NICHE RELATIONSHIPS WITHIN A GUILD OF UNGULATE SPECIES IN YELLOWSTONE-NATIONAL-PARK, WYOMING, FOLLOWING RELEASE FROM ARTIFICIAL CONTROLS

Niche relationships and diet overlaps were compared among elk (Cervus elaphus), bison (Bison bison), bighorn sheep (Ovis canadensis), mule d eer (Odocoileus hemionus), and pronghorn antelope (Antilocapra america na) between 1967-1970 and 1986-1988, a period when total ungulate numb ers nearly tripled on Yellowstone's northern range. Ungulate species r atios on Yellowstone's northern winter range during the latter period were 100 elk : 10 mule deer : 3 bison : 2 pronghorns : 1 bighorns. Elk numbers were positively correlated to bison, mule deer, and pronghorn numbers (r(2) = 0.76, 0.97, and 0.48, respectively, P < 0.01). Few ot her changes in habitat use or habitat overlap occurred, and diets for only 2 of the 10 species pairs, elk-bighorn (Spearman's rank order coe fficient (RHO) = 0.55, P < 0.05) and mule deer - pronghorn (RHO = 0.64 , P < 0.05), were significantly associated with each other. Bison cons umed more grass and fewer sedges, mule deer more fringed sage (Artemis ia frigida) and more rabbit-brush (Chrysothamnus spp.), and bighorn sh eep more grasses and fewer sedges, while pronghorns ate less saltsage (Atriplex nuttalli) but more big sagebrush (Artemisia tridentata) duri ng 1986-1988 than during 1967-1970. Bison expanded their range and bis on and bighorn sheep used a wider variety of habitats. We found little evidence of change in competitive interactions between ungulate speci es. A few diet and habitat overlaps increased, the opposite of the pre diction from the competitive exclusion principle amongst species, sugg esting that intraspecific competition was more important. Several expl anations are proposed for the lack of changes in niche relationships d uring a period of near tripling in density of the ungulate guild.