Factors influencing survival and reproduction of Virginia opossums (Didelphis virginiana) at their northern distributional limit
To understand how species' distributions vary with landscape and climatic changes, we must first understand the proximate mechanisms responsible for distributional boundaries. A direct physiological link between the Virginia opossum's (Didelphis virginiana) winter energetics and its northern geographical range limit has been hypothesized. However, opossums now occur well beyond the predicted limit. Though opossums commonly occur in the Connecticut River Valley of central Massachusetts, I found in a road-kill survey that opossums are rare in undeveloped areas. These findings suggest that the region is a local distributional edge. Demographic models indicated that northern population persistence depends upon high (0.67–0.81) over-winter survival of subadult females. Revisiting winter energetics, I found that average-weight subadult females should not, under natural conditions, survive most Amherst, Massachusetts winters at these rates. This model explains the failure of opossums in natural areas but not their presence in developed areas. I compared winter temperatures across the region; while urbanized areas had the warmest winter nights (and hence the environment in which opossums have the highest chance of survival), urban temperatures should still have been too cold for sufficient subadult female survival. Radio-monitoring free-ranging opossums, I found that use of anthropogenic resources allowed subadult females to obtain the necessary extra energy required to survive winter. The actual winter survival estimate for subadult urban female opossums was 0.697. This survival rate was high enough to contribute to a growing urban population, because both reproduction and juvenile pre-winter survival were higher than expected. “Rural” animals, which did not use anthropogenic resources, did not persist through the winter. Many individuals shifted to using urban resources before the onset of winter, and the remainder died, primarily of predation in autumn or starvation in winter. Though the demographics for the total sample population indicate a declining population, the opossums in Massachusetts actually could be in a stable “source-sink” dynamic, with populations in urbanized areas growing and supporting declining populations in rural areas. By exploiting anthropogenic resources, Virginia opossums already occur in northeastern North America where they otherwise would not persist, and further northward expansion through an increasingly urbanized landscape is thus expected.