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

Abstract

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.

Local stressors mask the effects of warming in freshwater ecosystems

“…We show the limits of our current comprehension of how stressors interact, as residual, unexplained heterogeneity emphasises the need for further development of a mechanistic framework taking additional context‐dependencies into account (Boyd et al, 2018 ; Burgess et al, 2021 ). For example, the cumulative effects of stressors and type of interaction may depend on stressor magnitudes, duration order of exposure (MacLennan & Vinebrooke, 2021 ) and type of response (i.e. organismal group, or level of biological organisation; Jackson et al, 2021 , Orr et al, 2022 ).…”

“…Cote et al 2016 ; Jackson et al, 2016 ; Piggott et al 2015 ; Tekin et al, 2020 ). Given the frequency with which stressors interact, a mechanistic understanding is needed if we are to predict future impacts in a rapidly changing—and warming—world (De Laender, 2018 ; MacLennan & Vinebrooke, 2021 ; Schäfer & Piggott, 2018 ). Instead of assuming additivity, it is increasingly argued that researchers should base predictions on null models selected using an established framework (e.g.…”

Variability of functional and biodiversity responses to perturbations is predictable and informative

“…Furthermore, although chemists, biologists and ecotoxicologists can sometimes gather detailed knowledge of the mechanisms of stressor interactions at the individual and population levels (Boyd & Brown, 2015; Didham, Tylianakis, Gemmell, Rand, & Ewers, 2007; Rillig, Lehmann, Orr, & Waldman, 2021), research into stressor interactions at the community level is underdeveloped (Orr et al, 2020). Some progress has been made, for example towards understanding the role of species co-tolerance (Vinebrooke et al, 2004; MacLennan & Vinebrooke, 2021) and species interactions (Thompson, MacLennan, & Vinebrooke, 2018; Beauchesne, Cazelles, Archambault, Dee, & Gravel, 2021), but fundamental knowledge gaps remain.…”

Similarity of anthropogenic stressors is multifaceted and scale dependent

“…7 Second, the order of exposure to sequential stressors will only matter when those stressors' effects are dissimilar. 8 If stressors have identical effects, and they do not interact with each other, their order will be irrelevant. Third, the similarity of stressors can provide insights into how populations adapt to multiple stressors.…”

“…Indeed, selection of the most appropriate multiple‐stressor null model (for predicting the combined effects of stressors) should be guided by the similarity of how stressors act 7 . Second, the order of exposure to sequential stressors will only matter when those stressors’ effects are dissimilar 8 . If stressors have identical effects, and they do not interact with each other, their order will be irrelevant.…”