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Linking key environmental stressors with the
delivery of provisioning ecosystem services in the
freshwaters of southern Africa
Michelle C. Jackson1,2,3, Darragh J. Woodford2,4 and Olaf L. F. Weyl2,5
Societies’ growing global footprint is causing a rapid increase in the demand for natural resources (i.e. ecosystem
services), while also reducing the capacity of ecosystems to provide them. Freshwater ecosystems contribute disproportionately to ecosystem services but are also particularly vulnerable to global environmental change. The provisioning of freshwater services, such as water and food production, is especially important in developing countries.
Here, we review the evidence which demonstrates the impacts of key environmental stressors on these two important provisioning services in southern Africa. Land use change, species invasions and climate change can all be
linked to a loss of the provisioning services provided by freshwater ecosystems in southern Africa. Water resources
for drinking, agriculture, sanitation and power are expected to decline as a result of both climate and land use
change. Fish production may be negatively or positively affected by the different stressors, highlighting the high
context-dependency associated with their impacts. Evidence also suggests that these stressors can interact to alter
one another’s impacts or promote the proliferation of further stressors. For instance, land use change can promote
aquatic plant invasions and, subsequently, the stressors may interact synergistically to cause fish kills. Stressors may
also interact to mitigate one another’s impact, for instance fish invasions may enhance total fish catch following a
pollution event. Since stressors are unlikely to occur in isolation and multiple stressors frequently result in complex
‘ecological surprises’, it is urgent that we increase research effort on the links between multiple stressors and the
loss of ecosystem services. Future research should, therefore, focus on the combined impacts of multiple environmental, social, and economic stressors on natural resources and provisioning ecosystem services in southern Africa.
Key words
climate change; fisheries; freshwater; invasive species; land use change; multiple stressors
1
Centre for Invasion Biology, Department of Zoology and Entomology, University of Pretoria, Private Bag X 20, Hatfield 0028, South Africa
South African Institute for Aquatic Biodiversity, Grahamstown 6140, South Africa
3
Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot, Berkshire SL5 7PY
4
Centre for Invasion Biology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, Wits,
Johannesburg 2050, South Africa
5
Centre for Invasion Biology, South African Institute for Aquatic Biodiversity, Grahamstown 6140, South Africa
E-mail: [email protected]
2
Revised manuscript received 7 September 2016
Geo: Geography and Environment, 2016, 3 (2), e00026
Introduction
Ecosystems provide people with numerous important
‘goods and services’ such as food, water and fuel, while
also providing indirect benefits including flood mitigation and climate regulation (MA 2005). Societies’ growing global footprint is causing a rapid increase in the
demand for these natural resources, while also reducing
the capacity of ecosystems to provide them ( Schmidhuber
and Tubiello 2007; Mooney et al. 2009). Despite covering less than 1% of the earth’s surface, freshwater
ecosystems contribute disproportionately to ecosystem
services (Dudgeon et al. 2006). Lakes, rivers and
wetlands are vital for human survival and sanitation,
for safe drinking water and through the provisioning
of food and power. Freshwater systems are also
especially vulnerable to environmental stressors and
over exploitation, with water and fish protein growing
in importance as commodities, and average species
population declines since 1970 estimated at 76%
(WWF 2014). This vulnerability means that freshwater
ecosystems can act as local ‘ecosystem canaries’, or
The information, practices and views in this article are those of the author(s) and do not necessarily reflect the opinion of the Royal Geographical
Society (with IBG). ISSN 2054-4049 doi: 10.1002/geo2.26 © 2016 The Authors. Geo: Geography and Environment
published by John Wiley & Sons Ltd and the Royal Geographical Society (with the Institute of British Geographers)
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and
distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
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Michelle C. Jackson et al.
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sentinels to environmental perturbations occurring at
larger scales (Woodward et al. 2010).
The provisioning of ecosystem services is especially
important in developing countries, where many rural
and poor communities rely heavily on natural resources
for survival and income (Egoh et al. 2012). Communities in southern Africa rely on freshwater ecosystems
for critically important provisioning services, such as
drinking water and food. Inland fisheries in Africa produce the second highest catch by continent (Figure 1a)
and are particularly important in Uganda, Tanzania,
The Democratic Republic of Congo and Kenya
(Figure 1b). Furthermore, only 58% of the population
in sub-Saharan Africa are thought to have access to safe
drinking water (Banerjee and Morella 2011), and delivery of such services is becoming progressively more at
risk due to stressors such as land use change, species
invasions and climate change, as well as increasing
demand (Schmidhuber and Tubiello 2007; Egoh et al.
