Reciprocated competition between two forest carnivores drives dietary specialization

Competition shapes animal communities, but the strength of the interaction
varies spatially depending on the availability and aggregation of
resources and competitors. Among carnivores, competition is particularly
pronounced with the strongest interactions between similar species with
intermediate differences in body size. While ecologists have emphasized
interference competition among carnivores based on dominance hierarchies
from body size (smaller = subordinate; larger = dominant), the reciprocity
of exploitative competition from subordinate species has been overlooked
even though efficient exploitation can limit resource availability and
influence foraging. Across North America, fishers (Pekania pennanti) and
martens (Martes spp.) are two phylogenetically related forest carnivores
that exhibit a high degree of overlap in habitat use and diet and differ
in body size by a factor of 2–5x, eliciting particularly strong
interspecific competition. In the Great Lakes region, fishers and martens
occur both allopatrically and sympatrically; where they co-occur, the
numerically dominant species varies spatially. This natural variation in
competitors and environmental conditions enables comparisons to understand
how interference and exploitative competition alter dietary niche overlap
and foraging strategies. We analyzed stable isotopes (δ13C and δ15N) from
317 martens and 132 fishers, as well as dietary items (n = 629) from 20
different genera, to compare niche size and overlap. We then quantified
individual diet specialization and modeled the response to environmental
conditions that were hypothesized to influence individual foraging.
Martens and fishers exhibited high overlap in both available and core
isotopic δ-space, but no overlap of core dietary proportions. When the
competitor was absent or rare both martens and fishers consumed more
smaller-bodied prey. Notably, the dominant fisher switched from being a
specialist of larger to smaller prey in the absence of the subordinate
marten. Environmental context also influenced dietary specialization:
increasing land cover diversity and prey abundance reduced specialization
in martens whereas vegetation productivity increased specialization for
both martens and fishers. Despite an important dominance hierarchy,
fishers adjusted their niche in the face of a subordinate, but superior,
exploitative competitor. These findings highlight the underappreciated
role of the subordinate competitor in shaping the dietary niche of a
dominant competitor.