Linking key environmental stressors with the delivery of provisioning ecosystem services in the freshwaters of southern Africa

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.