Rapid evolution occurring during biological invasions has contributed to the recognition that evolution can influence contemporary ecological dynamics. Invasive species often evolve clines in traits such as size and phenology as they spread along environmental gradients in their new range, leading to the concern that evolution could increase the impact that they have on native communities. This could happen by increasing the fitness of locally adapted non-native genotypes relative to native species, and/or by altering the degree of niche overlap between the invader and native species. The net effect of these changes could affect the outcome of competition by changing the way in which they compete for resources, either making coexistence more or less likely.
Lactuca serriola is an annual plant native to Eurasia, which has a global distribution and has evolved its flowering phenology as it spread along climate gradients in the new range. We combined field experiments with mathematical theory to ask how the evolution of phenology in Lactuca has affected its potential for coexistence with a native plant community in California. Knowledge of the introduction history of this species makes it possible to demonstrate the impact of evolution on competitive outcomes by comparing ancestral and descendent populations.
Lactuca populations originating from different climates varied greatly in their phenology. The populations from the regions with the warmest climate, including the local non-native population from California, bolted and flowered earlier than those from cooler climates from the native range – except for a few individuals, plants from the population originating from the coolest climate (from Switzerland) failed to bolt at all. Variation in phenology affected the ability of Lactuca to suppress native competitors, with early flowering plants exerting a much stronger competitive effect on native species than late phenology plants from the native range. This suggests that evolution of phenology during invasion has reduced the potential for the native and introduced species to coexist, and demonstrates more generally that evolution can affect contemporary community dynamics.