Philip J. Manlick, Shelby M. Petersen, Katie M. Moriarty and Jonathan N. Pauli

What an animal eats often determines its role in an ecosystem. This is especially true for predators who can limit the numbers of their prey through consumption. For example, snakes eat mice and wolves eat deer, thereby limiting the abundance of herbivores, with potential cascading effects on plants. Scientists often assume that these roles are fixed, so the reintroduction of predators is increasingly promoted as a means to re-establish lost predator-prey interactions. Variable foraging, however, could undermine such proposals. Moreover, it’s been hypothesized that closely related species exhibit similar diets because of shared traits, but this is also not well-documented.

To assess potential variation in predator-prey interactions, we quantified the diets of two closely related forest carnivores, American and Pacific martens, across a range of environments in the Pacific Northwest. We used naturally varying stable isotopes to compare marten diets at four paired sites – two island and two mainland – that varied in habitat and number of competitors. You are what you eat, at least isotopically, so we used stable isotopes signatures in marten hair to estimate the proportion of berries, marine prey (e.g., salmon), and terrestrial vertebrates (e.g., mice, birds) that were eaten across sites. We found that marten diets varied widely, with significant differences among all populations. Mainland martens ate mostly terrestrial vertebrates, while island martens ate all three prey types equally, likely due to limited competition for food (i.e., less carnivores) on these remote islands. Conversely, we found that American and Pacific marten diets were more similar to their own species than to their relatives, suggesting that genetics may limit diet variability.

These results highlight the dynamic nature of foraging, even among closely related species at similar sites, and this variation could also have consequences for the roles animals play in ecosystems. For instance, island martens eating more berries or salmon likely play important roles in dispersing seeds and nutrients, while mainland martens eating small vertebrates likely have a more classical role as terrestrial predators. As we continue to reintroduce species to restore their impacts on ecosystems, the drivers of foraging variation should be considered.

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