Coexistence of white footed mice and deer mice may be mediated by fluctuating environmental conditions

White-footed mice, Peromyscus leucopus noveboracensis, and deer mice, P. maniculatus nubiterrae, occur sympatically throughout much of the Appalachian Mountains of the eastern United States. Previous studies have shown that these two species are behaviorally and ecologically similar and do not partition food or microhabitat. In this paper I use a 14-year data set to demonstrate that the two species have differential population growth and survival rates in response to food abundance and season, which may mediate their coexistence. The ratio of white-footed mice to deer mice ranged from 0.5:1 to 6:1. During summer and times of food abundance, white-footed mice gained a numerical advantage over deer mice in 10 of 14 years, whereas following winter and poor food production, deer mice had higher survival in 9 of 13 years. The major decline in white-footed mice, but not deer mice, was associated with poor mast (acorn) production in autumn. Differential survival of deer mice may be due to their physiological adaptations to cold temperature and efficient use of torpor during food shortage. Inter-year variance in food production and climatic conditions appear to change the competitive advantage of each species often enough to permit coexistence. Coexistence of these two ecologically similar species may be maintained by different physiological limitations in a fluctuating environment.