Estimating the risk of contact and disease transmission between bighorn and domestic sheep due to foray movements by bighorn rams

Background/Question/Methods

Bighorn sheep (Ovis canadensis) once numbered an estimated 2 million in North America but have been reduced to fewer than 70,000 animals at present. Frequent outbreaks of bacterial pneumonia among free-ranging bighorn sheep have contributed to both the initial decline and subsequently slow recovery of the species. The bacteria associated with respiratory disease in bighorn sheep typically live as commensals in domestic sheep (Ovis ovis), which are much less susceptible to pneumonia than their wild congeners. Evidence that herd-level outbreaks often follow contact of bighorn sheep with domestic sheep has led most researchers and wildlife managers to conclude that prevention of contact between the two species is essential where maintenance of bighorn sheep populations is a goal. Two aspects of bighorn behavior – their attraction to domestic sheep and the tendency of rams to make long-distance exploratory movements, particularly during the rut – complicate efforts to keep the two species separate.  

We present a method for constructing models of bighorn home ranges and of foray movements outside of those home ranges, developed as part of a process aimed at ensuring viability of bighorn sheep on the Payette National Forest in Idaho. Our objective was to estimate the probability that bighorn sheep would contact active domestic sheep allotments located on the Forest.

Results/Conclusions

Analyses were based on a set of 54,000 telemetry locations, collected between 1997 and 2009 from 470 animals. Separate analyses were carried out for the summer season (May-Oct) and for the winter (Nov-Apr), which includes the November rut period. On average, 14.1% of rams left the home range each summer, and 17.8% each winter. In both summer and winter, >50% of ram forays extended at least 7 km beyond the home range boundary, and >25% extended at least 15 km. We combined estimates of foray distance distributions with an analysis of bighorn sheep habitat preferences to produce maps estimating the relative probability with which each point would be contacted by a bighorn sheep on a foray. These maps, together with estimates of herd size, were in turn used to calculate the annual probability of contact with each domestic sheep allotment, with its attendant risk of disease transmission. Domestic sheep are grazed extensively on public lands in the western US, throughout the current and former range of bighorn sheep. The methods and results of this research have implications for management of the two species across much of that area.