The influence of evolution on population spread through patchy landscapes

Background/Question/Methods

The spread of a population across a landscape underlies two pressing ecological concerns: biological invasions and range shifts induced by climate change. Predicting how fast a population will spread is complicated by heterogeneous landscapes and evolution during the invasion. Despite the possibility for selection pressures on traits to vary across positions along the invasion front, how landscape patchiness may change the potential for evolution has yet to be well explored. We used a theoretical approach to explore the nature of selection on populations spreading through continuous versus patchy landscapes and how evolution over the course of an invasion can influence spread velocity. Specifically, we used integro-difference equations and simulations to model the movement of an annual plant population through a linear environment. We focus on a tradeoff between two demographic traits: the ability of an individual to produce high numbers of seeds when growing at low density (λ) and its ability to cope with competitors when growing at high density (α), but we do not let dispersal traits evolve.

Results/Conclusions

We found that the optimal strategy at the leading edge of an invasion depends on the degree of landscape patchiness and on the strength of the tradeoff between λ and α. When the tradeoff between the demographic rates was strong, greater distances between patches led to the strategy with the least sensitivity to competition leading the invasion front. Conversely, in continuous landscapes, the strategies with high population growth rates, but low sensitivity to competition dominated. These results were consistent between the analytical solution for the evolutionary stable strategy and simulation models. We also found that evolution can increase the spread velocity under some conditions. However, the optimal strategy is not always in the leading edge of the invasion due to priority effects. Our results indicate the importance of landscape structure in determining how evolution may influence invading or range-shifting populations.