Multiple stressors and ecological surprises

The expanding global human population, now about 7.5 billion, is increasing the pressure that we as a species put on the environment.  2016 was the warmest year ever recorded, and temperature records continue to be exceeded. Each year, more natural ecosystems are lost to dam construction, deforestation and urbanisation. Rates of species invasion are increasing, and pollution events continue to pressure native wildlife. Many ecosystems are now threatened simultaneously by these multiple human-caused stressors, yet we still know very little about their combined interactive impacts.

In our paper in Geo (Linking key environmental stressors with the delivery of provisioning ecosystem services in the freshwaters of southern Africa) we review the impacts of multiple stressors on ecosystem services in freshwater ecosystems in southern Africa (e.g. the Okavango Delta; see photo). We chose these systems because freshwaters contribute disproportionately to ecosystem services despite covering less than 1% of the earth’s surface. 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 81% (WWF Living Planet Report, 2016). Communities in southern Africa rely on freshwater ecosystems for critically important provisioning services, such as drinking water and food (e.g. inland fisheries)

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The Okavango Delta

We found evidence that water resources for drinking, agriculture, sanitation and power are declining because of both climate and land use change. In some areas, fish production increased because of dam construction or species invasions, but these stressors can have negative impacts elsewhere. Evidence also suggests that stressors can interact to alter one another’s impacts or promote the proliferation of further stressors.

Multiple stressors often cause impacts which are hard to predict because of both complex interactions between the stressors themselves, and interactions within communities (such as those between species in a food web). These unpredictable impacts have been termed ‘ecological surprises’ and global analyses indicate that they are very common (e.g. http://onlinelibrary.wiley.com/doi/10.1111/gcb.13028). This creates problems for decision makers when prioritising which stressors to manage or control, especially when it comes to the supply of the goods and services which we rely on from natural ecosystems.

We provide a framework to categorise multiple stressor effects on ecosystem services where they can either be additive (i.e. predictable and the sum of their independent effects) or four different types of non-additive ecological surprises.  For instance, nutrient enrichment in Lake Victoria (because of high nutrient inputs from the surrounding catchment) causes low oxygen levels, killing fish (Photo 2). At the same time the nutrients promote growth of invasive aquatic plants (water hyacinth) causing a successive and synergistic multi-stressor interaction whereby the increase in plant biomass triggers further fish kills in the lake. In addition, the introduction of non-native fish (Nile perch) caused a dramatic decline in native fish biodiversity but boosted the overall fishery catch in the lake, benefiting the surrounding populations (see figure below).

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With the growing population, it is becoming difficult to protect biodiversity and rely on our planet’s natural ecosystems for food and water security. Multiple stressors are causing a downward spiral, where our use of ecosystem services threatens the environment and therefore impairs the delivery of these services for future generations.  We need more research into multiple stressors and ecological surprises, and much more needs to be done to reduce the impact that humans have on the environment.

Michelle Jackson is a Researcher at Imperial College London.