977 estimates of the original elephant enclosure. Barratt and
Hall-Martin17 hypothesized that ‘areas exposed to 20 years of
elephant utilization appeared to reach a loose equilibrium
between plant biomass loss due to elephant browsing and total
biomass regeneration’. Cowling and Kerley,34 however, pointed
out that it would be difficult to test this statement as elephant
density and, therefore, browsing intensity, fluctuated in these
areas over this period. The percentage of plants newly damaged
by elephants was species dependent, with P. afra experiencing
significantly more damage than S. afra, E. undulata, A. tetracantha
and C. sepiaria. Barratt and Hall-Martin17 confirmed the disappearance of A. africana from the elephant enclosure as well as a
significant decrease in species richness.
The disappearance of V. rotundifolium from the elephant enclosure, observed by Penzhorn et al.,33 prompted an investigation
into the size and frequency of the common mistletoes
(Moquinella rubra, V. rotundifolium, V. crassulae, V. obscurum) and
their associated host plants inside and outside the enclosure.
Midgley and Joubert51 observed that despite the high incidence
of host plants in the enclosure, all mistletoes were nearly locally
extinct, while the frequency of mistletoes outside the enclosure
ranged from 7% (V. rotundifolium on Acacia karroo) to 77%
(V. rotundifolium on Maytenus heterophylla). Given their high
nutritional status, mistletoes are thought to be selected by
browsing herbivores and may therefore be useful indicators of
browsing intensity.51
Stuart-Hill10 expanded the debate on the impact of elephants,
contrasting the effect of elephants and goats on the shrub
component of Succulent Thicket. Using a snapshot natural
experiment, he identified three browsing treatments in and
around the AENP (predominantly elephant or goat browsing
Elephant Conservation
and browsing exclusion in botanical reserves), and compared
the frequency, percentage canopy cover and density of 23 shrub
species within each treatment. Results suggested that elephants
and goats had a noticeable effect on the density and cover of
trees and shrubs, with goat browsing being considerably more
detrimental. Relative to botanical reserves, elephant browsing
significantly increased (with the exception of P. afra) the density
of the woody component, while significantly decreasing woody
canopy cover. Although elephant browsing did not reduce
woody species richness, it limited the variation in floristic
composition to only a few species (P. afra, C. sepiaria, E. undulata,
S. afra) and caused a significant decrease in the frequency of
Euphorbia mauritanica (82%). Rhigozum obovatum and Crassula
ovata showed a relative decrease of greater than 50% in response
to elephant browsing, but this was non-significant. In contrast to
these findings, Stuart-Hill10 argued that Succulent Thicket is
adapted to the ‘top-down’ browsing by elephants, thereby
maintaining thicket regeneration by protecting cover at ground
level, and facilitating coppicing and vegetative reproduction,
particularly for P. afra.
An important component of Succulent Thicket in conservation
terms is the endemic-rich geophyte and low succulent flora,
particularly Crassulaceae and Mesembryanthemaceae. 23
Moolman and Cowling40 contrasted the impact of elephant and
goat browsing relative to control sites on the cover and richness
of this endemic component in three micro-sites: open, under
P. afra shrubs, and under E. undulata shrubs. Because thicket is
thought to be adapted to the ‘top-down’ browsing by elephants,
but not the ‘bottom-up’ browsing by goats,10 Moolman and
Cowling40 postulated that elephants will have less impact on the
endemic-rich flora than goats. Furthermore, the mosaic of open
and dense thicket associated with elephant-browsed thicket10,33
was thought to result in a more diverse endemic-rich flora than
would be found in either control or goat-browsed sites. Due to
their large bite sizes, elephants were expected to be less likely
to select low succulents and geophytes. The results, however,
indicated that only 12 of the 19 endemics identified occurred in
elephant-browsed sites. Generally, species richness, density
and cover were lower in elephant-browsed than in control
sites. The cover of the Crassulaceae was, however, higher in
the elephant-browsed treatments, possibly due to their
palatability and ability to reproduce vegetatively with a
consequent resilience to elephant browsing. Based on the
above findings, Moolman and Cowling40 suggested that elephant densities should be reduced to alleviate the impact they
have on the geophyte and low succulent shrubs of Succulent
Thicket.