2012). Southern Africa also has low food security and
little legislation to control the use of natural resources
(Egoh et al. 2012), while also experiencing high rates
of human population growth (World Bank 2015).
With the growing demand for resources and an
increase in environmental perturbations, it is ever more
important that we understand the impacts of
Figure 1 Trends in inland freshwater fish catch by (a)
continent, and (b) the 10 countries in Southern Africa with
the highest catch
Source: Data taken from The Food and Agriculture Organization of the United Nations’ global capture production
database FishStatJ (http://www.fao.org/fishery/statistics/
software/fishstatj/en)
environmental stressors on ecosystem services. Here
we give an overview of the impact of three key stressors
in freshwaters (land use change, species invasions and
climate change) on provisioning ecosystem services in
southern Africa; before discussing how these stressors
may interact. We provide the first framework which
considers how multiple stressors interact to alter provisioning ecosystem services and discuss management
applications. We focus on access to water (for drinking,
power, agriculture, sanitation) and food (i.e. fisheries)
as provisioning ecosystem services because of their
importance, particularly in developing countries, and
consider any country in mainland Africa which is fully,
or in part, in the southern hemisphere.
Land use change
The impacts of land use change, in the form of both
physical habitat change and pollution, have been the
largest drivers of losses in global biodiversity and
ecosystem function in inland freshwater ecosystems
over the last 50–100 years (MA 2005). Southern
Africa, historically a region with relatively low population densities where food production was based on subsistence agriculture and pastoralism (Bloom et al. 1998;
DeFries et al. 2004) has over the last century seen rapid
shifts in economic activities and increased urbanisation
(Bryceson 1996; Darkoh 2009). These altered economic
activities have precipitated rapid changes in land use,
often resulting in measurable degradation of both terrestrial and aquatic ecosystems (Reardon et al. 1999;
Wasige et al. 2013). While land use change is rapid
and severe in some places (Reid et al. 2000; Wasige
et al. 2013), the economic activities underpinning urban
development are stunted in many regions by
unfavourable geography and other demographic drivers
such as infectious diseases (Bloom et al. 1998). This
means there remain many rural and peri-urban communities that directly depend on the provisioning ecosystem services provided by local aquatic ecosystems,
which are in turn, threatened by regional developments.
Land use change in rural areas is characterised by
deforestation and expanding land under cultivation
(Wasige et al. 2013). This form of land cover change
can have several direct and indirect impacts on aquatic
ecosystems. The most direct impacts of expanded or
intensified agriculture are experienced by wetlands,
which provide a wide range of ecosystem services that
are directly affected by the removal or degradation of
the wetlands (Rebelo et al. 2010). In East Africa,
wetlands are a critical source of potable water to rural
communities, in one documented case (the Yala Swamp
in Kenya) providing 100% of drinking and washing
water (Schuyt 2005). These ecosystems also provide
raw materials for roofing, furniture and fish traps, as well
as cropping and grazing land in the dry season and
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Environmental stress and ecosystem services
seasonal or permanent fisheries (Chapman et al. 2001;
van Dam et al. 2011). The rapid encroachment of intensified agriculture, characterised by removal of wetland vegetation and hydrological alterations, is leading to a
degradation of these resources (van Dam et al. 2011).
The direct utilisation of water resources, by way of
water abstraction or diversion, can play a substantial
role in degrading downstream provisioning ecosystem
services. Irrigation has historically been the primary recipient of such diverted supplies in Africa, comprising
80% of all water abstractions (Machibya and Mdemu
2005). While irrigated agriculture can drive significant
socio-economic development in rural communities,
downstream users can be negatively impacted, if they
are disenfranchised from the upstream irrigation
scheme, or depend on the system’s natural, preabstraction hydrology to receive the ecosystem services
(Oosterbaan 1984). For example, a government scheme
to upgrade traditional rice paddy irrigation infrastructure in the Usanga Basin, Tanzania resulted in an
inequitable allocation of water resources favouring
upstream users at the expense of downstream users
(Machibya and Mdemu 2005). While many forms of water utilisation infrastructure exist, it is the construction
of on-channel dams, a major consequence of agricultural intensification and urban expansion throughout
southern Africa, which has had the most widely
documented negative impact on downstream services.
