Data and code for: Multiple genetic impacts of immigration interact to shape local population persistence versus extinction

Major ongoing theoretical and empirical challenges are to predict the impacts of immigration on extinction probabilities of remaining populations within fragmented habitats. Comprehensive prediction requires considering multiple genetic effects on demography, including inbreeding and resulting inbreeding depression, additive genetic variance in fitness and resulting adaptive micro-evolution, and local adaptation and resulting migration load. However, all such effects have not been quantified or modelled simultaneously, especially for small wild populations experiencing regular natural immigration. We used quantitative genetic individual-based simulations parameterised using long-term data from song sparrows (Melospiza melodia) to show that, contrary to broad expectation, increasing immigration could slightly increase short-term extinction probability. This outcome arose because, while immigration reduced inbreeding and resulting expression of inbreeding depression, migration load stemming from apparent local adaptation was substantial and counteracted local adaptive micro-evolution, especially given heterosis-enhanced introgression. However, alternative self-reinforcing outcomes of rapid extinction due to an inbreeding-induced extinction vortex or migrational meltdown, and persistence due to micro-evolution of increased population growth, commonly arose. These results imply that altering dispersal rates among populations will not necessarily predictably affect local population persistence over short eco-evolutionary timeframes, and highlight how remaining populations can lie on a knife-edge between persistence and alternative routes to genetically-induced extinction.