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Summary Accumulating evidence suggests that the average body size of many organisms is declining in response to climate warming. This phenomenon has been suggested to represent a universal response to warming that may impose significant adverse effects on ecosystem functioning and services. However, we do not have a thorough understanding of why body sizes are commonly declining, and why some organisms show the opposite response. Because ectotherms constitute the vast majority of organism biomass and about 99% of species worldwide, it is particularly important to understand how ectotherms respond to a warming climate. This review discusses the underlying physiological mechanisms of changes in ectotherm body size and addresses observed responses within a broad ecological context at different levels of organization, from individuals to communities, particularly in aquatic systems. Warming‐induced responses in average body size are not only determined by changes in rates of individual growth and development, but also mediated through size‐dependent feedbacks at the population level, as well as competitive and predatory interactions within the community. Emergent properties at higher organizational levels have already been observed in both experimental and natural systems. Various approaches will be required for enhancing our knowledge about the importance of such processes in natural systems. These include controlled semi‐natural experiments and phylogenetic comparisons as well as statistical models of time‐series data and theoretical models linking climate effects at the individual, population and community levels. Understanding causes of observed changes in organism body sizes and how these depend on the ecological context is essential for improving our predictions and the management of ecosystems in the face of a warming climate.
Jan Ohlberger (Mon,) studied this question.