Range shift promotes the formation of stable range edges

Abstract

Aim: I investigate the counter-intuitive possibility that range shift promotes the formation of stable range edges. This might be expected because: (1) range-shifting populations typically evolve increased dispersal on the expanding range edge; (2) increased dispersal steepens the relative slope of environmental gradients (gradients appear steeper to a more dispersive population); and (3) environmental gradients that are steep relative to dispersal encourage the formation of stable range edges (when gradients appear steep, adaptation on the range edge is swamped by maladapted genes).

Methods: I test the idea that populations take longer to evolve across an environmental gradient when those populations have already undergone a period of spread. I do this using an individual-based coupled map lattice simulation, in which individuals carry heritable traits for dispersal probability and environment-specific fitness.

Results: Numerous simulations across parameter space confirm that a period of range shift almost always results in a longer time to evolve through an environmental gradient. This occurs because of both the mechanism described above and the erosion of adaptive variation resulting from the serial foundering that occurs during range advance.

Main conclusions: This result suggests that species may often shift their range due to intrinsic changes in the population rather than extrinsic changes in the environment. The result also suggests a new mechanism regulating the speed of invasion, and sounds a cautionary note for climate change impacts: the longer a species tracks climate change, the less able it may be to track that change into the future.