Life-history trade-offs and environmental variability shape reproductive demography in a mountain ungulate

Alpine ecosystems are changing rapidly with implications for the demography of alpine organisms. Here, we studied a sentinel species of mountain environments-the mountain goat (Oreamnos americanus)-to examine hypotheses about intrinsic and extrinsic drivers of reproductive demography using long-term data collected from individually marked animals across a broad spatiotemporal extent (n = 180 females, 3 study areas, 17 years) in coastal Alaska. Our analyses revealed the importance of life-history trade-offs and environmental variability on reproductive performance. The cost of reproduction, defined as the impact of reproducing the previous year on the probability of current year parturition, was high, especially for young, largely primiparous females (13%-32% reduction) and old, senescing individuals (27%-43% reduction); parturition of prime-aged individuals was relatively unaffected (2% reduction). Winter snow accumulation, which alters energetic expenditure and forage availability, exerted strong negative effects on reproduction (20%-35% reduction, depending on age). The relationship between temperature during the preceding summer's growing season and reproduction was likewise negative, although weaker and more variable (10%-15% reduction). Demographic modelling indicated that snow exerted stronger effects on population growth than summer temperature in part due to greater variability in snow versus temperature among years. Our analyses further revealed that reproductive performance did not affect subsequent survival of mothers or offspring, suggesting mountain goats employ a 'risk-sensitive', conservative reproductive strategy that prioritizes survival over reproduction. Taken together, these results fill an important knowledge gap by providing novel insights about the interplay between life-history trade-offs and environmental variation, and how they shape the reproductive demography of climate-sensitive wildlife.