Competition and intraguild predation among three sympatric carnivores

We examined the relative roles of dominance in agonistic interactions and e nergetic constraints related to body size in determining local abundances o f coyotes (Canis latrans, 8-20 kg), gray foxes (Urocyon cinereoargenteus, 3 -5 kg) and bobcats (Felis rufus, 5-15 kg) at three study sites thereafter r eferred to as NP, CP, and SP) in the Santa Monica Mountains of California. We hypothesized that the largest and behaviorally dominant species, the coy ote, would exploit a wider range of resources (i.e., a higher number of hab itat and/or food types) and, consequently, would occur in higher density th an the other two carnivores. We evaluated our hypotheses by quantifying the ir diets, food overlap, habitat-specific abundances, as well as their overa ll relative abundance at the three study sites. We identified behavioral do minance of coyotes over foxes and bobcats in Santa Monica because 7 of 12 r ecorded gray fox deaths and 2 of 5 recorded bobcat deaths were due to coyot e predation, and no coyotes died as a result of their interactions with bob cats or foxes. Coyotes and bobcats were present in a variety of habitats ty pes (8 out of 9), including both open and brushy habitats, whereas gray fox es were chiefly restricted to brushy habitats. There was a negative relatio nship between the abundances of coyotes and gray foxes (P=0.020) across hab itats, suggesting that foxes avoided habitats of high coyote predation risk . Coyote abundance was low in NP, high in CP, and intermediate in SP. Bobca t abundance changed little across study sites, and gray foxes were very abu ndant in NP, absent in CP, and scarce in SP; this suggests a negative relat ionship between coyote and fox abundances across study sites, as well. Bobc ats were solely carnivorous, relying on small mammals (lagomorphs and roden ts) throughout the year and at all three sites. Coyotes and gray foxes also relied on small mammals year-round at all sites, though they also ate sign ificant amounts of fruit. Though there were strong overall interspecific di fferences in food habits of carnivores (P<0.0001), average seasonal food ov erlaps were high due to the importance of small mammals in all carnivore di ets [bobcat-gray fox: 0.79+/-0.09 (SD), n=4; bobcat-coyote: 0.69+/-0.16, n= 6; coyote-gray fox: 0.52+/-0.05, n=4]. As hypothesized, coyotes used more f ood types and more habitat types than did bobcats and gray foxes and, overa ll, coyotes were the most abundant of the three species and ranged more wid ely than did gray foxes. We propose that coyotes limit the number and distr ibution of gray foxes in Santa Monica Mountains, and that those two carnivo res exemplified a case in which the relationship between their body size an d local abundance is governed by competitive dominance of the largest speci es rather than by energetic equivalences. However, in the case of the inter mediate-sized bobcat no such a pattern emerged, likely due to rarity or inc onsistency of agonistic interactions and/or behavioral avoidance of encount ers by subordinate species.