Urban heat and latitude: contrasting effects on ant diversity across cities

Increasing urban temperatures due to the urban heat island effect are associated with widespread biodiversity declines. Because cities are distributed across diverse climatic zones, the effects of urban heat on biodiversity may vary geographically. Here, we examined how urban warming influences ant communities across four U.S. cities that span 6.6° of latitude and differ by 4.1 °C in average summer temperature. We sampled urban ants using baits placed at the base of a single common tree species along an urban temperature gradient within each city. When pooling results across all cities, ant species richness increased towards the equator, consistent with broader macroecological trends. Within cities, however, ant richness consistently declined with increasing urban temperatures, regardless of latitude. In addition to temperature, we observed that ant richness was positively correlated with humidity and negatively correlated with impervious surface cover. Our findings contrast with studies of other urban arthropods, where responses to urban heat vary by latitude. The consistent negative effect of urban warming on ant diversity suggests that urban heat stress may be a conserved factor affecting biodiversity across temperate latitudes, particularly within eastern North America. While we did not detect an increasing sensitivity to urban warming at lower latitudes, the greater overall biodiversity in low-latitude cities implies that urbanization could pose a significant threat to these species-rich communities. Given the rapid expansion of urban areas, especially in warm regions, understanding the combined impacts of urbanization and climate change on biodiversity remains critical for conservation efforts.