Evolution at shifting range margins

The speed at which biological range expansions occur has important consequences for species experiencing climate change, and for invasions by exotic organisms. There is growing empirical and theoretical evidence that during range expansions there is selection for increased dispersal, and that this can result in faster rates of spread. However, few models consider whether increased dispersal comes at a cost. In this thesis I investigated how two different trade-offs between dispersal and other traits affected the rates of range expansions. The first modelled a direct trade-off between dispersal and reproduction, and the second incorporated a trade-off so that adaptation to an environmental gradient came at a cost to dispersal. The first trade-off was investigated by modelling a population that consisted of two dispersal phenotypes, one that has a higher population growth rate and one that has a higher dispersal rate. Using a simple deterministic model it was found that when there was a big trade-off between the morphs in terms of these traits, anomalous invasion speeds were observed whereby a population consisting of both phenotypes invades at a speed faster than either single phenotype. It was found that these anomalous invasion speeds were robust to demographic stochasticty. Adding a shifting climate to the model revealed that a trade-off between dispersal and establishment ability can help a species to keep up with climate change. The second trade-off was investigated using a quantitative trait model, which revealed that a trade-off between dispersal and adaptation can result in the formation of range margins. Introducing a shifting environment allowed a species to expand its range at a speed determined by the steepness of the gradient and the size of the trade-off. These models reveal that trade-offs can alter range shifting dynamics, the consequences of which for predicting rates of range expansions were discussed