Expanding horizons of ecological and evolutionary research in urban heat islands

Because temperature has profound effects on most aspects of an organism's biology, it is clear that climatic changes will disrupt associations between organisms and their environment. These disruptions increase the risks of population decline and extirpation in the absence of compensatory responses. Although these processes have been explored in the context of global climate change, other sources of change can threaten populations. Cityscapes generally increase environmental temperature through the retention of heat from impervious surfaces, that is, the urban heat island effect. There are now many studies documenting trait responses to urban heat islands through both plastic and evolutionary mechanisms. Similarly, many other studies have linked urban heat islands to changes in insect community structure within the city footprint. Progress in these research areas has relied heavily on theory and empirical foundations from thermal biology. However, despite a burgeoning literature of temperature effects on fundamental processes that generate (epi)genetic variation and eco-evolutionary dynamics and feedbacks across levels of biological organization from genes to ecosystems, there are large knowledge gaps in how urban warming affects these processes. Here, we summarize the current state of knowledge on the effects of urban heat islands on responses at the level of organismal traits and community structure. We then explore recent advances in temperature effects on genetic and epigenetic sources of variation and eco-evolutionary dynamics to highlight the potential for using cities as venues to understand the role of contemporary warming in each process.