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

Abstract

Summary1. 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. 2. Although the biodiversity impacts of urbanization are most commonly investigated at the assemblage level, these impacts are mediated through organismal physiology and behaviour. 3. 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. 4. 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. 5. 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. 6. 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. 7. 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.

Urban warming favours C4 plants in temperate European cities

“…The background climate variation on which urban areas are overlaid (Zhao et al . ; Chown & Duffy ) cannot, however, explain the observed patterns of high C 4 relative abundance in urban plant assemblages. Including high‐resolution remotely sensed temperature data, in which UHIs are identifiable, significantly improves the model fit of a zero‐censored tobit regression over a null model that represents regional climate but not localized UHI warming.…”

Expanding fundamental ecological knowledge by studying urban ecosystems

“…,b, ). This type of research needs to be expanded over a range of continents, biomes, city shapes and sizes to investigate questions that cannot be addressed by research conducted in a single urban setting (McDonnell & Hahs ; Chown & Duffy ; Harrison & Winfree ; LaPoint et al . ).…”

“…Although cities share many similarities in terms of the built infrastructure, human population densities and associated environmental stressors (Rebele ), they are distributed across a wide range of biogeographic and climatic zones. Chown & Duffy (), for example, highlight that large cities span mean annual temperature gradients of 30 °C and total annual precipitation gradients of 4000 mm. Therefore, the likelihood of complete convergence in the environmental conditions or biotic homogenization across all cities is very low (Aronson et al .…”

“…), and whilst not necessarily new, improvements in fluorescent dye techniques can contribute to improved understandings of plant–pollinator interactions (Harrison & Winfree ). Examples of important opportunities for experiments include the use of cities as natural experiments for understanding the impacts of climate change (Chown & Duffy ; Harrison & Winfree ), testing laboratory‐based knowledge on the impacts of environmental pollutants on organisms in actual urban landscapes (Harrison & Winfree ; Isaksson ), common garden experiments and long‐term studies of multiple populations or study organisms with pedigree data to test evolutionary responses (Isaksson ) and Before After Control Impact Reference studies on the efficacy of efforts to restore connectivity in urban landscapes (LaPoint et al . ).…”

“…Slabbekoorn ) pollution. Chown & Duffy () use thermal tolerance as a mechanism to explore entry, exit and transformation rules for organisms exposed to urban heat islands. Isaksson () explores mechanisms through which oxidative stress and inflammation determines the capacity of organisms to adapt, acclimatize or otherwise cope with urban environments.…”