Urbanization interacts with annual temperature to predict butterfly phenology in a long-term survey of Ohio

Background/Question/Methods

Ecologists should look to cities to predict the future. Urbanization has selected organisms not only for their ability to survive in degraded habitat but also for their adaptation to increased temperatures.  Cities typically are warmer and drier than their rural surroundings, leading to an urban heat island effect. Organisms track suitable thermal conditions through their phenology, or timing of life events. By determining which species respond to elevated temperatures in urban areas with a shift in phenology, we may be able to predict which species will persist as the climate warms. In this study, we test how the phenology of 20 butterfly species responds to the urban heat island effect using 13 years of surveys from 81 sites in Ohio across a rural-urban gradient. We test how mean annual temperature and impervious surface cover, our measure of urbanization, affect different aspects of butterfly flight phenology. We also examine whether species traits and phylogenetic relationships predict interspecific variation in phenological responses to urban warming.

Results/Conclusions

Across all species, the degree of urbanization and the mean annual temperature interacted to predict peak abundance and emergence phenology. At lower temperatures, flight phenology was earlier at sites with greater urbanization. At higher temperatures, more urbanization was associated with delayed flight phenology. Insect physiology may explain this interaction, as optimal temperatures for development fall between low and high temperature thresholds. Urban heat islands may speed up development for insects in cooler areas but impede development in warm areas where additional heat pushes temperatures above the high-end threshold. In agreement with previous studies, we observed interspecific variation in phenological responses to urbanization and temperature. Overwintering stage was a key trait in determining phenological responses and was phylogenetically conserved. Our study increases understanding of how species might cope with the combined impacts of global climate change and increasing urbanization.