Graduate Thesis Or Dissertation
 

Ecological relationships between Columbian white-tailed and black-tailed deer in southwest Oregon

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  • I examined ecological relationships and mechanisms of coexistence for sympatric populations of Columbian white-tailed (Odocoileus virginianus leucurus) and black-tailed deer (Odocoileus hemionus columbianus) in Douglas County, Oregon, from September 1997 to August 1998. Horseback transects were used to describe spatial distributions, population overlap, and habitat use for both species. Behavioral observations were conducted to examine intraspecific and interspecific social interactions. Diets were studied with microhistological analysis, and fecal nitrogen and neutral detergent fiber in fecal samples were used as indices of diet quality. Interspecific differences in foraging micro-site use and selection were investigated using vegetative surveys. Distribution patterns indicated that white-tailed and black-tailed deer maintained a degree spatial separation during most seasons, with spatial overlap ranging from 5 to 40% seasonally. White-tailed deer were more concentrated and tended to occur in the southern portions of the study area, which was characterized by lower elevations, more gradual slopes, and closer proximity to streams. Black-tailed deer were more wide ranging and tended to occur in the northern portions of the study area, which had higher elevations and greater topological relief. Coefficients of species association were negative suggesting that the species maintained spatial separation through mutual avoidance. In regions of population overlap, the species maintained separation by choosing different micro-sites in which to forage. Habitat use patterns were similar seasonally between white-tailed and black-tailed deer with overlap ranging from 89 to 96%. White-tailed deer used nearly all habitats available on the study area except those associated with conifers. They used oak-hardwood savanna shrub, open grassland, oak-hardwood savanna and riparian habitats the most. Black-tailed deer exhibited high use for open grassland and oak-hardwood savanna shrub habitats and lower use of all others. Columbian white-tailed and black-tailed deer exhibited strong seasonal similarities in diets with overlap ranging from 89 to 95%. White-tailed deer diets were dominated by forbs, shrubs, grasses, and other food sources (e.g., nuts, lichens). Columbian black-tailed deer diets were dominated mostly by forbs and other food sources. Seasonal diet diversity followed similar patterns for both species with the most diverse diets occurring in the fall and the least diverse diets in the spring. Detailed observation of behavioral interactions among white-tailed and black-tailed deer groups revealed that intraspecific interactions were more likely than interspecific interactions. Interactions among white-tailed deer groups were equally likely to be passive or active, while those among black-tailed groups were more likely to be passive. Interspecific interactions between white-tailed and black-tailed deer were infrequent. When they did occur, little aggression was observed and evidence of consistent dominance by either species was lacking. High overlap in habitat use and diets resulted in high trophic overlap (81 to 85% seasonally) between white-tailed and black-tailed deer; however, the low spatial overlap reduced the potential for exploitative competition between the species. High habitat heterogeneity on the study area created diverse niche characteristics that allowed white-tailed and black-tailed deer to have strong similarities in diets and habitat use, while coexisting. I hypothesized that the two species were competitively excluding each other.
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