Evolution in response to management and propagule size interact to increase invasiveness among experimental populations of aquatic plants.

Background/Question/Methods

Once established, invasive populations often require repeated removal (management) attempts to reduce their ecological impact on invaded communities and ecosystems. Whether these repeated removal efforts influence the invader’s evolution and, subsequently, its future invasiveness remains to be thoroughly and empirically investigated. To address this gap, I conducted an experiment evolving genetically diverse populations of the duckweed Lemna minor to the presence or absence of repeated reductions in population size (simulating management) in a semi-natural field setting over the course of nine weeks (approximately 14-16 generations). Following experimental evolution, these populations were isolated in common garden environments for one week (1-2 generations) to reduce the impact of maternal effects on invasiveness. These evolved populations were then introduced to diverse communities of other duckweed species in replicated, experimental invasions. To compare the overall effects of evolution, we included an additional treatment reconstructing the initial genetic compositions of populations prior to the nine-week evolution phase. During these invasions, initial population size of L. minor was also manipulated to identify interactions between evolutionary history and propagule size. To assess each population’s invasiveness, we measured L. minor’s growth rate and impact on community biomass following the two-week experimental invasion.

Results/Conclusions

Our results show that populations evolving to management regimes were significantly more invasive at the highest propagule sizes, demonstrating an average increase in population growth and biomass of 44% relative to populations evolving in non-managed conditions or non-evolved populations (p = 0.0378). That the impacts of evolution were only observed in larger propagule sizes could be the result of potential sampling or complementarity effects occurring within these populations. Our result hints at the potentially powerful evolutionary effects that humans might have on invasive populations over time. If populations are unable to be successfully eradicated, it’s possible that management selects for more invasive genotypes within populations. This could require more extensive management, thus creating a positive evo-eco feedback cycle. These results also illustrate that evolution (and not necessarily genetic diversity alone) could be an important underlying mechanism in many biological invasions and require more empirical investigation to distinguish the relative contributions of ecological versus evolutionary forces (as well as their interactions) in determining invasion outcomes.