Do Snow-Adapted Prey Facilitate Coexistence of the Sierra Nevada Red Fox With Sympatric Carnivores?

Abstract

Specialist species in alpine ecosystems may be increasingly threatened by climate-driven habitat loss and encroachment by generalist competitors. Ecological theory predicts that niche differentiation through dietary specialisation can facilitate coexistence with generalist competitors. We quantified dietary overlap between a high-elevation specialist, the Sierra Nevada red fox (SNRF; Vulpes vulpes necator) and a widespread generalist, the coyote (Canis latrans), as well as other sympatric carnivores. We were especially interested in dietary items that were themselves specialised to alpine habitats, as we expected them to be most critical to SNRF. To characterise diet, we used DNA metabarcoding for vertebrate and plant-based food items of 789 carnivore scats collected from the sites of two SNRF populations (Lassen, Sierra Nevada). As expected for potential competitors, SNRFs exhibited substantial dietary overlap with coyotes overall. Dietary niche overlap was lower between SNRF and both bobcats (Lynx rufus) and martens (Martes caurina). Compared to coyotes, however, SNRF more frequently consumed snow-adapted prey, including white-tailed jackrabbits (Lepus townsendii) and American pika (Ochotona princeps) (SIMPER p ≤ 0.005), especially during periods of deep snow. Whitebark pine (Pinus albicaulis; presumably seeds) also appeared more regularly in SNRF winter diets compared to coyotes. These findings support the hypothesis that co-adapted subalpine prey facilitate coexistence between specialist and generalist carnivores by increasing the competitive advantage of specialists under snowier conditions. This environment-mediated shift in competitive dynamics implies that the fates of locally adapted predator and prey may be tightly linked, an important consideration for conservation planning in alpine ecosystems.

Keywords: Sierra Nevada red fox; climate change; competition; coyote; diet; metabarcoding.

FIGURE 1

Lassen and Sierra Nevada mesocarnivore diet study areas, along with locations of 925 mesocarnivore scats (brown circles) collected in California, USA. The Lassen area included 133 Sierra Nevada red fox (SNRF) scats collected from 2011 to 2015 in Lassen National Park and Lassen National Forest. The Sierra Nevada area included 317 SNRF, 329 coyote, 88 marten and 58 bobcat scats collected from 2011 to 2017 north of Yosemite National Park in the Stanislaus and Humboldt‐Toiyabe National Forests.

FIGURE 2

Dietary composition in terms of estimated proportional biomass contributions of prey calculated as frequency of occurrence multiplied by a correction factor (Table S2) for mesopredators including 103 Sierra Nevada red fox (SNRF) scats from Lassen from 2011 to 2015 and 296 SNRF, 279 coyote, 61 marten and 50 bobcat scats from the Sierra Nevada from 2011–2017. Rings from inner to outer represents taxonomic class, order, family (or subfamily) and species (or lowest known operational taxonomic unit). For convenience, common names and abbreviations were used at the species level and for some higher‐order taxa, for example, to indicate Arvicolinae (voles), Mammalia (mammal), Aves (bird), Rodentia (rodent) or white‐tailed jackrabbits (WTJR). Complete taxonomic information for prey items is provided in Table S2.

FIGURE 3

Estimated proportional biomass contributions of prey species to the diets of Sierra Nevada red fox (SNRF) and coyote prey based on frequency of occurrence in scats multiplied by a prey body‐size correction factor (see Table S2), illustrating that most prey species constitute similar proportions of the diets of both canids, with exceptions noted in blue and red symbols. Red symbols indicate prey species with specialised adaptations to the subalpine zone of the Sierra Nevada on which SNRF may be especially well‐adapted to capture; blue symbols represent prey species likely to differ between carnivore diets due to body sizes. Pocket gophers contributed the greatest biomass to the diets of both carnivores. Data were based on 296 SNRF and 279 coyote scats from the Sierra Nevada from 2011 to 2017.

FIGURE 4

Seasonal frequency of occurrence of (A) white‐tailed jackrabbit and (B) pine for Sierra Nevada red fox (SNRF) and coyote scats in the Sierra Nevada from 2011 to 2017. Numbers of scats analysed for vertebrates were 23 and 52 winter, 41 and 29 spring, 130 and 94 summer and 100 and 101 fall SNRF and coyote scats, respectively. Numbers of scats analysed successfully for plant dietary items were 26 and 47 winter, 38 and 30 spring, 119 and 85 summer and 94 and 77 fall SNRF and coyote scats, respectively. Bars in both panels denote standard errors.

FIGURE 5

Annual frequency of occurrence of (A, B) whitetail‐jackrabbit and (C) pine in relation to annual snowpack for 293 Sierra Nevada red fox (SNRF) and 262 coyote scats in the Sierra Nevada from 2012 to 2017. Bars in all panels denote standard errors. Snowpack was derived from the California Department of Water Resources (https://cdec.water.ca.gov/reportapp/javareports?name=DLYSWEQ.20180401).