ECOLOGICAL RELATIONSHIPS BETWEEN MULE DEER AND WHITE-TAILED DEER IN SOUTHEASTERN ARIZONA

Niche relationships between the desert mule deer (Odocoileus hemionus crooki) and Coues white-tailed deer (Odocoileus virginianus couesi) were studied in the San Cayetano and Dos Cabezas mountains in southeastern Arizona from September 1969 to March 1972. Walk transects were established on the study areas on which deer were observed. Altitude, slope exposure, and habitat selection were used to describe distributional patterns of the two species. Food habits were studied by analyzing fecal samples microscopically for epidermal fragments of important forage plants. Behav- ioral interaction between mule deer and white-tailed deer, browse utilization on Cercocarpos brevi- florus, and recruitment were also investigated. Spatial distributions according to altitude and slope exposure indicated that a "buffer" zone existed between populations of mule deer and white-tailed deer in the San Cayetano Mountains. Mule deer were associated with fewer vegetational types than white-tailed deer on this study area; Prosopis juliflora and Fouquieria splendens were indicative of mule deer habitat. White-tailed deer utilized a greater diversity of vegetational types in the San Cayetanos and did not appear to be associated with any particular plant species, although Quercus oblongifolia, Dasylirion wheeleri, and Eysenhardtia polystachya were important. The most important forage plants for white-tailed deer in the San Cayetano Mountains were E. polystachya, Eriogonum wrightii, Krameria parvifolia, and Fendlera rupicola. Similarly, the primary food items of mule deer were E. polystachya, Calliandra eriophylla, and K. parvifolia. There was much overlap in food preferences of mule deer and white-tailed deer (55% to 67% seasonally) particularly on E. polystachya and K. parvifolia. Populations of mule deer and white-tailed deer were sympatric in the Dos Cabezas Mountains; the coefficients of association were highly positive. There was also a great similarity in habitat selection between the two species. Both species were associated with those habitat types dominated by Quercus emoryi, Juniperus deppeana, C. breviflorus, Quercus gambelil, Nolina microcarpa, Symphoricarpos ereophilus, and/or Bouteloua spp. White-tailed deer displayed a greater preference for the Quercus- Symphoricarpos and Quercus-Cercocarpus habitat types than mule deer, whereas Juniperus- Cercocarpus habitats were preferred more by mule deer. Food habits of mule deer and white-tailed deer were very similar; all plant species important to white-tailed deer were also important to mule deer. The most frequent forage plants, in order of importance, were C. breviflorus, E. wrightii, J. deppeana, and Q. gambelii. Recruitment to the population was higher for mule deer than for white- tailed deer. Utilization of the annual leader growth on the key browse plant (C. breviflorus) was extremely high. Mule deer were dominant over white-tailed deer in all behavioral interactions observed during the study. In half of these instances, mule deer displayed aggressive threat in asserting their dominance; the rest were of a passive nature. The low coefficients of distributional overlap and high indices of overlap in food habits for the San Cayetano Mountains lead us to conclude that mule deer and white-tailed deer competitively exclude each other. The high overlap in spatial distributions, habitat selection, and food habits coupled with the high utilization of the key forage plant (C. breviflorus) in the Dos Cabezas Mountains during the winter months lead to the conclusion that direct competition between the two species probably was occurring during that time. This direct competition can be explained on the basis that it is a temporary phenomenon. It is hypothesized that the factors that have brought mule deer and white-tailed deer into direct competition in this area have been vegetational changes, livestock overgrazing and/or range fire suppression.