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

Predicting population responses to environmental changes requires understanding interactions among environmentally induced phenotypic variation, selection, demography and genetic variation, and thereby predicting eco-evolutionary dynamics emerging across diverse temporal and spatial scales. Partially migratory metapopulations (PMMPs), featuring seasonal coexistence of resident and migrant individuals across multiple spatially distinct subpopulations, have clear potential for complex spatio-seasonal eco-evolutionary dynamics through impacts of selection on migration on spatial population dynamics, and feedbacks resulting from ongoing micro-evolution. However, the key genetic and environmental conditions that maintain migratory polymorphisms, and eco-evolutionary dynamics of PMMPs under stochastic environmental variation and strong seasonal perturbations, have not yet been resolved. Accordingly, we present a general individual-based model that tracks eco-evolutionary dynamics in PMMPs inhabiting spatially structured, seasonally varying landscapes, with migration formulated as a quantitative genetic threshold trait. Our simulations show that such genetic and landscape structures, which commonly occur in nature, can readily produce a variety of stable partially migratory systems given diverse regimes of spatio-seasonal environmental variation. Typically, partial migration is maintained whenever sites differ in non-breeding season suitability resulting from variation in density-dependence, causing ideal free non-breeding distributions where residents and migrants occur with frequencies that generate similar survival probabilities. Yet, stable partial migration can also arise without any fixed differences in non-breeding season density-dependence among sites, and even without density-dependence at all, through risk-spreading given sufficiently large stochastic environmental fluctuations among sites and years. Finally, we show that local non-breeding season mortality events, as could result from extreme climatic events, can 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. Such effects result from spatially divergent selection on both the occurrence and destinations of migration. Our model thus shows how facultative seasonal migration can act as a key mediator of eco-evolutionary dynamics in spatially and seasonally structured environments, providing key steps towards predicting responses of natural partially migratory populations to ongoing changes in spatio-seasonal patterns of environmental variation.