|Abstract or Summary
- I investigated survival, movements, home range sizes, and habitat selection of pygmy rabbits (Brachylagus idahoensis) in southeastern Oregon and northwestern Nevada from June 2005 to June 2007. I trapped 298 rabbits on four sites and fitted each with radio transmitters. More than 13,000 locations of telemetered rabbits were recorded. I used known fate models in program MARK to estimate survival of radio-marked pygmy rabbits from September 2005 – August 2006. The most appropriate model, based on Akaike's Information Criterion (AICc), indicated survival rates varied among study sites, sexes, and with monthly interval in a parallel pattern [model S(area * sex + t)]. The estimated annual survival rate on the four study sites was notably low, varying from 0.003 (SE = 0.003) to 0.173 (SE = 0.066). Predation on collared rabbits was high for both adult (88.6%) and juvenile (89.4%) rabbits. When the cause of mortality could be determined, the most common predators of pygmy rabbits were coyotes (Canis latrans)–19.6%, avian predators–18.5%, and weasels (Mustela spp.)–9.8%.
Spatial use by pygmy rabbits was strongly influenced by sex and season. Males used larger annual (mean = 1.70 ha ± 0.69 SE) and breeding (1.67 ± 0.27 ha) home ranges than females (Annual: 0.90 ± 0.26 ha; Breeding: 0.92 ± 0.16 ha), likely
influenced by mate-searching behaviors exhibited by males. Both sexes utilized larger home ranges during the breeding season than during the nonbreeding season (Male: 0.63 ± 0.11 ha; Female: 0.50 ± 0.09 ha). Core area sizes of males (0.10 ± 0.04 ha) also were larger than females (0.07 ± 0.03 ha). Twenty-four radio-marked individuals dispersed greater than 0.5 km, with a maximal observed dispersal distance of 8.5 km. The majority (62.5%) of these long distance movements were by juvenile males, which likely represented dispersal from natal areas. Many of these individuals apparently crossed low sagebrush (Artemisia arbuscula) communities and relatively open areas. The ability of this species to cross unsuitable habitats, previously considered barriers to movement, may suggest that fragmented populations of pygmy rabbits may not be as isolated as once thought.
To examine resource selection by pygmy rabbits, I sampled vegetative and soil characteristics at locations used by radio-marked rabbits (n = 178) and available (n = 100) sites and compared them using logistic regression. The top model, based on Akaike's Information Criterion (AICc), indicated that the increased density and height of live shrubs, higher silt content of soil, and lower clay content of soil were the primary variables related to pygmy rabbit occupation relative to available habitat. Habitat-use patterns also varied among study sites as I did not find a consensus model that described habitat selection among pygmy rabbits for each study site. Further, my analysis demonstrated that soils at pygmy rabbit burrows were deeper, had greater subsurface strengths, lower strengths at the surface, and lower clay composition than was randomly available in the study area.
The findings in this study provide needed information to management agencies making land-use decisions. Given the concern over the status of this species throughout its range, a more thorough understanding of survival rates, spatial use,
dispersal capabilities, and habitat associations of this sagebrush obligate species is essential to identify factors influencing their conservation.