Cumulative impacts of multiple stressors: improving temporal and biological realism

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In our changing world there is an increasing urgency to understand interactions among multiple environmental stressors, such as pollution and warming. Much of the concern surrounding multiple stressors is due to their potential to interact, creating more severe impacts than they would do independently. Freshwater ecosystems are particularly vulnerable and freshwater biodiversity is the most threatened across the globe: a recent report estimated average population declines of >80% among freshwater vertebrate species compared to <40% in terrestrial and marine species (since 1970; WWF Living Planet Report, 2018). Although the combined impacts of multiple stressors has started to receive more attention, our knowledge on their interactive effects still remains almost non-existent. In reality, stressors are unlikely to occur in the same space at exactly the same time, yet studies that measure the combined effects of multiple stressors often assume this to be the case. In other words, they lack temporal realism. Most of these studies also lack biological realism by quantifying the effects of stressors on model species at lower levels of organisation (e.g. range shifts, survival, abundance) and ignoring feeding interactions. Here, we will consider how the order, or sequence, of stressor events alters individual-to-ecosystem responses of freshwaters, with a focus on food web interactions. Ecosystems will have multiple responses to the multiple stressors they face, including changes in diversity, abundance, body size and feeding behaviour. Even minor alterations to any of these can shift food web structure, with implications for the effects of future stressors, yet these critically important interactions have been largely ignored to date. This leaves us with little or no predictive ability about the consequences of future change in natural systems. Therefore, here we will use mesocosm experiments to quantify the combined effects of staggered nutrient pollution and warming events (i.e. previous exposure) on freshwater ecosystems, and scale our results up to the catchment level by adapting a suite of dynamic water quality models. Our experimental results will be used to parameterize temperature and nutrient controlled population sizes and growth rates, and to simulate how these changed rates will alter food web structure at the larger river system scale. This interdisciplinary study will generate an unprecedented breadth and depth of data: from individual changes in fitness and population shifts in size structure to food web complexity. We will show how the order of multiple stressor events (i.e. previous exposure) affects community resistance and resilience to change. These unique data sets will allow us to ask numerous novel questions in pure and applied ecology, and to characterise the little known multiple impacts of multiple stressors on freshwater food webs. Such a comprehensive coverage of responses has never been attempted before and this study will address this glaring gap in our knowledge of stressor impacts.