In the 1960s and 1970s, many countries invested in
large dams to promote water security and produce
hydropower (Schuyt 2005). While in some cases these
projects have given rise to new provisioning ecosystem
services above the dam wall, such as the commercial
lift-net and small-scale light fisheries which have been
established to harvest some 35 000 t year 1 kapenta
Limnothrissa miodon (an introduced small pelagic fish
species), from reservoirs in the Zambezi Basin
(Ellender and Weyl 2014), the net change in ecosystem
services below the wall appear to be consistently
negative. The major impact on provisioning ecosystem
services derives from reduced floods and sediment
transport. In the Tana River delta in Kenya (Leauthaud
et al. 2013), the Zambezi River in Mozambique
(Davies et al. 1975) and the Kafue River in Zambia
(Richter et al. 2010), for example, altered river
hydrology has resulted in collapsed fisheries, degraded
grazing land productivity and, in some cases, the need
for local communities to shift from migrant livestock
grazing to crop farming. Small, privately built dams
can also have negative effects on downstream
ecosystem services. A decrease in natural flooding
caused by a proliferation of small upstream dams was
shown to exacerbate bush encroachment in floodplain
grazing land in semi-arid tributaries of the Limpopo
River, South Africa, thus accelerating the depletion of
this ecosystem service (O’Connor 2001).
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While large dams can have the impact of decreasing
the supply of sediments to downstream rivers, broader
landscape changes such as deforestation, overgrazing,
and poor farming practices that cause erosion of
topsoil, all increase sediment loads to water bodies
(Olago and Odada 2007). In the tributaries of Lake
Victoria for example, navigability for fishing boats is
disrupted directly by the sedimentation of stream channels (Olago and Odada 2007) while in Lake Tanganyika,
sediment loading has depleted both fish diversity and
densities in shallow shoreline areas (Cohen et al. 1993).
In Lake Malawi, fish populations that are already under
severe pressure of overfishing are increasingly pressured
by increased sediment inputs resulting from poor land
use practices (Weyl et al. 2010). Worst affected were
the endemic potamodromous cyprinids, Labeo mesops
and Opsaridium microlepis, whose decline since the
1930s has been linked primarily to alterations in flow
regime and sedimentation of spawning areas resulting
from catchment degradation (Weyl et al. 2010).
Lake-dwelling fish species are negatively affected by
increased turbidity which decreases light penetration
and reduces overall productivity, as well as from the
smothering of demersal algae and fauna by settling
sediment (Weyl et al. 2010).
Eutrophication, the oversupply of organic nutrients,
is another major problem in Lake Victoria, caused by
the inflow of fertile top soils eroding from the transformed farmland, as well as sewage effluents from
urban developments along the shoreline (Olago and
Odada 2007; Nyenje et al. 2010). It results in high
biological oxygen demands from organisms such as
blue-green algae, which can lead to hypoxia-driven fish
kills, threatening local fisheries (Olago and Odada
2007). Similar fish kills have been reported in Lake
Tanganyika (Odada et al. 2003), and are also recognised
as a threat to Lake Malawi’s fisheries (Dudgeon et al.
2006). Additionally, eutrophication is recognised as a
major threat to potable water supplies to communities
along lake shorelines (e.g. Odada et al. 2003). The two
major sources of this pollution are treated urban
wastewater, which remains high in organic nutrients
such as nitrogen and phosphorous; and untreated
sewage emanating from damaged or overwhelmed
wastewater treatment infrastructure (Nyenje et al.
2010). The latter brings an additional threat to the
safety of drinking lake water by increasing microbial
toxicity (Odada et al. 2003) and increasing the risk of
exposure to infectious diseases (Momba et al. 2006).
Despite the proportionately high impact of urban
wastewater on water resources, certain industrial
effluents also have significant negative impacts on water
supply in certain regions. For instance, the capital of
Uganda, Kampala, has several industries, including
abattoirs and small-scale metal industries, which
discharge untreated waste directly into local water
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bodies (Matagi 2002). Another significant industrial
threat to water resources across southern Africa is that
of mining, which has a long history of creating environmental problems (Boocock 2002). For example, the
Kafue River in the Copper Belt of northern Zambia
receives significant inputs of copper, cobalt and iron
oxides from mine tailings, often exceeding international
water quality standards (Sracek et al. 2012). Similarly, in
the Witwatersrand gold-mining region in South Africa,
water bodies are being contaminated by heavy metals,
including uranium radio-isotopes, emanating from
mine tailings deposits (Winde and van der Walt 2004),
and acid mine drainage, whereby chemical reactions
between water and exposed iron pyrite lower the pH
of receiving streams to toxic levels (Akcil and Koldas
2006; McCarthy 2011). All of these chemical pollutants
are likely to increase the burden on water treatment
works in the future, depleting the ability of these
aquatic ecosystems to safe drinking water to ever
growing human populations.
