Thermal physiology and urbanization: perspectives on exit, entry and transformation rules

More than 50% of the global human population lives in urban settings, which, for urban agglomerations with >1 million inhabitants, span a 30 °C range in mean annual temperature and 4000 mm annual precipitation range. Although the biodiversity impacts of urbanization are most commonly investigated at the assemblage level, these impacts are mediated through organismal physiology and behaviour. At the individual level, mechanistic models, which provide an explicit means to understand how organisms meet the requirements of heat, water and nutrient balance, offer a useful way to assess physiological and behavioural responses to urban environments and their consequences for population dynamics. Physiological determinants of assemblage variation can usefully be investigated by examining entry, exit and transformation rules. These are biases in the processes that determine which individuals join or leave a population or assemblage, and processes that act on individuals, which remain part thereof. Direct evidence for physiological adaptation (broadly construed as phenotypic plasticity, genotypic change or some interaction between them) to urban environments is remarkably limited, owing largely to low numbers of investigations, given that most studies that have been conducted reveal some form of adaptation. A warming tolerance approach, applied to insects globally, demonstrates that transposing sampled populations to the nearest urban area reduces warming tolerance by several degrees, though this effect is on average more marked for the Southern than the Northern Hemisphere. The lowest warming tolerance is found between 30 and 35°N or S, suggesting that in the absence of mitigation, the urban heat island effect of cities here will have the most pronounced impacts on insect populations. Physiological approaches offer a useful means for understanding the urban–rural gradient across a wide range of climatically variable settings during a period of background environmental change.