Matthew J. Ferrari

Brucella abortus, a bacterium which causes disease in livestock, wildlife, and humans, has become the target of a large scale eradication program in the wild species of the Greater Yellowstone area. An understanding of the spatial and temporal associations of bison (Bison bison) and elk (Cervus elaphus), the two major vertebrate hosts, is essential in assessing the risk of inter-specific transmission and the long term efficacy of proposed management scenarios to eradicate the disease. The Madison-Firehole drainage of Yellowstone National Park supports high densities of elk and bison during the winter and spring when B. abortus can be shed by females through birth or abortion. I utilized 4,526 telemetry locations of cow elk collected between 1991-1998 and conducted 30 ground censuses of bison between 1997-1998 to assess the distribution and spatial and temporal associations of elk and bison on the winter range. The Madison-Firehole bison winter range is entirely contained within the winter range used by elk. Elk and bison, which normally display significant spatial separation, were found to have high levels of association on the winter range. Increasing snow pack increased the density of bison on the winter range as bison moved into the Madison-Firehole from the Hayden Valley summer range, and deep snow restricted elk to the valley bottoms. Range overlap varied between 53 and 76% (ANOVA, P = 0.09) and tended to increase from December to May. The percent of radiolocations in which instrumented cow elk were =< 100m of bison was 13-30% between April and May, 1991-1998, the peak time of bison calving. Regression analysis indicated that snow water equivalent, a measure of snow depth and density, was positively correlated with elk/bison association and was the strongest predictor of association (P < 0.0001). Despite close association between the two species, a sample of 73 adult cow elk indicated that the prevalence of sero-positive animals in the Madison-Firehole was not significantly different (P > 0.05) from other elk populations that do not associate closely with bison. However the sero-prevalence in the Madison-Firehole was lower (P < 0.05) than the sero-prevalence in populations associated with winter feeding operations. Thus I conclude that the close contact between bison and elk during the winter and spring does not result in increased levels of disease exposure in elk.