From coastlines to mountain crests: Regional and continental variation in North American marten diet and foraging

Abstract
Dietary plasticity is an important trait in an increasingly dynamic world and can reveal how species respond to changes in resource availability, intra- or interspecific interactions, and landscape structure. Small-bodied carnivores occupy a unique trophic position as both consumers and prey while also filling critical roles in species interactions and provisioning of ecological services. Nonetheless, dietary patterns and factors that influence small carnivore foraging are often poorly understood, despite the ecological importance of carnivoran taxa. Here, we examined the diet and foraging of North American martens (Martes spp.), small-bodied mustelids often described as dietary generalists whose foraging patterns are assumed to be influenced by intrinsic and extrinsic factors (e.g., habitat and geography). We used complementary analyses to consider marten diet and foraging at two spatial scales: (1) regionally, using new empirical data collected in the coastal and montane forests of western Oregon and northern California; and (2) continentally, by reviewing and synthesizing previous studies of American (Martes americana) and Pacific martens (Martes caurina) across their North American distributions. Regionally, taxonomic richness and focal prey species varied among sampling sites, while forest cover and edge density had strong, contrasting effects on marten foraging patterns. Continentally, marten diet differed among coastal and interior ecoregions, but foraging did not vary substantially among studies when considering study characteristics. Martens exhibited high dietary plasticity and consumed a remarkable diversity of food items across new and synthesized data, although small mammals and birds were omnipresent prey. A diverse diet and flexible foraging, resulting in facultative variation among and within populations rather than obligate dependence on certain prey taxa, may be an important mechanism by which martens segregate niches with other carnivores or adjust to changing biotic or abiotic conditions. Our findings highlight the need to increase information on sensitive carnivore species, with particular regard to ecological and energetic requirements, to support their continued persistence.