Ecological surprises: why multiple stressors in freshwaters may cancel each other out
Stressors are environmental changes that place stress on the health and functioning of an ecosystem. There is increasing evidence – largely from marine environments – that multiple stressors may interact to produce unexpected effects on aquatic ecosystems. However, there is a pressing need to better understand the ‘ecological surprises’ caused by multiple stressors in freshwater ecosystems (a point made in papers by MARS scientists Steve Ormerod in 2010 and Daniel Hering and colleagues in 2015).
Existing scientific literature from marine environments show that multiple stressors can have effects that are greater than the sum of those caused by individual stressors. This ‘synergistic’ interaction poses important questions for environmental managers and policy makers. In short, it is difficult enough to manage individual stressors such as pollution, habitat destruction and overfishing, without the unexpected and, as yet, largely unpredictable interactions and effects these stressors might have.
In the light of this uncertainty, a team of researchers from the University of Pretoria in South Africa and the University of Alberta in Canada analysed data from 88 existing scientific studies that show the responses of freshwater ecosystems to pairs of stressors. The team, led by Michelle Jackson from the University of Pretoria, brought together the findings of these studies to investigate the characteristics and effects of different stressor interactions; and the extent to which interactions vary between different stressor pairs and response measurements.
Freshwater stressors may cancel each other out
Recently published in Global Change Biology, the team’s findings are perhaps surprising, at least initially: the environmental effects caused by pairs of stressors in freshwater was most often less than the sum of their single effects. This is known as an antagonistic interaction, where two or more stressors interact to cancel out some or all of their individual effects.
Across the 88 surveyed studies, stressors included acidification, increased water temperatures, ultraviolet radiation, contamination, nutrification, habitat alteration and invasive species. Antagonistic interactions were found in the majority (41%) of surveyed studies, affecting animal abundance, biomass, condition, growth/size and survival and plant diversity. The authors suggest that this widespread antagonism could be due to the effects of the stronger stressor overriding and negating those of the weaker one. Here, environmental managers might seek to rank the effects of the strongest stressor in an antagonistic interaction in order to forecast the cumulative impacts of multiple stress.
Another suggestion is that exposure to one stressor can result in greater tolerance to another. An important point made here is that there may exist a potential for co-adaption within freshwater ecosystems to minimise the net effects of multiple stressors. However, despite the predominantly antagonistic interactions, net effects of multiple stressors were still mostly negative, underlining the widespread threats faced by freshwater ecosystems.
Multiple stressors in freshwaters: the overall picture
Overall, the net effects of stressor pairs were frequently more antagonistic (41%) than synergistic (where the overall effect is more than the sum of individual stressors: 28%), additive (overall effect is equal than the sum of individual stressors: 16%) or reversed (overall effect is opposite to the positive / negative effect of individual stressors: 15%).
Synergistic interactions between multiple stressors such as increasing sea temperatures, species invasions and habitat destruction have been observed in a number of studies of marine ecosystems. Why is the picture different for freshwaters? The authors suggest that this is due to greater inherent environmental variability in smaller aquatic ecosystems, which fosters higher potential for ecological acclimation and co-adaptation to multiple stressors.
Antagonistic interactions were most frequently observed in affecting animal condition; synergies and reversals with plant growth and size; and additive effects with plant diversity. This variability shows that it is important to consider the ecological metrics used to measure the impacts of multiple stressors.
Different effects on ecosystem diversity and function
Interestingly, multiple stressors had different effects on the diversity and function of ecosystems. Multiple stressor interactions affecting species diversity were most often additive. This suggests that the species eliminated by one stressor were often not the same as those eliminated by the second, and additional stress causes additional species loss.
On the other hand, the most common interaction affecting the functional performance of an ecosystem was antagonism. This may be a result of a process known as compensatory species dynamics. Here, the remaining species in the stressed ecosystem may compensate functionally (e.g. in nutrient cycling) for species loss. This in turn points to the idea that an ecosystem’s functional resilience to stress is not simply dependent on biodiversity, but instead determined by species identity and traits. In short, the findings here suggest that freshwater biodiversity is more sensitive than ecosystem function to the impacts of multiple stressors.
The authors identified that 15% of studies showed reversal effects from multiple stressors. These reversals are termed ‘ecological surprises’, in that they reverse the environmental impacts – negative to positive (or vice versa) – caused by the individual stressors. Whilst reversals were the least common type of interaction observed, their existence has potentially important effects for environmental management.
The stressor most commonly associated with reversal interactions was climate warming. For example, a study by Patrick Thompson and colleagues in 2008 found that warming reversed the negative effects of excess nitrogen supply on phytoplankton growth, possibly as a result of increased conversion of nitrates and ammonia by enzymes as a result of increased temperatures. Ecological responses to temperature change are complex, but the evidence here on multiple stressors causing reversals suggests that there may be ever more ‘ecological surprises’ in a warming world.
Lessons for freshwater conservation management
The paper contains potentially important lessons for freshwater conservation management. For multiple stressors that generate additive or synergistic interactions, management that focuses on a single stressor should cause a positive outcome. However, in ecosystems affected by antagonistic stressor interactions, both stressors may need to be removed in order to foster any significant ecological recovery.