Environmental change is multivariate. Understanding the joint effect of multiple drivers of environmental change on communities and ecosystems is a key challenge for science. The dominant approach today is to compare observed joint effects with predictions from various types of null models. Drivers are said to combine synergistically (antagonistically) when their observed joint effect is larger (smaller) than that predicted by the null model. In this paper, I integrate multiple stressors into resource uptake theory to investigate if community and stressor characteristics predict the prevalence of synergistic/antagonistic effects on species richness. I focus on a community of two species that stably coexist because of a trade-off in resource use. Two stressors independently affect the competitive ability of both species, but to variable extents (Tilman’s R*). The characteristics I consider are the size of the trade-off (how much species differ in resource use) and the intensity of the stressors (to what extent stressors affect competitive ability). I use algebra and simulations.
Results/Conclusions
Simulations indicated that the probability for antagonistic/synergistic effects on species richness varied between 5 and 80%, depending on the community and stressor characteristics. Antagonism, which is most frequently concluded in meta-analyses of multiple stressor effects on community variables, is less probable when trade-offs are more pronounced. Algebra shows that this result is not related to the parameter range selected for the simulations, but due to the independence of the stressors at the level of competitive abilities. This analysis maps for the first time the biology of multiple stressors (e.g. (in)dependence among physiological effects) to community-level effects. I also find that the probability for synergistic/antagonistic effects on species richness peaks at intermediate stress levels. My results can inspire future experimental studies in global change ecology to test these predictions. Importantly, these results show that reporting the joint effects of multiple drivers of environmental change should be complemented by characteristics describing the ecology of species interactions and the intensity of the environmental changes.