Reproductive elasticity increases with rising temperature in Arctic charr, Salvelinus alpinus (Salmonidae)

Northern post-glacial fish are valuable natural systems for studying life-history responses to climate due to their recent evolutionary history, replicated spatial structure, and strong temperature dependence of physiological processes. From published data we developed matrix models for Arctic charr (Salvelinus alpinus) populations distributed across a latitudinal air temperature gradient spanning the species’ full thermal range. Using elasticity analysis, which estimates the proportional change in the population growth rate for a proportional change in a vital rate, we examined the importance of reproduction, juvenile survival, and adult survival for mean fitness (population growth rate) across this gradient. Reproductive elasticity increased significantly with temperature, with regression models estimating an ∼150% rise from −22.1 to 7.8°C. This trend paralleled earlier maturation, shorter lifespan, faster juvenile growth, and more frequent breeding in warmer environments. The temperature responses of survival elasticities were less clear: in anadromous populations (N  =  12), juvenile elasticity increased while adult elasticity decreased with temperature; in lake-resident populations (N  =  41), juvenile elasticity showed the opposite trend and adult elasticity was unresponsive to temperature. Despite substantial variation in life-history traits, the relative ranking of life-stage elasticities—an important consideration for wildlife management—was largely consistent among populations across most of the temperature range, supporting the evolutionary conservation of elasticity rankings within species and the use of data-rich populations to inform the management of data-limited populations. Our findings indicate that as climates warm, natural selection on reproduction will probably intensify.