In addition to those already proclaimed, Lombard et al.3
identified a system of potential botanical reserves in the AENP
that would conserve the components of the flora with high
conservation value mostly low-succulents and geophytes,
which are apparently particularly vulnerable to elephant browsing.40 They showed that the length of exposure to elephant
browsing had a strong impact on the richness and abundance of
75 special species (Albany Centre Endemics, Red Data Book taxa,
taxa very rare within the AENP)52 and two indicator species
(V. rotundifolium, V. crassulae)51 of elephant browsing intensity. In
general, species abundance decreased with increasing length
of exposure to elephant browsing, with most special species
recorded at low densities after 20 years of browsing, and more
than half of the these species confined to small populations after
42 years of browsing. Thus, they showed that species richness for
Spekboomveld declined exponentially with length of exposure
to elephant browsing, halving approximately every 7 years.
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South African Journal of Science 102, September/October 2006
Similarly, Bontveld exposed to 13 years of browsing showed a
7-fold decrease in species richness when compared with
unbrowsed Bontveld. Lombard et al.3 concluded that at current
densities, elephants had an adverse effect on plant species diversity, with the endemic and threatened component (special
species) of Succulent Thicket being most vulnerable to elephant
impacts. An important point is that 168 plant species identified
as being entirely reliant on the AENP for their conservation52 are
potentially vulnerable to elephant-driven extinction.
It is generally assumed that elephant herbivory is the primary
mechanism responsible for plant extirpation in the AENP. The
diet of elephants in Succulent Thicket has largely been inferred
from plant-based studies,17,33 assuming that differences between
botanical reserves and the elephant enclosure are due to elephant
browsing. However, plants previously thought to disappear due
to herbivory3,40,52 do not occur in the diet of elephants despite
being available. Hence, it may not be valid to attribute the
disappearance of these plant species from the elephant enclosure to elephant herbivory, and alternative mechanisms of
impact should be identified (see later).
Seed dispersal
Despite their dietary breadth, elephants are relatively poor
seed dispersers, spreading only 21 plant species through
endozoochory.53,54 In this regard they are nearly matched by
black rhinoceros (20 species) and eland (20 species53). The mechanism for this relatively poor seed dispersal in terms of species
richness is not clear but may reflect the rarity of most plant
species in the diet (only 25 out of 146 species comprise 71% of the
diet43), selective foraging behaviour in terms of plant growth and
reproduction patterns, the complete loss of propagules during
digestion or inadequate sampling.
The large volume of forage intake, and hence faecal output, by
elephants,41 however, allows them to disperse large numbers of
seeds.54 It may therefore be predicted that elephants play an
important role in the regeneration of those plant species for
which they do disperse seeds (cf. La Cock55 for the role of black
rhinoceros), but this remains to be quantified.
Insects
Musgrave and Compton56 measured phytophagous insect
feeding damage on five plant species inside the AENP elephant
enclosure and compared this to data for the botanical reserves.
They demonstrated a significant increase in feeding damage in
the presence of elephants and attributed this to an increase in
the quality of browsed plants through a decline in secondary
chemical compounds, for example, tannins. This hypothesis is
yet to be tested. Nor was it shown which insect species were
involved in this feeding process and what their population or
overall insect biodiversity responses were to the presence of
elephants. The postulated increase in nutritional quality of
plants needs to be weighed against the significant decline in
overall plant phytomass (mentioned above).
Vertebrates
Besides the work by Landman on black rhinoceros foraging
opportunities (see below), the effects of elephants on other
herbivores have not been directly investigated.
Kerley et al.37 hypothesized that elephants may increase habitat
availability for tortoises through their creation of open habitat
patches and paths. This was based on Mason’s57 demonstration
that leopard (Geochelone pardalis) and angulate tortoises (Chersina
angulata) both preferred thicket margins and open habitats and
avoided dense thicket. While it is clear that tortoise densities are
399
relatively high in the AENP, this hypothesis has yet to be tested.
Kerley et al.37 further suggested that some of the changes in the
plant community attributed to elephants (declines in geophytes
and small succulent shrubs), may in fact be due to an increase in
tortoise access. This followed from observations that many of the
plant species identified as being at risk of extinction in the AENP
are preferred dietary items for tortoises.38 If this is correct,
the ‘elephant problem’ may in part translate into a ‘tortoise
problem’!
Chabie58 showed that in transformed thicket in the AENP, there
were significant changes in the bird communities. At the guild
level, there was a shift from frugivores in intact thicket to a
community dominated by insectivores and granivores in
opened-up thicket. In addition, there was a shift to larger species
in transformed thicket. The hypothesis that elephants drive
these changes needs to be further tested.
Sigwela59 compared the diet of kudu in the elephant enclosure
and botanical reserves and showed that elephants had no
apparent effect on kudu diet selection. This is surprising given
that extensive vegetation changes have occurred in the elephant
enclosure, that kudu diet (28 species) includes many of the plant
species recorded as being affected by elephants, and elephants
consume all the plant species recorded in the diet of kudu. This
suggests that dietary items are not limiting to either kudu or
elephant at the present densities of vegetation and browsers at
these sites.
