Similarity of anthropogenic stressors is multifaceted and scale dependent

Abstract

Human influences have created a world where all ecosystems are exposed to multiple stressors with diverse characteristics and impacts. Organizing this ever-expanding list of stressors presents the opportunity to derive general rules, to understand effects and to improve predictions. Assessing the similarity of anthropogenic stressors can be approached from different perspectives, which we synthesize here. Stressors can be compared by asking what they are (traits), why they are present (sources), where they occur (spatial overlap), when they occur (temporal overlap), how they affect organisms (mode of action) and who they affect (co-tolerance). Some concepts of stressor similarity are thus related to intrinsic features of the stressors themselves (e.g. sources and traits), while other concepts pertain to the similarity of the effects of stressors and are therefore dependent on the ecological scale at which the effects of stressors are measured (e.g. mode of action and co-tolerance). Viewing stressor similarity as a multifaceted and scale-dependent concept will open new avenues for stressor classification, will enhance predictions of the combined effects of stressors and may even inform the management of novel or emerging stressors. Keypoints:• The anthropogenic stressors that are impacting biodiversity and ecosystem functioning worldwide can be physical (e.g. warming), chemical (e.g. pesticides) or biological (e.g. disease) in nature.• We show that stressors can be compared by asking what they are, why they are present, where they occur, when they occur, how they affect organisms and who, or which organisms, they affect.• Understanding the similarity of stressors requires interdisciplinary collaboration and will help us to manage species and habitats threatend by a growing number of stressors.

Local stressors mask the effects of warming in freshwater ecosystems

“…In 42% of cases, the observed effect was less than predicted by all null models, supporting the previously documented prevalence of antagonistic stressor interactions in freshwater ecosystems (Jackson et al, 2016 ; Lange et al, 2018 ; Tekin et al, 2020 ). This was particularly apparent when temperature was combined with invasion or change in food quantity, which might be related to the similarity of their effects (Orr et al, 2022 ). For example, invasive predators and warming have similar size‐selective impacts (favouring smaller species; e.g.…”

“…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 ). Further research under different stressor scenarios will be needed to disentangle the variation in effect sizes revealed by our analysis.…”

Fluctuating fortunes: Stressor synchronicity and fluctuating intensity influence biological impacts

“…Identifying the mechanisms by which an organism responds to stressors, and how similar the processes are, can help detect and predict novel stressor outcomes (Orr, Rillig, & Jackson, 2022 ). Multiple stressors may elicit a shared response meaning similar cellular pathways and physiological processes are triggered to mitigate effects, where overlap in responses may enhance or reduce tolerance to stressor exposure (Gunderson et al, 2016 ; Pandey et al, 2015 ; Todgham & Stillman, 2013 ).…”

What does the future look like for kelp when facing multiple stressors?

“…These stressors, which operate from local to global scales, can interact to determine their combined cumulative effect (Figure 2; Falkenberg et al, 2012). Growing evidence suggests that the combined effect of two stressors is rarely additive (i.e., additive = the sum of their parts; Orr et al, 2020Orr et al, , 2022. Instead, stressor pairs frequently result in impacts which are more or less than the sum of their parts (i.e., non-additive responses).…”