Age, sex, density, winter weather, and population crashes in Soay sheep

Quantifying the impact of density, extrinsic climatic fluctuations, and dem ography on population fluctuations is a persistent challenge in ecology. We analyzed the effect of these processes on the irregular pattern of populat ion crashes of Soay sheep on the St. Kilda archipelago, United Kingdom. Bec ause the age and sex structure of the population fluctuates independently o f population size, and because animals of different age and sex respond in different ways to density and weather, identical weather conditions can res ult in different dynamics in populations of equal size. In addition, the st rength of density-dependent processes is a function of the distribution of weather events. Incorporating demographic heterogeneities into population m odels can influence dynamics and their response to climate change.