Most ecosystems are subject to a multitude of anthropogenic environmental changes simultaneously. Research in the field of multiple stressors usually finds that stressor richness decreases species richness as well as ecosystem function. However, these studies typically vary the number of stressors, here termed stressor richness, but without controlling for overall stressor intensity. They therefore assess the combined effect of stressor richness and stressor intensity. Here, we examine the separate effects of stressor richness and intensity on biodiversity and ecosystem functions. This study contributes to basic ecological understanding, investigating the influence of a variety of modifying factors on community composition and ecosystem functioning. It also has important ramifications for applied ecology: is it preferable to limit stressor richness but increasing the individual stressor intensity, or to reduce the individual intensity of a larger number of stressors?
Results/Conclusions
Using community models, we examined the effects of increasing the stressor richness, while keeping the community-level stressor intensity constant. We considered three models, ranging from broadly-applicable and phenomenological to highly mechanistic and system-specific, to ensure general applicability of the results. We found that increasing stressor richness aggravated joint stressor effects on community productivity, but reduced effects on species diversity. The heterogeneity of stressor effects on species, termed here stressor diversity, explained the latter result: increasing stressor richness decreases stressor diversity, and it becomes less likely for the stressor mixture to disrupt coexistence. All species’ growth rates are similarly affected, giving no species a clear competitive advantage due to stressor presence. While fewer extinctions occur, compensation by other species is restricted, themselves being similarly affected by stressor action, and overall productivity does not recover. High stressor richness is commonplace globally, and its importance thus far has been overshadowed by the increased stressor intensity which usually accompanies. Demonstrating the effects of stressor richness in isolation is an important step in disentangling how exactly multiple stressors impact ecosystems.