Invasive species
As is the case elsewhere, alien invasive species are
having increasing impacts on aquatic ecosystems in
southern Africa. Most severe are the negative impacts
of invasive terrestrial and aquatic plants. In South
Africa, for example, colonisation of riparian zones by
invasive trees, such as wattle Acacia mearnsii, gums
Eucalyptus spp. and pines Pinus spp. are an ongoing
problem (Richardson et al. 2010). Impacts on freshwater
systems are linked to the higher evapo-transpiration
rates of alien trees relative to those of the indigenous
vegetation that they replace, resulting in reduced river
flows and groundwater reserves (Malan and Day
2003). For river catchments receiving >800 mm rainfall
per year, Cullis et al. (2007) estimated that the total loss
of usable water resulting from alien plant invasions was
equivalent to 4% of the total registered water use in
South Africa. In a water-poor country such as South
Africa, such reductions in flow can have severe
consequences for water provisioning in both rural and
urban areas.
Aquatic plant invaders include water hyacinth
Eichhornia crassipes, water lettuce Pistia stratiotes,
Kariba weed Salvinia molesta and red water fern
Azolla filiculoides, all of which increase rapidly in
biomass resulting in the formation of dense mats in
nutrient loaded environments (Chamier et al. 2012).
While this can decrease nutrient loads, the resultant
mats reduce light penetration and photosynthesis,
reduce water circulation and the diffusion of air into
water, and increase organic detritus loads which are
broken down by anaerobic processes (Chamier et al.
2012). Collectively this often results in anoxic conditions that are uninhabitable by aquatic invertebrates
Michelle C. Jackson et al.
and fishes. On the Ugandan side of Lake Victoria,
for example, stationary fringes of invasive plants
covered much of the shoreline in the mid 1990s and
resulted in significant socioeconomic and environmental impacts, including disruption of transport, tourism
and fishing, negative impacts on water quality, and
spread of waterborne diseases (Balirwa et al. 2003).
This increased disease incidence is linked to improved
habitat (and therefore increased abundance) for snails
(Biomphalaria spp. and Bulinus spp.) which act as
intermediate hosts for schistosomiasis (Masiwa et al.
2001), and mosquitoes (e.g. Anopheles spp.), which
are vectors for diseases such as dengue fever,
elephantiasis, encephalitis and malaria (Oliver 1993).
Non-native fishes have been introduced into southern Africa for more than two centuries. In South
Africa alone, a total of 55 fishes have been introduced
into novel environments for angling, aquaculture,
fisheries enhancements, as aquarium fish, and for the
control of mosquito larvae (Ellender and Weyl 2014).
While many introduced fishes now contribute
significantly to local economies through fisheries and
aquaculture, they also impact on native biota through
direct predation, competition, hybridisation introduction of pathogens and environmental modification
(Ellender and Weyl 2014).
Nile perch Lates niloticus were introduced into Lake
Victoria in 1954, and in the 1980s their populations increased explosively and their predation on native fishes
resulted in the extinction of approximately 200 species
which is considered to represent the largest extinction
event among vertebrates during the twentieth century
(Goldschmidt et al. 1993). The species has however
had considerable economic impact and its introduction
resulted in an increase in fisheries production
from around 100 000 t year 1 in 1980 to about
1 000 000 t year 1 at present (Mkumbo and Marshall
2015). The fisheries as a whole now supports some
4 million people and Nile perch exports, mainly to
Europe, are worth about US$350 million annually
(Mkumbo and Marshall 2015).
In South Africa, introduced common carp Cyprinus
carpio are the main target species for an estimated 15
million freshwater anglers, who target this species for
both recreation and subsistence (Ellender and Weyl
2014; Weyl and Cowley 2015). Carp therefore contribute not only to the local and national economy but are
also a source of food security for the rural poor
(Ellender and Weyl 2014). However, in South Africa,
common carp is the primary vector for introduced parasites and diseases which affect native sport fishes such
as the smallmouth yellowfish Labeobarbus aeneus which
are also economically important (Ellender and Weyl
2014). Although the effects of C. carpio have not been
quantified in southern Africa, the bottom-grubbing
feeding strategy employed by this species alters invaded
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Environmental stress and ecosystem services
habitats through the resuspension of sediments which
increases nutrient loading (i.e. eutrophication) and increases turbidity (Lougheed et al. 1998), which has
resulted in additional costs associated with water treatment in South Africa (De Moor and Bruton 1988).