Cape grysbok (Raphicerus melanotis), bushbuck and bushpig
(Potamochoerus porcus) numbers in the AENP elephant enclosure
have declined over the last decade;60,61 this is postulated to result
from observed changes in habitat structure brought about by
elephants.60 It is not, however, known whether populations of
these species outside the elephant enclosure remained constant
over this period, or whether the changes in habitat structure are
definitely the result of elephant impacts or whether some other
process, such as global climate change, is responsible. Despite
these caveats, Novellie et al.60 claimed that ‘elephants could
reduce the diversity of mammal species.’
Elephants may impact biodiversity directly by killing individuals of other animal species. The killing of white and black
rhinoceros in savannas by elephants has been documented and
attributed to aberrant behaviour brought about by unusual
population structures.62,63 Elephants are known to kill black
rhinoceros in the AENP (two black rhinos were so killed in 1997
and 1998), which depressed rhinoceros population growth rates.64
There is no other evidence for aberrant elephant behaviour
in the AENP, so this killing may represent an extreme form of
interference competition, in which access to limiting resources is
curtailed through behavioural interactions between competitors. The causes and significance of this interaction need to be
further examined.
The reduction of vegetation cover and density by elephants
results in a change in potential browse availability for black
rhinoceros. The increase in elephant paths associated with
expanded elephant densities, initially facilitates access to browse
by black rhinoceros, but the subsequent dominance of the landscape by these paths results in a loss of foraging opportunities
(M.L., unpubl. data). Given that kudu display similar feeding behaviour to that of black rhinoceros,26 a similar response may be
predicted for them. The over-utilization of thicket vegetation by
elephant, therefore, compromises the potential of this vegetation type to contribute towards black rhinoceros foraging, and
hence conservation opportunities. Conflict prevails between the
management and conservation of these two megaherbivore
species, which needs to be recognized and managed.
400
South African Journal of Science 102, September/October 2006
Elephant Conservation
Ecological cascades
Given the abundance and generalist feeding behaviour of
elephants and the resultant diversity of impacts in the AENP, it
can be expected that elephants have a number of cascading
effects on their ecosystems. One example of such an effect could
be the improved access for tortoises to otherwise impenetrable
thicket resulting from elephant behaviour which, in turn,
increases browsing pressure on the low-growing succulent and
geophyte flora.38
The oft-repeated statement (e.g. Chown et al.39) that the persistence of the endangered flightless dung beetle (Circellium
bacchus) is dependent on the presence of elephants and their
dung is, however, unfounded. There are populations of this
species in the Gamtoos River valley and sections of the Sundays
River valley (personal observation), which have not had
elephants for more than 150 and 70 years, respectively.39 This is
longer than the pupation period of even the most recalcitrant
beetle larvae, indicating their ability to utilize non-megaherbivore dung.
Surprisingly, in the light of the history of concerns about
elephant impacts in the AENP, little attention has been paid to
the phenomenon of ecological cascades. Understanding these
cascading effects will be critical to appreciating the influence of
elephants on these landscapes.
Contribution of elephant research in the Eastern Cape
to understanding elephant impacts in South Africa
A recent workshop on elephant impacts on biodiversity,
hosted by South African National Parks, reviewed the available
evidence in South Africa. Written submissions to this workshop
focused on the effects of elephants in the Kruger National Park
and the AENP.7 From these submissions, it is clear that despite
the shortcomings in our understanding of the consequences of
elephants in Addo and the methodological constraints, the findings emerging from the park are the most comprehensive and
robust available. Thus, the AENP has made a disproportionate
contribution to generating solutions to the elephant problem in
South Africa. For example, Addo has led the way in the establishment of botanical reserves,33 which effectively conserve floristic
diversity,3 and has been expanding elephant habitat since 1977,17
thereby reducing density. More recently, elephants have been
translocated out of the AENP.29 The current review demonstrates
the depth of, and highlights some of the shortcomings in,
our understanding of elephant impacts. These can be used to
generate biodiversity change indicators that managers can use
to make decisions to intervene in elephant populations. The
nature of these interventions (such as culling, contraception,
metapopulation management) is currently being hotly debated
at a national level.36
Do elephant impacts bring about density dependence?
For more than 50 years, elephant densities in the AENP have
consistently exceeded published estimates of carrying capacity
(Fig. 1), and there is a well-documented decline in nutritional
resources.10,17,33,34 The population has not been able to disperse,
nor has it been subject to regulation by management. Despite
this, there is no evidence of density-dependent population
regulation29 through decreased fecundity or increased mortality.
