Adaptive evolution and plasticity define the distribution of European weeds across elevation gradients in Hawai'i

Background/Question/Methods Climate matching between the site of origin and site of introduction is one approach to predicting the likely range of an introduced species, but invasive species may also expand their ranges into novel environments through adaptation. The unique Hawaiian ecosystems allow insights into the most significant driving forces of plant invasions along steep environmental gradients - selection of preadapted genotypes or adaptation? Which functional and lifehistory traits affect the invasion success of individual species? To investigate the role of evolutionary adaptation in determining invasive species ranges, we studied six European plant species that have invaded along steep environmental gradients of two Hawaiian volcanoes. These European species were introduced to Hawai‘i about one hundred years ago. We studied invasion patterns of all species to investigate the effect of inbreeding vs. outbreeding and annual vs. perennial and clonal species. Results/Conclusions Field surveys suggested the occurrence of different ecotypes adapted to differing temperatures. Seeds were collected across elevation gradients from both volcanoes and grown in a greenhouse at cold and warm temperatures over two generations. Populations of some species, especially Hypochaeris radicata, Plantago lanceolata, Rumex acetosella and Holcus lanatus, had a clear home site advantage in germination and growth, indicating a genetic basis for the observed ecotypes, rather than phenotypic plasticity. The pattern was consistent over several generations. Ecotypic differences were more pronounced in outbreeding than in inbreeding species, suggesting that the genetic diversity and recombination is a key factor that has promoted ecotypic differentiation. These results demonstrate the potential for rapid genetic differentiation during invasion across environmental gradients, resulting in patterns of ecotypic differentiation similar to those often reported for native species.