Climate change
The Earth’s surface has already experienced warming of
0.85 °C in the period from 1880 to 2012 (calculated from
global combined land and ocean surface temperature;
IPCC 2013), causing shifts in species geographic ranges,
seasonal activities, migration patterns, abundances and
interactions (Parmesan and Yohe 2003; Bellard et al.
2012; IPCC 2013). Future projections vary based on different socio-economic scenarios, but suggest warming of at
least 2 °C by 2100 (IPCC 2013). Climate change will
cause shifts in the seasonal patterns and amount of precipitation, while also increasing the number and severity
of extreme weather events, such as drought and flooding
(IPCC 2013). In southern Africa, these changes are
expected to create stress on water resources, crop
productivity and food security, and an increase in waterborne diseases (IPCC 2013). Moreover, Africa is considered one of the most vulnerable regions to global climate
change (Magadza 1994; Schmidhuber and Tubiello 2007;
Egoh et al. 2012; IPCC 2013) and communities are
already experiencing its affects (e.g. Bunce et al. 2009).
By the end of the twenty-first century most of the
region is expected to experience a 10% decline in
annual rainfall, with this being as high as 20% in
Namibia (de Wit and Stankiewicz 2006). This will reduce access to fresh drinking water, impair sanitation
and perhaps increase conflict as water becomes an
increasingly valuable resource (de Wit and Stankiewicz
2006). However, projections for the north-eastern
equatorial parts of southern Africa suggest that the
region will experience benefits from climate change,
through increased precipitation, river runoff and fresh
water availability (de Wit and Stankiewicz 2006;
Doherty et al. 2010). Nonetheless, this may also increase
flood risk, soil erosion and pathogen loads (Doherty et al.
2010; IPCC 2013), and climate change is still likely to
exacerbate the broader region’s overall water scarcity
considerably (Milly et al. 2005; Schewe et al. 2014).
Water shortages and accessibility to water are
already ongoing problems in many parts of southern
Africa (Banerjee and Morella 2011). For instance, in
the Usangu plain in south-western Tanzania, only 13%
of households have full-time access to safe drinking
water while the national average is just 66% (Malley
et al. 2009). Stressors associated with climate change
are blamed for this scarcity (Malley et al. 2009), and
the same water shortage scenario occurs in many
communities throughout the arid regions of southern
Africa. A global prioritisation scheme, which selected
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the 10 most vulnerable watersheds for ecosystem service
delivery, ranked 50% of them in Africa, including Rufiji
in Tanzania (Luck et al. 2009). Like many watersheds
in the region, this floodplain is vital for the food security
and nutrition of local communities, but is expected to
be impacted by global climate change.
A change in the availability of ground and surface
water, due to shifts in the amount or timing of seasonal
rainfall, also negatively affects agriculture. Many rural
communities in southern Africa rely on annual floods
to increase soil productivity, and rain or local water
bodies for irrigation (Cooper et al. 2008). In the past,
severe droughts have caused a decline in commercial
crop yields, impacting the economies of South Africa,
Malawi and Zambia (Jury 2002; Thurlow et al. 2012).
This is expected to be aggravated by increases in
temperature, which may increase irrigation requirements and further water stress (Morton 2007). Future
projections of important crop yields (including maize
and millet) under warming scenarios suggest a considerable decline in productivity (Schlenker and Lobell
2010). Alternatively, climate change may cause extreme
flooding events, resulting in widespread devastation to
agricultural land and threatening food security (Conway
et al. 2015). A decline in water resources may also have
implications for electricity production from dams
(Harrison and Whittington 2002; Conway et al. 2015).
For example, Yamba et al. (2011) predicted a reduction
in hydropower generation over the next 60 years for the
Zambezi River Basin due to droughts and reduced
reservoir storage capacity.
Fish protein is important for both food and income
in many parts of southern Africa (Figure 1), and globally, the countries with fisheries which are the most vulnerable to climate change are also the poorest,
including many in this region (Allison et al. 2009).
Although annual catch in inland fisheries has increased
over time (Figure 1), this is due to increased effort and
the growing population escalating demand. At a local
scale, many fisheries have seen a decline in fish stocks
due to this growing demand, which is exacerbated by
climate change. Nine countries in southern Africa
(Angola, Burundi, Democratic Republic of Congo,
Mozambique, Malawi, Uganda, Tanzania, Zambia and
Zimbabwe) have fisheries ranked as highly vulnerable
to climate change in a global assessment (Allison et al.
