Effects of disease, dispersal, and area on bighorn sheep restoration

We simulated population dynamics of bighorn sheep (Ovis canadensis) inhabit ing six discrete habitat patches in the Badlands ecosystem, South Dakota. M odeled populations were subjected to a range of potential management action s and rates of disease-causing infection. Simulated disease varied in sever ity from mild (similar to 12% mortality) to severe (similar to 67% mortalit y), with infections imposed once, at regular intervals, or with a fixed pro bability each year. In the absence of disease, 200-year extinction rates we re uniformly low and insensitive to changes in colonization rate or area of suitable habitat. A single infection, accompanied by change in the area of suitable habitat or colonization rate, resulted in extinction rates of up to 40%, and large changes in average population size (up to 10-fold with ch anges in area; 4-fold with changes in colonization rate). Simulations with multiple infections, which are probably most realistic, generally resulted in extinction rates that exceeded 20% over a 200-year period. Model results clearly showed that efforts directed toward reducing the frequency or seve rity of disease are of highest priority for improving the success of attemp ts to restore bighorn sheep populations. Increases in areas of suitable hab itat or improvements to corridors between existing habitat patches were far less likely to improve persistence of simulated sheep populations than red uctions in the impact of disease. Although theory predicts that enhanced mo vements may exacerbate effects of disease, increased colonization rates res ulted in relatively small but consistent increases in persistence and avera ge population size for all combinations of parameters we examined.