Interacting effects of urbanization and coastal gradients on ant thermal responses

Urban-to-rural gradients intersect with other, often unmeasured, environmental gradients that may influence or even supersede species responses. Here we use coastal-to-interior and urban-to-rural gradients to investigate woodland ant response (physiological thermal tolerance, community structure and spring phenology) to two overlapping thermal gradients, the Great Lakes (Erie and Ontario) and the Buffalo, NY urban center (USA). Woodland ant physiological and behavioral responses, and community responses, shifted along the coastal-to-interior and urban-to-rural gradients, but they were generally best explained by lake effects (though urban ants tolerated higher temperatures than rural ants). The relatively colder spring temperatures in coastal areas (as compared with inland) corresponded with higher physiological cold tolerance in the ants, even though the coastal areas are annually warmer. The coastal spring temperatures also influenced ant phenology so that, in a warm year, the coastal ants began foraging considerably earlier than inland ants, likely due to their lower physiological cold tolerance. Ant community responses also shifted with proximity to the lakes and urban areas, but those changes appeared more linked with land use than climate. These results suggest that species responses to urbanization gradients may be influenced, or even superseded, by the impacts of proximate large water bodies. Our results suggested that spring coldness nearer the Great Lakes may select for cold tolerance in ants (despite that the coastal areas are relatively warmer annually), whereas urbanization selected for greater heat tolerance.