Ecology of Red Foxes and Niche Relationships with Coyotes on Mount Desert Island, Maine

Fuller, Angela; Harrison, Daniel

A description of the dietary overlap between red foxes and coyotes. Conclusions: 'Coyotes and foxes on Mount Desert Island exhibited high levels of resource overlap. Colwell and Futuyma (1971) suggested that the only way to truly demonstrate interspecific competition is to compare the niche overlap of the species under natural conditions (actual) compared to the niche overlap in the absence of the species putative competitor (virtual). If the actual and virtual overlaps are both zero, or they do not change, then there is no evidence for competition at that time and place. Because our study began during a period when it was unknown if a resident population of coyotes existed on the island, we were unable to sample the fox population prior to the invasion by coyotes. Based on our results of high dietary overlap, and spatial avoidance of foxes by coyotes, we suggest that there is evidence to suggest both exploitative and interference competition between the two species. Increasing coyote density has been suggested as a reason for decreased density of foxes (Robinson 1961, Linhart and Robinson 1972, Voight and Earle 1983, Sargeant et al. 1987, Harrison et al. 1989, Sovada, et al. 1995). We were unable to assess whether density of foxes decreased after colonization of coyotes, but we were able to document evidence of interspecific competition. Although we did not have a complete census of foxes on MDI, it appears that foxes position their territories surrounding coyote territories to avoid competition. Additionally, foxes have adapted to the recent colonization of coyotes by positioning territories in areas of higher elevation to avoid competition with coyotes. Coexistence is likely possible because potential competition is reduced via niche partitioning. We documented that both foxes and coyotes consume deer fawns during summer, and deer fawns may be a limiting resource when both coyote pups and fox pups are weaning, especially given that deer populations on MDI are declining (Fuller and Harrison 2003). Species with similar diets may diverge in resource use and hence overlap when food becomes scarce to reduce interspecific competition (Lack 1946, DuBowy 1988). If deer populations in MDI continue to decline, competition for fawns will increase, probably resulting in foxes having to depend more heavily on alternative prey items during pup weaning times. If deer decline to levels that force coyotes to become more dependant on small mammals, the result could be rescued deer for foxes and hence greater competition for small mammals with possible declines in density of foxes.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/