Mortality in an alpine ungulate increases nonlinearly with snow depth.

Abstract

Heavy snowpack can be a severe constraint for alpine herbivores, yet adult survival in long-lived temperate ungulates is typically canalized against environmental variation. Using a 45-year individual-based dataset on bighorn sheep (Ovis canadensis) and historical snow reconstructions at Ram Mountain, Canada, we evaluated the direct and indirect effects of snow conditions on age- and sex-specific survival. For both sexes, survival of lambs and senescent individuals decreased with increasing average snow depth, whereas non-senescent adults (aged [≥] 1 year and < 8 years) showed high survival under mean conditions. However, this resilience broke down once specific thresholds of snow depth were crossed, with sharp survival declines beyond 90 cm of snow depth for females and 100 cm for males. Lamb survival, already reduced by increasing average snow depth, declined further once snow depth exceeded 90 cm. Crucially, the annual probability of snow depth exceeding 90 cm increased markedly over the study period, along with a density-independent temporal decline in survival across all demographic groups. Lamb survival declined by 0.37, and adult female survival, the most critical vital rate for population growth, dropped by 0.13 over the study period. Our findings challenge the established paradigm of demographic buffering in prime-aged adults, demonstrating that their resilience to environmental variability is fundamentally non-linear and breaks down once a changing climate regime breaches adult physiological boundaries, severely compromising population viability.