Population spread lies at the heart of two great threats to ecological systems: invasions by exotic species, and the need of many native species to shift their ranges to track climate change. In order to make reliable predictions of population spread rates, it is essential to understand both their ecological and evolutionary dynamics. Recent work has shown that populations spreading through favorable habitat may rapidly evolve higher rates of dispersal and reproduction at the expansion front, accelerating their spread velocity. However, as species expand their range, they will likely eventually encounter stressful environments. In this study, we asked how prior evolutionary change during spread through benign environments affects subsequent population performance under harsher conditions.
Using replicate experimental invasions of Arabidopsis thaliana, we examined evolutionary change in performance under drought, interspecific competition and heat stress after six generations of spread through landscapes of favorable habitat. We tested the performance of individual genotypes under stress and combined these data with knowledge of the changes in genotype frequencies over the course of the replicate invasions, to quantify changes in performance between leading edge and founding populations. Finally, we examined to which degree this evolutionary change reflected changes in intrinsic performance or stress tolerance.
Results/Conclusions
Over six generations of spread through favorable habitat, average silique production under drought or interspecific competition declined in leading edge populations as compared to the founding population. This evolutionary change was driven primarily by a negative correlation between intrinsic silique production and seed size, which increased over the course of spread. Tolerance to drought or interspecific competition did not change markedly, resulting in an overall decrease in silique production under stress. In contrast, heat tolerance did increase in leading edge populations, and was associated with the evolution of increased plant height.
We conclude that evolution during spread through favorable habitat may leave a legacy on population performance in stressful environments encountered in the expanded range. These changes can be driven by altered intrinsic fecundity and/or stress tolerance. Depending on the traits under selection during invasion, further spread into stressful environments may thus be accelerated or slowed down. Forecasts of population spread rates may be improved by considering the joint evolution of the ability of populations to spread under favorable conditions and to grow in stressful environments, as both contribute to the eventual limits of range expansion.