Density dependence shapes life-history trade-offs in a food limited ungulate population

Abstract

Quantifying trade-offs within populations is an important goal in life-history and evolutionary theory. However, most studies focusing on life-history variation assume trade-offs to be static. In this paper, we provide a framework for understanding life-history variation at different densities while enabling us to reveal trade-offs that are often masked due to individual heterogeneity. We use published individual-based data from a population of Soay sheep and find density dependence strongly shapes life-history trade-offs and the distribution of lifetime reproductive success. We find trade-offs between juvenile survival and growth structures life-history variation for Soay sheep at all densities, but the intensity of the trade-offs increases with population density. At carrying capacity (K), a new trade-off appears between reproduction and juvenile survival. In addition, we find the distribution of LRS to be highly constrained at K, with mothers of prime adult sizes ([~] 25kg) contributing the most to reproduction. We also find the effects of density-dependence on demographic measures such as net reproductive rate, stable size distribution, and average vital rate functions. Our results suggest high density limits the diversity of individual life history strategies and has implications for better understanding the evolution of reproductive tactics via density-dependent selection and management of ungulates in food limited environments.