The expansion of a species’ range underlies two pressing ecological issues reshaping patterns of global biodiversity: biological invasions and climate change. Expansions of invasive species into new areas are occurring at unprecedented rates and climate change is already causing many terrestrial species to alter their ranges, often upward in latitude and elevation. Traditional theory considers range expansion as the outcome of the basic demographic processes of birth, death, and dispersal. However, recent research suggests that spatial evolutionary mechanisms can play critical roles in determining the speed of expansions and the traits present at the expanding edge. We tested the role of spatial evolution in range expansions using 60 experimental microcosms of the red flour beetle (Tribolium castaneum) divided between two treatments: shuffled and structured. In shuffled landscapes, we prevented the evolution of spatial genetic structure by spatially randomizing the beetles once per generation. In structured landscapes, beetle populations were allowed to form spatial genetic structure via spatial assortative mating and differing selection pressures between the range core and edge. Range expansions proceeded for eight generations, after which we used offspring of beetles from the edge and core of each landscape to test for evolved differences in growth rate and dispersal.
Results/Conclusions
The speed of expansion was significantly higher in structured landscapes compared to shuffled landscapes. Our results suggest that differential trait evolution between edge and core habitats could be responsible. Edge populations evolved a significantly greater low-density dispersal response compared to core and shuffled populations. This tendency could increase the speed of range expansion as even at the low population densities characteristic of the range edge, individuals would be more likely to disperse and thus push the range boundary further. In addition to increased speed, structured landscapes displayed significantly more variance in expansion rate. The observed increase in expansion speed due to spatial evolution is consistent with previous research, however it is currently unclear why variance in speed should be so strongly affected. Our results suggest that stochasticity in trait evolution at the expanding edge could be responsible. Growth rates of beetles descended from edge populations were significantly lower compared to core and shuffled populations. Stochasticity in the countervailing forces of increased dispersal tendency and decreased growth rate could therefore be responsible for the observed increase in variance of expansion speed. Our results demonstrate that incorporating evolutionary processes will be vital to better understand and predict the outcomes of range expansions.