Low individual diet variation and high trophic niche overlap between the native polecat and invasive American mink

Introductions of invasive species increase competitive interactions in a community proportionally to the similarity between the ecological niches of invasive and native species. Food represents one of the most important resources influencing competition, and differences in the trophic niche of native and invasive species can affect their ability to coexist. Moreover, in sympatric native and invasive species of similar body size and high sexual dimorphism, competition for food between individuals of the same sex may be higher than intraspecific competition between males and females, thus making the overall competitive interactions more complex. The aim of this study was to investigate the intraspecific and intrasexual competition for food resources between two sexually dimorphic mustelids, the native polecat Mustela putorius and invasive American mink Neovison vison, in riparian habitats of Bialowie(z) over dota Forest (Poland). Based on an analysis of 1215 scats, we studied the food habits of eight polecat and 24 mink individuals. The trophic niches of the polecat and mink were narrow and overlapped considerably. The diet of both predators was dominated by amphibians (mainly the common frog Rana temporaria), which comprised up to 94.1% and 89.7% of polecat and mink prey biomass, respectively. Individual diet variation was low within each sex group of both species. Interspecific similarities in body mass of the sexes and intraspecific differences in body mass between the sexes did not affect pairwise niche overlaps. However, we observed some mechanisms of food segregation between the sexes and species that reduced intra- and interspecies competition. Polecat males hunted more common toads Bufo bufo than polecat females, mink males and mink females. They also hunted larger frogs than polecat females, and the opposite pattern was recorded in mink. We conclude that two predator species exploiting abundant prey can coexist despite a very high overlap in their food niches.