Competitive overlap between martens Martes americana and Martes caurina and fishers Pekania pennanti: a rangewide perspective and synthesis

Competition is a major determinant of where species occur and how species interact. Among carnivorans, interspecific competition is particularly apparent, as many of these species have evolved to be efficient killers. Theoretically, phylogenetically related carnivorans that occupy seasonal habitats, share common resources, and differ in body size by a factor of 2.5-10x should exhibit the most interference competition. Fishers Pekania pennanti and martens Martes americana and Martes caurina are members of the subfamily Guloninae (Mustelidae, Carnivora) that occupy forests throughout northern North America. These taxa occur sympatrically throughout much of their range, utilise similar habitats, and consume similar prey; fishers and martens also differ in body size by a factor 2-5x. Consequently, these two taxa appear to be locked in particularly strong interspecific competition and should attempt to limit competitive overlap. We review the current knowledge of this dyadic interaction in the framework of ecological niches and niche partitioning. In particular, we explore the three critical niche axes of diet, space, and time. We found that, in contrast to the traditional view of them being highly specialised, both martens and fishers are dietary generalists; however, they also appear to be specialists in complexity, at least in space and habitats. Collectively, martens and fishers exhibit high degrees of diet and habitat niche overlap across their ranges, and this overlap is likely to have the greatest fitness consequences for the smaller and subordinate martens. Nevertheless, fine-scale habitat and prey partitioning, and especially partitioning along snow clines, seem to be the mechanisms by which these two taxa can coexist. We predict that rapid ecological change - especially from increasingly homogenised forests and prey communities, as well as from declining snow cover and snowpack due to climate warming - is likely to destabilise marten-fisher coexistence. As the climate continues to change, fishers and martens are likely to experience distributional and numerical shifts and increased isolation at their southern range boundaries, and vulnerable populations - especially of martens - will be driven to local extirpation.