Desert bighorn sheep forage relationships in the Virgin Mountains, Arizona.

Abstract

Twelve desert bighorn sheep (Ovis canadensis nelsoni) were captured 12-14 November 1979 in the Black Mountains, western Arizona, and translocated to a 283 ha enclosure in the Virgin Mountains, northwestern Arizona. I studied habitat and foraging relationships of the population from November 1979 to December 1981. My objectives were to investigate productivity, group characteristics and habitat use, intraspecific comparisons of diet, diet overlap with cattle, forage availability and use, diet diversity, and plant quality. Seven and 8 females in the enclosure had lambs in 1980 and 1981, respectively. Mean group size was largest in spring, coincident with the peak of lambing. The number of mixed sex groups decreased in spring and summer when adult females did not associate with adult males. The number of mixed sex groups were highest in fall and winter due to rut and post-rut aggregations. The 4 vegetation associations in the enclosure were used in proportion to availability except by adult females and lambs in spring-summer 1981. Grasses, forbs, and browse were equally important in the annual diets of bighorn sheep; the use of succulents was secondary. Browse and grasses comprised most of the cattle diet (45.4% and 40.1%, respectively) followed by forbs (13.1%). Intraspecific differences in bighorn sheep diets were not significant. Bighorn sheep and cattle diets did not overlap significantly and bighorn sheep diets were more diverse. Bighorn sheep did not eat 8 plant species in proportion to their occurrence in the enclosure. Habitat conditions and behavior patterns of bighorn sheep in the enclosure were similar to free-ranging populations. However, range conditions in the enclosure were excellent, predators were controlled, and potential competitors were excluded. The reproductive potential of desert bighorn sheep was obtained. Although I confirmed a relationship between bighorn sheep diet and plant nutrition, no intraspecific differences in seasonal nutrition requirements were established. In addition, dietary overlap between bighorn sheep and cattle was not significant; these data have important management implications for future bighorn sheep introductions onto traditional livestock grazing areas.