Competition often occurs between sympatric species that exploit similar ecological niches. Among canids, competition may be reduced by partitioning resources such as food, time, and habitat, but the mechanisms of coexistence remain poorly understood, particularly among fox species. We described the food habits of two foxes that live sympatrically across northern and central Asia, the corsac fox (Vulpes corsac) and red fox (V. vulpes), by analyzing scats collected during a field study in Mongolia. We analyzed 829 corsac and 995 red fox scats collected from April 2005 to August 2007 and tested the extent to which food partitioning occurred. The diets of both species consisted mainly of insects followed by rodents, but also included birds, reptiles, large mammal remains (carrion), plant material (including fruits and seeds), and garbage. Despite high overlap in the proportion of food items consumed, differences existed between species in overall diet with corsacs more frequently consuming beetles, but proportionally fewer crickets and large mammal remains than red foxes. We detected interspecific differences during the pup rearing and dispersal seasons, when prey was abundant, but not during the breeding season, when prey was scarce and diet overlap highest. Each species' diet also differed seasonally and exhibited moderate overall breadth. Corsacs consumed proportionally more beetles and rodents during pup rearing and crickets during dispersal relative to other seasons, whereas red foxes consumed proportionally more crickets during pup rearing and dispersal and more rodents and large mammals during pup rearing and breeding relative to other seasons. Our results suggest that partitioning of food resources during most of the year facilitates coexistence, and that the potential for competition is highest during winter months.

Short CommunicationThe Endangered Siberian marmot Marmota sibirica as a keystone species? Observations and implications of burrow use by corsac foxes Vulpes corsac in Mongolia J a m e s D . M u r d o c h , T s e r e n d o r j M u n k h z u l , S u u r i B u y a n d e l g e r R i c h a r d P . R e a d i n g and C l a u d i o S i l l e r o -Z u b i r i Abstract The Siberian marmot Marmota sibirica is a social, colonial-living rodent that ranges widely throughout northern Asia. In Mongolia the species has declined substantially in recent years due to overharvesting for fur, meat and body parts, used locally and traded illegally in international markets. The Siberian marmot is often considered a keystone species because its burrows appear to represent an important resource for a variety of taxa, including carnivores. However, few studies have quantified marmot burrow use by other species, although such use may be important for developing conservation strategies. We monitored patterns of burrow use by 10 radio-collared corsac foxes Vulpes corsac during a study in Mongolia during May-November 2006. Corsacs used marmot burrows regularly and at rates greater than expected by chance, suggesting that burrows represent an important resource for foxes and supporting the notion of the Siberian marmot as a keystone species. As corsacs are also declining in Mongolia we contend that targeted patrols of marmot colonies in certain areas would provide a costeffective means of protecting both species.

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