2009). Given that six of these are land-locked countries
with high inland fishery yields (Figure 1b), this implies
that inland fisheries in southern Africa are especially
vulnerable to climate stressors.
A study in Tanzania, which used lake sediment cores
to measure changes in productivity over time in Lake
Tanganyika, suggested that climate change has caused
a 20% decrease in primary productivity, implying
around a 30% decrease in fish yields (O’Reilly et al.
2003). Similarly, Lake Chilwa in Malawi has seen fish
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production decline dramatically since 1970 (Njaya et al.
2011). Inland fisheries in smaller lakes will be affected
by declines in precipitation (and subsequently water
levels) because there will be fewer opportunities for
fishing and aquaculture (Allison et al. 2009). For
instance, Lake Naivasha in Kenya has unpredictable
lake level fluctuations which threaten the small fishery
(Becht and Harper 2002). The lake level has been
falling consistently since 1980 and there has been a
linked decline in annual fish catch (Hickley et al. 2004).
Other stressors associated with climate change, such
as sea-level rise, increasing salinity and changes in
weather patterns, will also have an impact on provisioning ecosystem services in southern Africa (Ngoran et al.
2015). Extreme weather events and unpredictable wet
seasons may cause damage to assets and decrease the
ability of communities to plan seasonal fishing activities
(Allison et al. 2009). Changes in wind patterns will also
have implications for inland fisheries, by altering lake
mixing and stratification. Moderate mixing can promote
fishery production by allowing nutrient influx into the
water column. Stronger or weaker winds, however, can
suspend anoxic sediments or cause low levels of dissolved oxygen, respectively (Kaufman et al. 1996; Ficke
et al. 2007). For example, in Lake Victoria decreased
mixing has led to low levels of dissolved oxygen and,
subsequently, fish kills (Kaufman et al. 1996).
Multiple stressors
With accelerating change in freshwater systems
(e.g. Aeschbach-Hertig and Gleeson 2012; Jackson
and Grey 2013) it is becoming increasing unlikely that
environmental stressors will occur in isolation (Jackson
et al. 2016). Many freshwater ecosystems will
simultaneously be affected by local, catchment-level,
and global stressors, such as species invasions, land
use change and global warming, respectively. However,
if, how and when these stressors interact to impact
ecosystems is still largely unknown (Piggott et al. 2015;
Jackson et al. 2016). There are a variety of potential stressor interactions (Table I, Figure 2) which may
alter ecosystem services in different ways depending
on, for instance, stressor identity, community structure,
the type of service, and the lag period between stressor
events (Jackson et al. 2016). This high context
dependency means it is extremely difficult to predict
stressor interactions and the subsequent consequence
for ecosystem services.
Stressors can interact to create impacts which are
equal to (additive interactions), smaller than (antagonistic interactions), or larger than (synergistic interactions)
their single effects (Table I, Figure 2). Non-additive
antagonistic interactions can occur when the impact of
one stressor is so large, it overshadows that of the
second stressor (Jackson et al. 2016). For instance, a
severe drought might overshadow any impacts of
invasive trees on water availability. Alternatively,
antagonistic interactions can result from co-tolerance
or acclimation of service delivery to stressors (Jackson
et al. 2016). Synergistic interactions of stressors are often
considered the most damaging because of the amplification of their independent impacts (Jackson et al. 2016).
These can occur when the impact of two stressors are
larger than predicted, perhaps because an ecosystem
service is only vulnerable when both stressors are
present (Table I, Figure 2). For example, flooding of
cropland from extreme weather events might worsen
with land cover change from deforestation because of
reduced ability of the soil to hold water (Douglas et al.
2008; Wheater and Evans 2009).
More complex stressor interactions may mitigate one
another’s impacts on ecosystem services (Table I,
Figure 2). For example, in Lake Victoria pollution was
causing a decline in native fish stock and, although the
introduction of tilapia and Nile Perch exacerbated this
at a species level (Witte et al. 1992), their introduction
boosted fishery catches overall (Mkumbo and Marshall
2015). However, a recent review by van Zwieten et al.
(2016) reassessed Lake Victoria’s Nile perch takeover
and concurrent reduction in native fishes and suggested
that increased eutrophication (driven by catchment
degradation) resulted in changes in algal and
zooplankton composition, decreased water transparency, and widespread hypoxia, which most likely caused
an initial native fish population crash. This then
facilitated the rapid increase in Nile perch by reducing
predation on larval Nile perch by native zooplanktivorous cichlids. Eutrophication in Lake Victoria has
also promoted the proliferation of invasive water
hyacinth, causing further anoxic fish kills in the lake
(Figure 3; Odada et al. 2004; Olago and Odada 2007).
