Demographic stochasticity and the evolution of life history traits during range expansion
Evolution has been proposed to accelerate biological invasions through 'spatial selection'. During an invasion, individuals become spatially sorted by dispersal ability, with leading-edge populations being dominated by highly dispersive individuals that mate assortatively. When dispersal is heritable, this spatial sorting is expected to increase dispersal in leading-edge populations over generations, yielding evolutionarily-accelerated invasions. There has been substantial theoretical work to understand the impact of spatial selection on invasion speed, but few studies have explicitly considered how demographic stochasticity affects predictions for spatial selection.
Demographic stochasticity has been shown to slow invasions of non-evolving populations by reducing the number of individuals at the leading-edge. Demographic stochasticity may also slow invasions of evolving populations by weakening the signal of spatial sorting, since individuals with the most dispersive phenotypes will not always disperse the farthest. Allee effects may further weaken spatial selection by reducing the fitness of highly dispersive individuals at the leading-edge. Finally, spatial selection may be weakened as dispersal heritability decreases, since offspring of highly dispersive parents may not be highly dispersive themselves when dispersal heritability is low.
We used a stochastic simulation model to estimate how explicitly accounting for Allee effects, dispersal heritability, and demographic stochasticity alter invasion speed predictions.
Preliminary results show that Allee effects play a significant role in reducing the speed of spatially-selected biological invasions. During the initial stages of simulated invasions (i.e., the first 10 generations of an invasion), the strength of the Allee effect was a better predictor of invasion velocity than the magnitude of dispersal heritability. In these early stages, there was no significant effect of dispersal heritability (p > 0.10), but increasing the strength of an Allee effect resulted in a significantly reduced invasion velocity (p < 0.001). These results suggest that when attempting to predict early-stage invasions, it is more important to account for demographic effects than evolutionary effects.