The demographic legacy of founding events revealed by fitting population models to experimental data on growth rates and spread
Population establishment and spread are fundamentally important to ecology, particularly in light of species invasions, shifting species' ranges, and deliberate reintroductions. Empirical and theoretical studies have examined the effect of demographic and genetic factors in population establishment, showing the importance of population size and genetic diversity for successful founding events. However, the role of these factors—and the interplay between them—in the long-term demographic performance of populations over multigenerational scales is unknown. To address this question, we used the model organism Tribolium castaneum (red flour beetle) in a laboratory experiment with non-overlapping generations and discrete dispersal and reproductive periods. We created nine distinct genetic lineages ranging from inbred to outbred and used these to found replicate populations with 2, 4, or 32 individuals. After seven generations of spread through a linear landscape, dispersal was halted to assess demographic processes. We used hierarchical Bayesian techniques to fit a fully stochastic Ricker model to the data. Our model incorporates demographic and environmental stochasticity, phenotypic variation among individuals, and stochastic sex determination to fully capture the variation in this system. We compare estimated rates of intrinsic growth, density dependent competition, and phenotypic variation among populations founded with different genetic and demographic treatments.
Only two populations founded with inbred lineages survived for eight generations, highlighting the importance of a multigenerational approach when analyzing population establishment events. For populations that persisted past seven generations, we found an increased strength of competition in populations founded with only two individuals, but only in non-outbred lineages. This is potentially due to the small genetic diversity present in these founding populations, leading to increased intraspecific competition via increased similarity. Intrinsic growth rates were also higher in non-outbred populations founded by two individuals. These populations remained at lower densities for longer than other treatments, potentially allowing individuals with higher intrinsic growth rates to flood the gene pool with more offspring before an effect of density dependent competition. Surprisingly, the strength of demographic heterogeneity was uncorrelated with treatment, implying that demographic and genetic conditions at the time of population founding have little effect on phenotypic variation in later generations, despite their importance for other demographic parameters. By using a stochastic model incorporating the biological and environmental processes contributing to the observed variation, we are able to show a surprising relationship between demographic and genetic conditions during population founding and basic demographic parameters even after many generations of growth and spread.