Decomposing the variation in population growth into contributions from multiple demographic rates

Abstract

Summary The decomposition of variation in population growth into the relative contributions from different demographic rates has multiple uses in population, conservation and evolutionary biology. Recent research has favoured methods based on matrix models termed ‘life‐table‐response experiments’ or more generally ‘the retrospective matrix method’, which provide an approximation of a complete demographic decomposition. The performance of the approximation has not been assessed. We compare the performance of the retrospective matrix method to a complete decomposition for two bighorn sheep populations and one red deer population. Different demographic rates make markedly different contributions to variation in growth rate between populations, because each population is subject to different types of environmental variation. The most influential demographic rates identified from decomposing observed variation in population growth are often not those showing the highest elasticity. Consequently, those demographic rates most strongly associated with deterministic population growth are not necessarily strongly associated with temporal variation in population growth. The retrospective matrix method provides a good approximation of the demographic rate associated most strongly with variation in population growth. However, failure to incorporate the contribution of covariation between demographic rates when decomposing variation in population growth can lead to spurious conclusions.