Closely related species that use similar resources often differ in their seasonal patterns of activity, but the factors that limit their distributions across seasons are unknown for most species. One hypothesis to explain seasonal variation in the distributions of species involves a trade-off between competitive ability and cold tolerance, where tolerance to the cold compromises competitive ability in warmer (benign) temperatures, either at the level of the individual or population.
We tested both individual-level and population-level mechanisms of this hypothesis in two co-occurring species of temperate burying beetles (Silphidae: Nicrophorus sayi, N. orbicollis) that differ in their seasonal patterns of activity.
We measured cold tolerance, breeding activity as a function of temperature, and competitive ability as a function of temperature and season.
Consistent with our hypothesis, the mid-season N. orbicollis was less able to function at the cold temperatures that characterize early spring, when the early-season N. sayi is most active. The larger beetle, however, always won one-on-one competitive trials at warm temperatures, regardless of species, inconsistent with an individual-level trade-off. N. orbicollis was usually larger and successful when competing for the same carrion later in the season, mostly because of its larger population size, consistent with a trade-off between competitive ability and cold tolerance acting at the population level.
Our findings suggest that cold temperatures limit the mid-season N. orbicollis from earlier spring emergence, while competitive pressure from the more abundant, larger N. orbicollis constrains the early-season N. sayi from remaining active through the summer.