Cumulative impacts of multiple stressors: improving temporal and biological realism

在我们不断变化的世界中，越来越迫切地需要了解污染和变暖等多种环境压力因素之间的相互作用。围绕多重压力源的担忧很大程度上是由于它们相互作用的潜力，造成的影响比它们单独产生的影响更严重。淡水生态系统特别脆弱，淡水生物多样性在全球范围内受到最大威胁：最近的一份报告估计，淡水脊椎动物物种的平均种群数量下降了80%，而陆地和海洋物种的平均种群数量下降了40%(自1970年以来；2018年世界自然基金会生命地球报告)。尽管多种压力源的综合影响已经开始受到更多的关注，但我们对它们的交互作用的了解仍然几乎不存在。在现实中，压力源不太可能在完全相同的时间在同一空间发生，但衡量多种压力源的综合影响的研究往往假设情况就是这样。换句话说，它们缺乏时间现实主义。这些研究中的大多数也缺乏生物学现实主义，通过量化应激源对较低组织水平的模式物种的影响(例如范围转移、存活率、丰度)，并忽略摄食相互作用。在这里，我们将考虑应激源事件的顺序或顺序如何改变淡水个体对生态系统的反应，重点放在食物网相互作用上。生态系统将对它们面临的多种压力做出多种反应，包括多样性、丰富度、体型和摄食行为的变化。即使对其中任何一个的微小改变都可以改变食物网络结构，从而影响未来应激源的影响，然而这些至关重要的相互作用迄今在很大程度上被忽视了。这使得我们对未来自然系统变化的后果几乎没有预测能力。因此，在这里，我们将使用中观实验来量化交错营养物质污染和变暖事件(即先前的暴露)对淡水生态系统的综合影响，并通过采用一套动态水质模型来将结果扩展到集水水平。我们的实验结果将被用来对温度和营养控制的种群大小和增长率进行参数化，并模拟这些变化的速率如何在更大的河流系统尺度上改变食物网络结构。这项跨学科的研究将产生前所未有的广度和深度的数据：从个体健康状况的变化和种群规模结构的变化到食物网络的复杂性。我们将展示多重应激源事件的顺序(即以前的暴露)如何影响社区对变化的抵抗力和应变能力。这些独特的数据集将使我们能够在纯粹的和应用的生态学中提出许多新的问题，并描述鲜为人知的多种应激源对淡水食物网的多重影响。这种对反应的全面报道以前从未尝试过，这项研究将解决我们对应激源影响的知识中的这一明显差距。

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.