Interspecific competition reduces seed dispersal in an annual plant and slows simulated range expansions

Understanding the factors influencing how fast populations can spread
across the landscape will be crucial as species ranges shift due to
climate change. While the role of abiotic factors in determining expansion
speed has been well studied in theory and empirical research, how
competition impacts speed has received far less attention. Here, we
investigated how seed dispersal distances change in response to
competition and how these changes to distributions of dispersed seeds
impact expansion speed. We dispersed four genotypes of the annual plant
Arabidopsis thaliana with variation in life history traits into greenhouse
mesocosms of either empty habitat or habitat containing the annual grass
competitor, Lolium multiflorum. We found that competition decreased both
mean and maximum dispersal distance. We then built a simulation model of
range expansion with experimental data from this and a prior experiment to
explore whether competition slows species expansions primarily through
decreasing dispersal or fecundity. We found that competition primarily
slows expansion speed through decreases in dispersal, but that when
competition impacts both dispersal and fecundity, expansions slow more
than with dispersal alone. The genotype with traits associated with longer
distance dispersal was the most affected by competition in both
experimental dispersal and simulations. This research suggests that not
only does competition slow range expansions through decreases in both
fecundity and dispersal, but that there may be consequences for
evolutionary processes at the leading edge.