Simulated Range Expansion Suggests Rapid Change in Biotic Resistance to a Range-Shifting Competitor

Variable climate-change-driven range shifts will likely create novel species and population interactions. Most research has focused on how these interactions may impact expansion rates and adaptation of species on the move (range-shifters). However, slower-moving resident species could also respond to novel competitors, as seen in biological invasions. Whether resident adaptation can occur at initial, low-density phases of these range expansions remains unknown. Here, we simulate this scenario by constructing populations of eight duckweed genotypes (Lemna minor = resident) from various localities near and beyond the range edge of a potential range-shifting competitor, Spirodela polyrhiza, and introduce one genotype of S. polyrhiza at low density. After 14 weeks, we observed significant but subtle evidence for rapid evolution and phenotypic plasticity in the resident, including selection for resident genotypes with faster growth. These changes led to an increase in the production of dormant S. polyrhiza propagules (turions) when reintroduced to L. minor populations. However, whether turion production increases or decreases biotic resistance exerted by L. minor populations depends on the contribution of turion production to short- vs. long-term population growth in S. polyrhiza. Collectively, these results suggest that range-shifting species, even at low densities, may drive evolution, plasticity, and evolution of biotic resistance in resident competitors, but further study is needed to confirm the direction of change in biotic resistance as well as the existence and generality of these effects in a natural context.