Quantifying the per-capita contribution of all components of a migratory cycle: A modelling framework

Migratory species make use of different habitats and pathways at different life stages, and in different seasons. Ecological management strategies proposed for migratory species should acknowledge the importance of each component of the migratory cycle. Metapopulation approaches used to assess the quality of habitats do not capture key features of migratory populations. In recent years, contribution metrics – roughly, the annual contribution to a population from a single individual – that are suitable for migratory species and quantify the quality of habitats and pathways have been developed. However, existing contribution metrics are either not suitable for use on species that experience complex life histories or movement strategies; or are only able to differentiate between pathways taken during the first season of the annual cycle. Here, we develop a modelling framework to calculate contribution metrics that quantify the contribution of individuals migrating along specific pathways over any number of seasons in the annual cycle. Our framework yields easily-computable formulae, even for population models with complex migratory patterns. We illustrate our framework using hypothetical examples as well as a model inspired by the monarch butterfly, and highlight ecological insights that could not have been found using existing contribution metrics. We envisage our framework being used to identify the most important or vulnerable components of the migratory cycle, such that appropriate conservation strategies may be applied.