Exposure order effects of consecutive stressors on communities: the role of co-tolerance

Cumulative impacts of multiple extreme and novel environmental changes on communities are often the result of asynchronous rather than simultaneous exposures to such stressors. Yet, the importance of temporal dynamics remains a major knowledge gap in multiple stressor ecology, lacking theory or evidence. We provide a conceptual template for predicting the ecological importance of the order in which consecutive stressors occur (i.e. an exposure order effect) based on correlated species responses. Negative correlation of species responses is hypothesized to increase, while positive correlation is expected to reduce, exposure order effects of consecutive stressors on communities. Towards a proof of concept, we experimentally exposed planktonic communities from fishless mountain lakes to different temporal sequences of two stressors, namely invasive sportfish and elevated water temperatures. Both stressors suppressed the same resident top predator and large grazers while eliciting positive responses from smaller tolerant taxa, attesting to their interchangeable effects across species based on size selection. As a result, reversal of the order of exposure to the two stressors did not alter their combined effects on community composition and function. Our findings highlight how the order in which consecutive stressors occur may not matter to a community if ecological memory of either stressor induces tolerance of the other.