This type of multiple stressor interaction, which we
term successive interactions (Table I), involves one
Table I Types of multi-stressor interactive impacts on ecosystem services
Interaction type
Additive
Antagonistic
Synergistic
Mitigating
Successive
Description
The impact of two stressors occurring simultaneously equals the sum of their single effects
The impact of two stressors occurring simultaneously is less than the sum of their single effects
The impact of two stressors occurring simultaneously is more than the sum of their single effects
One stressor modulates the impact of a second stressor
One stressor causes a second stressor to occur
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Figure 2 A schematic highlighting some of the potential positive (‘Increased delivery’) and negative (‘Reduced delivery’)
effects of three key stressors on freshwater ecosystem services in Southern Africa. The ecosystem services considered are
(a) water resources, and (b) fisheries. Some of the possible consequences of interactions between the stressors are also
given (‘Example multi-stress interaction’)
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Institute of British Geographers)
Page 8 | 2016 | Volume 3 | Issue 2 | e00026
Michelle C. Jackson et al.
(3) monitoring of stressor events and ecosystem service
delivery over time (e.g. Durance et al. 2016); and,
modelling approaches which may provide theory which
can later be tested for realism (e.g. Segner et al. 2014).
Management of freshwater systems
Figure 3 A case study of the interactions between multiple
stressors and subsequent influence on fishery catch in Lake
Victoria, East Africa. Eutrophication in the lake, resulting
from high nutrient inputs in the surrounding catchment,
causes anoxic fish kills while at the same time triggering
the proliferation of invasive water hyacinth. This caused a
successive and synergistic multi-stressor interaction
whereby the increase in hyacinth biomass triggered further
anoxic fish kills in the lake. In addition, the introduction of
non-native Nile Perch caused a dramatic decline in native
fish biodiversity but boosted the overall fishery catch in the
lake, benefitting the surrounding populations
stressor causing a second stressor to occur, and in this
case, land use change promoting species invasions.
However, since the two stressors also caused fish kills
which were greater than the sum of their single effects,
this is also a synergistic interaction (Figure 3). Another
example of a successive interaction has been
documented in Morogoro Tanzania, where climate
change has forced households to extend or intensify
agriculture and migrate to new areas, causing water
resources to be depleted or degraded (i.e. climate
change promoting land use change; Paavola 2008).
These complex and non-additive impacts of multiple
stressors have been termed ‘ecological surprises’ since
they are hard to predict without context-dependent
data (Jackson et al. 2016). Furthermore, nonenvironmental stressors (e.g. social and economic
stressors) may also interact with the environment to
exacerbate or mitigate impacts on ecosystem services.
We suggest that future research goals should be
directed towards understanding the multiple responses
of ecosystem service delivery to these multiple, and
potentially interacting, stressors. A multitude of
approaches will be needed to achieve this goal including
(1) controlled factorial experiments involving more
than one stressor (Jackson et al. 2016); (2) space for
time substitution field studies (Newbold et al. 2015);
In southern Africa more than 50% of the population live
below the poverty line (World Bank 2015). Issues of
food security, livelihood provision, poverty alleviation,
and economic development are therefore national
policy objectives in many countries (Weyl et al. 2007).
Adaptation to global change is therefore an important
management issue, particularly with the multiple
responses that may arise from multiple stressors.
Aquatic resources provide considerable goods and
services and inland fisheries in particular are an
important safety net for the rural poor (Figure 1;
Welcomme 2011; Béné et al. 2016). In some countries
such as Malawi, the anthropogenic impacts are so
profound that recovery of the catchment may be an
unachievable management goal (Weyl et al. 2010).
Nonetheless, in other countries there may still be
opportunities to better manage catchments by involving
communities in such a manner that the benefits of
conservation outweigh the losses of their existing
unsustainable resource use (Weyl et al. 2010).
While the ecosystem services concept is rapidly
establishing itself as a framework through which to
implement the sustainable use of freshwater resources,
many of its principles already exist within southern
African water policy in the form of an older concept,
integrated water resource management (IWRM). Both
concepts approach the management of freshwater
resources, by addressing competing interests such as
socio-economic needs, environmental concerns, and
human health considerations (Jewitt 2002; Malan and
Day 2003; Cook and Spray 2012). There remains,
however, a significant knowledge gap in understanding
the fundamental interactions between environmental
stressors and freshwater resources that, if not
adequately addressed, could result in ineffective
implementation of sustainable water resource management objectives (Jewitt 2002). Moreover, a disconnect
between scientific knowledge, society and policy has
hampered the implementation of IWRM in many
southern African countries (van der Zaag 2005; Cook
and Spray 2012). Scientific understanding of the complex interactions between multiple stressors thus cannot
translate into improved management without strong
co-operative governance between national agencies
and local stakeholders, especially when the latter parties
may both depend upon and negatively affect freshwater
provisioning ecosystem services (van der Zaag 2005).
