Carrion from large carnivores and food from humans subsidize mesocarnivores year round

TL;DR

The study found that red foxes on Isle Royale rely on human-provided food in summer and wolf-killed carcasses in winter, which may explain why mesocarnivores aren't suppressed by large carnivores.

Contribution

The novel finding is that human and carnivore subsidies sustain mesocarnivores year-round, challenging predictions of suppression by large carnivores.

Findings

Red fox diets in summer are dominated by human-provided food.

Winter diets are specialized and rely heavily on wolf-killed carrion.

Meso-carnivores were not demographically suppressed despite wolf reintroduction.

Abstract

The return of large carnivores is predicted to suppress meso-carnivores, though it has only been observed in a minority of contemporary studies. Isle Royale is a remote island wilderness in Lake Superior, USA, managed for outdoor recreation. Following their natural extirpation, the National Parks Service translocated gray wolves (Canis lupus) in 2018–2019 and we expected the return of suppression and trophic facilitation of meso-carnivores. Nevertheless, we hypothesized that human resource subsidies led to a breakdown in meso-predator release. From Fall 2021-Winter 2024, we captured 16 individual red foxes (Vulpes vulpes), collected whiskers, and subsampled them to recreate the yearly diet of the population using stable isotope analysis. Notably, meso-carnivores were not demographically suppressed. Coinciding with the dates when Isle Royale National Park opened, fox diets were generalized, dissimilar between individuals, and primarily composed of human foods (0.28 ± 0.02) during summer. In winter, fox diets were specialized and exhibited high similarity, being composed primarily of carrion subsidized by wolves (0.62 ± 0.04). We propose that the complementarity of human resource subsidies to communities and ecosystems, broadly, may help explain the limited reach of the anticipated interactions following the return of large carnivores, such as meso-predator release.

The online version contains supplementary material available at 10.1038/s41598-025-15503-w.

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