Carnivore community reassembly provides a test of Eltonian niche conservatism

The repatriation of species is common, especially in conservation efforts aimed at restoring trophic interactions. Whether the Eltonian niches of restored species are conserved in reassembled ecological communities is largely unknown. Within mammalian carnivores, we hypothesized that the Eltonian niches of sympatric competitors would be structured by trophic facilitation from subsidies provided by large carnivores (i.e. carrion) and humans (i.e. food subsidies). Using stable isotopes (δ 13 C, δ 15 N), we quantified the Eltonian niches of an apex predator, the grey wolf Canis lupus and two subordinate guild members, coyotes Canis latrans and red foxes Vulpes vulpes , from Michigan, USA across three periods that differed in community composition: a ‘historical' carnivore community (1910–1930), a ‘reduced' community which lacked wolves (1950–1970), and a ‘reassembled' carnivore community featuring wolves as well as human food subsidies (2000–2020). In historical and reassembled communities, wolves constrained the niche space of coyotes via resource facilitation, regardless of human presence, such that the consumption of deer by coyotes increased by an average of 1.4–1.8‐fold in the presence of wolves. Without wolves, coyotes exhibited an isotopic (dietary) niche breadth ~ 2–5× greater (3.33 ± 0.74‰ 2 ) than in the historical (0.64 ± 0.11‰ 2 ) and reassembled (1.66 ± 0.63‰ 2 ) communities. Conversely, we found that the niche breadth of foxes was conserved across the historical (1.91 ± 0.41‰ 2 ) and reduced (1.98 ± 0.75‰ 2 ) communities but increased ~ 4‐fold in the presence of human resource subsidies (7.52 ± 1.95‰ 2 ). Notably, we found that trophic facilitation by wolves and resource subsidies from humans altered the foraging strategies of individuals, resulting in disparate foraging strategies for foxes in the reassembled community. Our results highlight that Eltonian niche conservatism in carnivore communities is driven by resource subsidies, both from the provisioning of carrion by large carnivores to meso‐carnivores and from human foods subsidizing small carnivores. More broadly, our work suggests that large carnivore repatriation can restore some species interactions, while human resource subsidies can decouple others.