Documenting trophic niche partitioning and resource use within a community is critical to evaluate underlying mechanisms of coexistence, competition or predation. Detailed knowledge about foraging is essential as it may influence the vital rates, which, in turn, can affect trophic relationships between species and population dynamics. The aims of this study were to evaluate resource and trophic niche partitioning in summer/autumn between the endangered Atlantic-Gaspésie caribou (Rangifer tarandus caribou) population, moose (Alces americanus) and their incidental predators, the black bear (Ursus americanus) and coyote (Canis latrans), and to quantify the extent to which these predators consumed caribou. Bayesian isotopic analysis showed a small overlap in trophic niche for the two sympatric ungulates suggesting a low potential for resource competition. Our result also revealed that caribou occupied a larger isotopic niche area than moose, suggesting a greater diversity of resources used by caribou. Not surprisingly, coyote consumed mainly deer (Odocoileus virginianus), moose, snowshoe hare (Lepus americanus), and occasionally caribou, while bears consumed mainly vegetation and, to a lesser extent, moose and caribou. As coyotes and bears also feed on plant species, we documented trophic niche overlap between caribou and their predators, as searching for similar resources can force them to use the same habitats and thus increase the encounter rate and, ultimately, mortality risk for caribou. Although the decline of the Gaspésie caribou population is mostly driven by habitat-mediated predation, we found evidence that the low level of resource competition with moose added to the shared resources with incidental predators, mainly bears, may contribute to jeopardize the recovery of this endangered caribou population. Highlighting the trophic interaction between species is needed to establish efficient conservation and management strategies to insure the persistence of endangered populations. The comparison of trophic niches of species sharing the same habitat or resources is fundamental to evaluate the mechanisms of coexistence or competition and eventually predict the consequences of ecosystem changes in the community.

We captured 44 caribou in the winter of 2013 and 2014 across the Gaspésie caribou range using a net gun fired from a helicopter. We collected caribou hair samples from the rump of the animal, and dried and stored it in paper bags at ambient temperature until processing. We used the same sampling and conservation protocol for hair samples collected from 90 moose, 127 coyote and 57 black bear carcasses across the three Wildlife Reserves that overlap the Gaspésie National Park (Fig. S1). Moose were harvested between September and October 2018 during the sport hunting season whereas coyotes and bears were trapped during the annual predator control program between June 2016 and October 2018 across the Gaspésie caribou range. The capture and manipulation protocols were authorized by the Animal Welfare Committee [Université du Québec à Rimouski (hereafter UQAR) certificate #CPA-52-13-112; Ministère des Forêts, de la Faune et des Parcs (hereafter MFFP) certificate #CPA FAUNE 13-08].

We collected samples opportunistically from all potential food sources consumed by moose, coyotes and bears to describe the composition of their diet. We collected hair samples from 22 individuals belonging to 6 different species that were accidentally trapped during the predator control program, including 4 white-tailed deer (Odocoileus virginianus), 1 snowshoe hare (Lepus americanus), 2 large rodents (groundhog, Marmota monax, North American porcupine, Erethizon dorsatum), 8 Canada lynx (Lynx canadensis) and 7 moose (Alces americanus). We also collected hairs from 22 individuals belonging to 6 species that were harvested by sport trappers or opportunistically collected in the ZEC Casault during summer, including five different species of rodents (1 red squirrel, Tamiasciurus hudsonicus, 2 common voles, Microtus arvalis, 5 deer mice, Peromyscua maniculatus, 6 Northern flying squirrels, Glaucomys sabrinus, and 1 pygmy shrew, Sorex minutus), and one species of large rodent (7 North American beaver, Castor canadensis). We also collected the feathers of 3 ruffed grouses (Bonasa umbellus) and hair from 4 snowshoe hares which were harvested by sport hunters in the ZEC Casault. We dried and stored hair and feathers in paper bags at ambient temperature until processing.

Finally, in the study area in 2017, we collected opportunistically six insects from three species belonging to the coleopteran (1 beetle) and hymenoptera orders (3 ants and 2 wasps). We also collected plant samples in July 2017 belonging to 46 different species in the montane boreal forest area of Mount Albert (n = 93) and Mount Logan (n = 72) in the Gaspésie National Park and of Petit Mount Ste-Anne (n = 53) in the Chic-Chocs Wildlife Reserves (see details in Appendix S1). Plant sampling was authorized by the Société des Établissements de Plein Air du Québec, which manages the Gaspésie National Park and the Chic-Chocs and Matane Wildlife Reserves [certificate # PNG-2017042703]. We randomly collected three replicates per species and froze them at -20°C until they were processed. We freeze-dried insect and plant samples for 48h, ground them into a fine powder using a CryoMill (Jardine et al., 2003), and stored them in a desiccator until the stable isotope analysis.

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