An example of the need to understand both ecological and social requirements when addressing interacting
ISSN 2054-4049 doi: 10.1002/geo2.26
© 2016 The Authors. Geo: Geography and Environment published by John Wiley & Sons Ltd and the Royal Geographical Society (with the
Institute of British Geographers)
Environmental stress and ecosystem services
stressors on water resources can be found in the management of invasive riparian trees in South Africa.
Research has demonstrated that these invasive species
reduce surface flows through evapotranspiration, thus
exacerbating the problems of equitable water allocation
to multiple competing users downstream (Richardson
et al. 2010; Forsyth et al. 2012). The government
responded to this threat with the Working for Water
Programme
(www.dwa.gov.za/wfw/)
which
was
launched in 1995 to alleviate the impacts of invasive
species on water resources through multiple control
measures (Richardson et al. 2010). This national-scale
initiative aims to protect water supplies and restore
productive land, primarily through labour-intensive
invasive plant-clearing operations (Richardson et al.
2010). Between 1995 and the end of 2008 the programme had cleared invasive plants from more than
1.8 million ha of invaded land. Nonetheless, factors
including the hiring of under-skilled contractors (in
aid of poverty alleviation) with resulting ineffective
clearing operations, and the emphasis on tackling as
many species in as many regions as possible to maximise
the social benefits of the programme, have led to significant inefficiencies in alleviating the stress on water
resources in key catchments (van Wilgen et al. 2012).
The Working for Water case study demonstrates that
it is possible to prioritise efforts to manage interacting
stressors (here, alien trees and water abstraction). In
this case a multi-criteria approach called the analytic
hierarchy process was used to gain consensus on which
catchments should be invested in first for alien clearing
based on current invasion levels, water demand from
irrigators, and the local contractor capacity for ensuring
successful clearing (Forsyth et al. 2012). In a broader
water resource management context, prioritising which
potentially interacting stressors to attack first will
depend on understanding which threatened provisioning ecosystem services are most needed by particular
communities. This presents a challenge, as different
catchments will face a unique combination of
interacting stressors, which will in turn affect provisioning ecosystem services that have greater significance to
some communities than others (e.g. impact of siltation
on floodplain farmers versus abstracting irrigators).
Even once the prioritisation of interventions is made,
the success of these interventions will still depend on
strong, co-operative governance between government
and local stakeholders to ensure the intended outcomes
are achieved (van der Zaag 2005).
Conclusions
Land-use change, species invasions and climate change
can all be linked to a loss of the ‘goods and services’
provided by freshwater systems in southern Africa.
Evidence suggests that all three stressors can
Page 9 | 2016 | Volume 3 | Issue 2 | e00026
independently reduce water availability and food security and may interact to alter these valuable provisioning ecosystem services. However, information on the
impacts of stressors for this region is rarely quantitative,
and there is an urgent need for more information in our
changing world. This is particularly urgent given that
stressors are unlikely to occur in isolation and multiple
stressors frequently result in unpredictable and complex
‘ecological surprises’ (Jackson et al. 2016).
It is also important to consider that the impacts of
environmental stressors can be perpetuated by other
concerns, such as war, economic policy, access to
information and poor governance (Reid and Vogel
2006; Zimmerman et al. 2008; Bunce et al. 2009).
Therefore, the large number of environmental, social
and economic stressors, including the potential interactions among them, must all be considered in attempts to
manage and conserve ecosystem services in southern
Africa. The prioritisation of management efforts to
mitigate against impacts of land-use change, invasive
species, climate change, or the emergent effects of
these stressors will first require a more explicit
understanding and appreciation of the social-ecological
context of these stressors at both local and regional
scales. We therefore suggest there is an urgent
requirement for research on the combined impacts of
these environmental, social and economic stressors in
southern Africa, and to effectively communicate the
management implications of such research to
policymakers across the region.
Acknowledgements
We thank Anson Mackay for inviting us to write this
paper and two anonymous reviewers whose comments
greatly improved the text. DJW (UID: 103581) and
OLFW (UID: 77444) acknowledge the support of the
National Research Foundation of South Africa.
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