Eco-evolutionary dynamics of partially migratory metapopulations in spatially and seasonally varying environments

Interlinked population dynamic and evolutionary responses to spatial and seasonal environmental variation, stemming from interactions and feedbacks among phenotypic variation, genetic variation, selection and demography, could generate complex eco-evolutionary dynamics that span temporal and spatial scales. Partially migratory metapopulations (PMMPs), featuring sequential seasonal sympatry and allopatry of different sets of resident and seasonally migrant individuals, have clear potential for such eco-evolutionary outcomes. This is because ongoing evolution of reversible seasonal migration affects spatio-seasonal population dynamics and densities, which could in turn shape forms and magnitudes of selection on migration, causing feedbacks on evolution. However, key environmental and genetic conditions that maintain migratory polymorphisms, and resulting eco-evolutionary dynamics of PMMPs given stochastic environmental variation and strong spatially restricted seasonal perturbations, have not been characterized. We built a general individual-based model that tracks eco-evolutionary dynamics in PMMPs inhabiting spatially structured and seasonally varying landscapes, with seasonal migration formulated as a quantitative genetic threshold trait. Simulations showed that such genetic architectures and landscape structures, which are common in nature, readily produce stable partially migratory systems given diverse regimes of environmental variation. Partial migration is maintained whenever sites differ in non-breeding season suitability, defined as variation in density-dependence, causing 'ideal free' non-breeding distributions where residents and migrants occur with frequencies generating similar survival probabilities. Further, bet-hedging can cause stable partial migration without any fixed differences in non-breeding season density-dependence among sites and even without density-dependence at all, given sufficiently large stochastic environmental fluctuations among sites and years. Importantly, major local non-breeding season mortality events, as could result from extreme climatic events, generate eco-evolutionary dynamics that ripple out to affect breeding and non-breeding season space use of subpopulations throughout the PMMP, on both short and longer timeframes. These effects result from spatially divergent selection on the occurrence and destination of migration. Our model thus shows how changing partial seasonal migration acts as a key mediator of eco-evolutionary dynamics in (meta)populations occupying spatially and seasonally varying environments. It thereby initiates new steps towards predicting responses of natural partially migratory populations to ongoing changes in spatio-seasonal patterns of environmental variation.