Partitioning resources through the seasons: a test of the competitive ability – cold tolerance trade-off hypothesis in seasonally breeding beetles

Abstract

Understanding the factors that maintain and constrain biodiversity and species coexistence is a major goal in ecology. Closely related species that use similar resources often differ in their seasonal patterns of activity, consistent with resource partitioning that is thought to facilitate coexistence. The factors that limit the distributions of these species across seasons, however, are unknown for the majority of species. In my second chapter, we conducted a large-scale survey of a diverse carrion beetle community in southeastern Ontario, Canada from April to October, and found evidence consistent with resource partitioning in Nicrophorus habitat generalists, but limited evidence for seasonal differences in abundance among habitat specialists. In my third chapter, we test one hypothesis that may explain seasonal differences in activity between two burying beetle species (Nicrophorus sayi, N. orbicollis) that co-occur, require carrion for food and reproduction, and differ in their seasonal patterns of activity. Specifically, we tested predictions of the competitive ability – cold tolerance trade-off hypothesis whereby adaptations to cold temperatures compromise competitive ability under warmer conditions, leading to the partitioning of resources along seasonal gradients. Consistent with our hypothesis, we found evidence that the late-season N. orbicollis is less able to function at the cold temperatures that characterize early spring, when the early-season N. sayi is most active. Contrary to our hypothesis, however, we found that the late-season N. orbicollis was not competitively dominant to the early-season N. sayi under warm conditions, but instead the larger beetle always won, regardless of species. Our survey data suggest that N. orbicollis is usually the larger species when competing for the same carrion later in the season, mostly because of its high abundance during this time. The larger size of N. orbicollis thus presents an intense competitive pressure that should limit N. sayi breeding later in the season. Overall, my thesis research provides evidence for seasonal partitioning of resources and the factors that constrain the seasonal distributions of two species. My work further suggests that seasonal partitioning of resources may contribute to maintaining local diversity within a diverse community of carrion beetles.