Abstract
We investigated influences of risk of predation by mountain lions (Puma concolor), topographic metrics at multiple scales, and vegetation, land, and snow cover on resource selection by Sierra Nevada bighorn sheep (Ovis canadensis sierrae), an endangered taxon, during winters 2002–2007, in the Sierra Nevada, California, USA. We hypothesized that those mountain ungulates would trade off rewards accrued from using critical low-elevation habitat in winter for the safety of areas with reduced risk of predation. Sierra Nevada bighorn sheep did not trade off benefits of forage for reduced risk of predation but selected areas of high solar radiation, a correlate of vegetation productivity, where risk of predation by mountain lions was greatest, while mitigating indirect risk of predation by selecting for steep, rugged terrain. Bighorn sheep selected more strongly for areas where mountain lions were active, than for low-elevation habitat in winter, likely because mountain lions were most active in those areas of bighorn sheep winter ranges overlapping ranges of mule deer (Odocoileus hemionus), where both ungulates accrued forage benefits. We demonstrated reduced benefit of migration to low elevation during drought years, providing an alternative explanation to the predator-induced abandonment hypothesis for the disuse of low-elevation winter range observed during drought years.
Highlights
Animals living in temperate or arctic environments, where a seasonal abundance of forage coincides with increased nutrient demands of late gestation and lactation [1,2,3], must balance the need to acquire nutrients against constraints from risk of predation [4,5,6]
The hypothesis predicting an influence by direct risk of predation on resource selection by male and female Sierra Nevada bighorn sheep (Table 1) was supported, but in a direction opposite to that predicted under the predation risk hypothesis
This outcome may have resulted from bighorn sheep selecting areas with suitable forage and escape terrain and mountain lions hunting in areas where mule deer overlapped winter range of bighorn sheep [50, 56]
Summary
Animals living in temperate or arctic environments, where a seasonal abundance of forage coincides with increased nutrient demands of late gestation and lactation [1,2,3], must balance the need to acquire nutrients against constraints from risk of predation [4,5,6]. Many populations of ungulates migrate between discrete seasonal ranges [7,8,9,10,11,12,13], with those occupying montane environments obtaining high-quality resources by selecting among elevations that enable exploitation of new growth in forage [11, 14]. Benefits of migration to areas of high-quality forage must outweigh increased risk of predation to comprise an evolutionarily stable strategy [15]. Populations of mountain sheep (Ovis spp.) occupying montane environments may migrate between high-elevation summer ranges and lower-elevation winter ranges, corresponding to the progression of new growth in grasses, forbs, and shrubs [16,17,18]. Differences between the sexes in risk tolerance and predator avoidance strategies [29, 32, 34, 35] must be accounted for in models of resource selection under risk of predation
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