Niche compression intensifies competition between reintroduced American martens (Martes americana) and fishers (Pekania pennanti)

Carnivores exhibit strong interspecific competition and partition niche axes to minimize agonistic interactions. Niche partitioning, though, is contingent upon resource heterogeneity, and recent landscape homogenization may limit the abilities of carnivores to partition niche space. The negative fitness consequences associated with niche overlap may be particularly problematic for repatriating carnivores, and could delay the recovery of rare or endangered species. American martens (Martes americana) and fishers (Pekania pennanti) are the most commonly translocated carnivores in North America and both were reintroduced to a highly modified landscape in Wisconsin, United States. To date, fishers have flourished while martens remain endangered. To assess the role of competition in marten recovery, we used a combination of occupancy modeling, point pattern analyses, and stable isotope analyses to assess 5 coexistence mechanisms: spatial segregation, dietary segregation, temporal avoidance, and differential use of habitat and snow features. Over 7 years, we observed consistently high fisher occupancy and consistently low marten occupancy. Moreover, martens and fishers overlapped in their use of space and time, and neither exhibited habitat preferences. Isotopic analyses revealed complete dietary overlap, with martens falling entirely within the isotopic niche of fishers. Deep, uncompressed snow, however, had a negative effect on fisher activity. We propose that extensive landscape homogenization has resulted in niche compression and that marten recovery has been limited by increased competition with fishers. Restoration programs often overlook competitive interactions, but our results emphasize the importance of interspecific competition for recovering carnivore populations and highlight the challenge of reconstructing carnivore communities in increasingly homogenized landscapes.