Multidimensional niche advantages reveal how the red fox stands out in mesopredator release within terrestrial island habitats

Global pressures such as climate change and habitat fragmentation have altered apex predator population sizes, spatial distributions, and trophic interactions. Consequently, this has triggered fluctuations and even restructuring of mesopredator populations. During the decades of lack of apex predators in the Helan Mountain of northwestern China, red fox (Vulpes vulpes) became dominant and persisted, leading to a classic ‘mesopredator release’. This study analyzes the niche characteristics of the red fox from temporal, spatial, and dietary dimensions, aiming to elucidate the factors underlying its emergence as the sole dominant carnivore. The results indicate that the red fox exhibits nocturnal activity patterns (β > 0.54), with activity peaks occurring around midnight (21:00–02:00). There was no temporal niche overlap with felids (Δ < 0.5), whereas the highest temporal overlap was observed with the asian badger (Δ > 0.8). Species distribution modeling (SDM) predicted that over half of the Helan Mountains (53.38 %) provides suitable habitat for the red fox, with vegetation and elevation identified as the primary determinants of its distribution. Compared with other mesopredators, the red fox exhibited the broadest vertical distribution range (1400–3500 m). The dietary profile of the red fox is omnivorous, with substantial seasonal variation in food composition; however, both dietary diversity and trophic niche breadth remained consistent across seasons. By characterizing the red fox’s niche across the three dimensions, we discuss the reasons for its prominence during mesopredator release. Furthermore, this study provides direct evidence of the ecological mechanisms driving mesopredator release within terrestrial ‘island’ habitat contexts. It also offers a generalizable predictive framework applicable to other closed, small- to medium-sized protected areas with similar habitat characteristics.