Between Rabbits and Rubbish: introduced prey dominates Andean fox diet and differs from free-ranging dogs in peri-urban Santiago city (Chile)

Ecological theory predicts that the coexistence of sympatric carnivores depends on how they partition limited resources and mitigate intraguild competition. In human-modified landscapes, altered prey availability and anthropogenic food subsidies can blur natural trophic boundaries, reshaping species interactions. We examined dietary relationships between the Andean fox (Lycalopex culpaeus) and free-ranging domestic dogs (Canis lupus familiaris) in a peri-urban landscape of central Chile, combining stable isotope analysis with Bayesian niche modeling to assess resource use and trophic overlap. Mean isotopic values for foxes (δ¹³C = –22.6 ± 1.0‰; δ¹⁵N = 6.1 ± 1.1‰) did not differ across sex or age classes, indicating uniform diets within the population. Baysian isotope mixing models revealed that introduced European wild rabbit (Oryctolagus cuniculus) dominated the fox diet (≈ 61% of assimilated biomass), with small mammals as secondary prey and minimal reliance on livestock or anthropogenic foods. Free-ranging dogs exhibited a narrower isotopic niche and distinct isotopic signatures, consistent with greater dependence on human-derived resources. Standard ellipse analyses showed negligible niche overlap (0% at 40% core; ~34% at 95% level), supporting strong trophic segregation between the species. This study reinforces key ecological theories of coexistence and intraguild competition, demonstrating that sympatric carnivores can partition trophic niches even under significant human influence. Moreover, it highlights the pivotal role of the introduced European rabbit as an abundant, cross-subsidizing prey shaping trophic dynamics in Chilean carnivore communities.