The population has thus been increasing very rapidly.14,29 Gough
and Kerley29 hypothesized that this may be due to two factors:
first, the evergreen, nutritional nature of thicket and its high
standing biomass provides year-round forage; and, second,
elephants have the ability to digest large volumes of low quality
food, due to their large body size, and to utilize the broadest
range of plants of any other herbivore in the system.11 Hence,
Gough and Kerley29 postulate that the Addo elephants are
buffered against a decline in nutritional resources, enabling
them to degrade the landscape without showing density
dependence. Other studies have shown that thicket, once
severely transformed, does not achieve a stable state but
continues to decline.25 Gough and Kerley,29 therefore, suggested
that these elephants would not respond to density-dependent
processes until the ecosystem is severely degraded and collapses.
In this respect, the AENP elephant/thicket system appears to be
analogous to Caughley’s64 ‘non-interactive system’, namely,
herbivore dynamics independent of plant dynamics. However,
elephants are not ‘non-interactive’ with plant production (as
defined by Caughley64) as they utilize accumulated biomass
rather than just annual production. Thus, once elephants
deplete accumulated biomass the production base will collapse
and the system will then conform to Caughley’s64 definition of
an ‘interactive system’, in which herbivory has a negative feedback on herbivore populations via plant dynamics. More specifically, the interaction will conform to his ‘interferential system,’
whereby co-existing herbivores will also be affected. An important consequence of this finding is that elephant demographic
data cannot be used to make management decisions about
elephant populations until it is too late for the plant communities and associated biodiversity.
Discussion and conclusions
The AENP provides the best information available in South
Africa on elephant impacts. This is based on more than 30 years
of relatively robust science, with the botanical reserves serving
as assumed controls34 for the study of the consequences of
elephants. The assumption that the botanical reserves serve as
appropriate controls has limited validity since the absence of
elephants is as much an artefact as their presence at high densities,11,34 and it has not been possible to control for elephant
density. A recent significant advance has been work using the
sequential expansion of the elephant habitat in reasonably
homogeneous landscapes and known elephant numbers, in
order to assess impacts in terms of the cumulative density of
elephants. This has the advantage of addressing both the
elephant density and duration of occupation, but does, however,
assume the homogeneous use of the available habitat. There is
a need for studies on landscape use by elephants in order to
validate this approach. The ideal research approach requires
long-term surveys of a replicated series of appropriately located
exclosures, which allow temporary access and monitoring of
elephants so that elephant densities, duration and season of
occupation can be controlled. Such a rigorous approach is especially necessary in the light of potential climate change impacts
on these landscapes, and the need to distinguish between
elephant and climate change effects as they are probably of the
same order of magnitude and scale. Enclosures alone, however,
will not provide a predictive understanding of elephant impacts,
which require a greater focus on the mechanisms of the effects of
elephants on biodiversity and ecosystem functioning so as to
demonstrate causal relationships. As pointed out above, there is
a need to develop such an understanding across the wide range
of potential elephant habitat in the Eastern Cape. The recent
establishment of a number of elephant populations in the Eastern
Cape provides new opportunities to address this challenge.
There have been changes, ranging from soil alterations to black
rhinoceros foraging opportunities, at all levels at which elephant
impacts on biodiversity and ecosystem functioning have been
assessed in the AENP. The clear trend is an overall decline in
biodiversity. We postulate that these effects, which have been
Elephant Conservation
South African Journal of Science 102, September/October 2006
shown on a spatially limited scale, can be extrapolated to
landscape-level impacts.
This review, as well as recent surveys of the effects of elephants
elsewhere in Africa,6,7 show that the Addo represents the only
current example where elephants may be driving plant species
to extinction. This reflects the narrow distribution range of many
thicket endemic plants22 and contrasts with the broader distributional range of most species impacted by elephants in savanna.
Finally, we conclude that there is a unique urgency in understanding and dealing with the elephant problem in the Addo
Elephant National Park. Short-term management measures to
reduce density, such as expanding the elephant range within
the park and translocation of elephants out of it, should be
implemented as a matter of priority.
The International Fund for Animal Welfare, National Research Foundation and
Nelson Mandela Metropolitan University provided financial assistance. The
Mazda Wildlife Fund provided field transportation. We are grateful to R. Grant for
her encouragement to produce this manuscript, to A.F. Boshoff, A. Shrader,
B. Reeves, S. Pimm and two anonymous reviewers for valuable comments on
improving this paper, and to M. Knight for his insights into elephant issues.
Received 21 April. Accepted 2 August 2006.
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