Anthropogenic environmental changes, or ‘stressors’, increasingly threaten biodiversity and ecosystem functioning worldwide. Multiple-stressor research is a rapidly expanding field of science that seeks to understand and ultimately predict the interactions between stressors. Reviews and meta-analyses of the primary scientific literature have largely been specific to either freshwater, marine or terrestrial ecology, or ecotoxicology. In this cross-disciplinary study, we review the state of knowledge within and among these disciplines to highlight commonality and division in multiple-stressor research. Our review goes beyond a description of previous research by using quantitative bibliometric analysis to identify the division between disciplines and link previously disconnected research communities. Towards a unified research framework, we discuss the shared goal of increased realism through both ecological and temporal complexity, with the overarching aim of improving predictive power. In a rapidly changing world, advancing our understanding of the cumulative ecological impacts of multiple stressors is critical for biodiversity conservation and ecosystem management. Identifying and overcoming the barriers to interdisciplinary knowledge exchange is necessary in rising to this challenge. Division between ecosystem types and disciplines is largely a human creation. Species and stressors cross these borders and so should the scientists who study them.

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Ecosystems experiencing global change are threatened by many different anthropogenic stressors. Predicting the combined effects of these multiple stressors is a deceptively challenging goal. A vast and diverse body of research has shown that the combined effect of stressors, or global change factors, often differs from what can be expected based on individual effects. This discrepancy is what defines interactions between stressors. Antagonistic interactions cause combined effects to be less than expected, whereas synergistic interactions cause combined effects to be more than expected

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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.