Multiple stressors in a warming world: Using thermal performance curves to improve predictions of non-additive outcomes
Climate change is occurring across a mosaic of human activities that affect species and ecosystems, however our ability to predict the combined ecological effects of environmental change and other stressors is limited due to the potential for non-additive interactions. Despite the demonstrated prevalence of non-additive interactions across ecosystems, our ability to predict the conditions under which multiple stressors will interact to either offset or exacerbate the effects of one another is very low. Here, we examine the effects of multiple stressors in cases where one stressor is temperature. Temperature is a fundamental determinant of all biological rates, and existing knowledge regarding thermal effects may allow for theoretical and empirical advances in our understanding of multiple stressor interactions. We hypothesize that warming can both offset and exacerbate the effects of other stressors, and that the outcome of this interaction (i.e., a super- or sub-additive effect) will depend on where the warming treatment falls on the thermal performance curve of the study species. To test this hypothesis, we use meta-analysis to examine the interactive effects of elevated temperature and other stressors (e.g., CO2 enrichment, nutrient loading, and UV exposure) in factorial laboratory experiments with marine and aquatic species.
Our results suggest that non-additive interactions can in part be explained by species’ responses to near-future warming scenarios. In particular, the type of interaction depends on whether the warming treatment imposed in the experiment causes an increase or decrease in the metabolism or growth of the study species. While the patterns differ among stressor combinations and species, we demonstrate that the negative effects of some stressors (e.g., ocean acidification) are predictably more pronounced as warming increases the metabolism or growth of the study species. This research highlights how future warming can influence vulnerability to other stressors and provides guidance for the spatial prioritization of local stressor mitigation to promote resilience to climate change.