COS 109-6
Can competitors coexist in the Anthropocene? Niche compression enhances competition between two reintroduced carnivores
Carnivores exert strong interspecific effects and partition niche axes to minimize competitive interactions. However, niche partitioning is often contingent upon resource heterogeneity, and current trends in landscape homogenization can dramatically alter realized niche axes. These altered landscapes are often targeted for species restoration, and reintroductions are an increasingly important tool for carnivore recovery. However, interspecific competition is routinely overlooked when reconstructing historic communities, and if competing carnivores are unable to partition the available niche space, coexistence can only be attained through spatial segregation or fitness consequences that reduce competition for shared resources. These mechanisms are likely problematic for recovering carnivores and could delay restoration efforts for many rare or endangered species.
American martens and fishers are the two most commonly reintroduced carnivores in North America, but they are also direct competitors that exhibit strong interference competition. Both species have been reintroduced to a highly modified landscape in Wisconsin, USA, and while fishers have flourished, martens remain a state endangered species. We assessed the role of competition in limiting marten recovery and tested four general carnivore coexistence hypotheses – 1) spatial niche partitioning; 2) temporal niche partitioning; 3) differential habitat selection; and 4) dietary segregation – using occupancy models and stable isotope analyses
Results/Conclusions
Over seven years, Martens and fishers exhibited a stable relationship with high fisher occupancy (x̄ = 0.86) and low marten occupancy (x̄ = 0.37). Contrary to previous research, they did not exhibit spatiotemporal segregation or differential habitat selection, and aggregated in both space and time. However, fishers responded negatively to snow cover, suggesting this interaction could be mediated by snow conditions. The two species also exhibited substantial dietary overlap with martens falling 99.99% within the isotopic niche of fishers. We propose that this unique lack of niche differentiation is ultimately limiting the recovery of martens via enhanced competition, and hypothesize that this mechanism is a function of Wisconsin’s homogenized landscape where historically important niche axes such as hemlock forest have decreased by 79%, total coniferous forests by 57%, deciduous forests by 40%, small mammals by 56%, and snow cover by 10%.