Incorporating foray behavior into models estimating contact risk between bighorn sheep and areas occupied by domestic sheep.

摘要

Respiratory disease epidemics are considered a primary factor limiting bighorn sheep (Ovis canadensis) populations in most of the western United States. Diverse sources of evidence indicate that domestic sheep harbor pathogens that cause fatal pneumonia in bighorn sheep, and that contact between the species often leads to respiratory disease and mortality in wild bighorn sheep populations. Managers have long argued that maintaining species separation is essential to conserving bighorn sheep. However, few studies have quantified probability of interspecies contact under different separation-based management scenarios. We present an approach to estimating relative risk of interspecies contact that uses telemetry data to characterize bighorn sheep home ranges, habitat selection, and the occasional long-distance exploratory movements (forays) characteristic of the species. We developed this approach to assess probability of bighorn sheep entering domestic sheep allotments on the Payette National Forest in central Idaho, USA, and we present that analysis as a case study. First, a large telemetry data set was used to identify bighorn sheep core herd home ranges and foray characteristics, including frequency, distance traveled, and habitat selection. Observed summer-season foray probabilities for males and females were, respectively, 0.141 and 0.015 forays per animal per summer, with 50% of foraying males traveling at least 8.1 km and 10% of foraying males traveling at least 21.7 km beyond their core herd home range boundary. Risk of bighorn sheep contact with a domestic sheep allotment was then modeled by combining observed bighorn habitat preferences and movement patterns with geographic data, including habitat suitability and distance of core herd home ranges from domestic sheep allotments. For the domestic sheep grazing alternatives analyzed by the Payette National Forest, modeled contact rates between foraying bighorn sheep and domestic sheep allotments ranged from 0 to 1.33 contacts per year. Managers elsewhere can apply this methodology to estimate risk of interspecies contact based on bighorn sheep space use and habitat preferences.