Expanding beyond their native ranges, some plant species become ecologically dominant in new areas, affecting biodiversity, ecosystem function, and human health. Thus, the ability to understand and predict invasive behavior is of great importance. Using weed-rich tribes of the cosmopolitan family Asteraceae as models, this project will investigate evolutionary relationships among native and invasive taxa, with the goal of facilitating development of predictive frameworks for screening potentially invasive taxa. Phylogenies will be reconstructed based on DNA sequence data and used to test how relatedness between natives and exotics determines invasive/weedy behavior in Mediterranean climates in the US and Europe.
Once exotic plants establish in new regions, they are difficult to control. Hence, identifying and preventing potentially invasive species from reaching ground zero is the most economically and environmentally desirable management method. This study will provide the basis for developing frameworks for screening potentially problematic species, as well as insights into traits that influence the success of exotic invaders. Mediterranean climate ecosystems are both highly diverse and highly endangered, rendering them ideal as study systems. The project will provide scientific training for high school and undergraduate students and outreach efforts aimed at raising interest in environmental sciences among Asian American youth.
Invasive species have great ecological and economic impacts and are difficult to control once established, making the ability to understand and predict invasive behavior of great import. Exotic plant invasions are amidst the greatest threats to natural ecosystems, as the number of exotic plant species introduced to the US far outnumber those of other organisms. In order to develop a framework to identify and screen such potentially problematic, invasive weeds, this study examines Darwin’s Naturalization Hypothesis, which posits that invaders less related to native flora are more likely to be successful than those that are closely related to natives. On one hand, invaders similar to native species may be at a disadvantage due to competitive exclusion and/or the presence of common enemies. On the other hand, these invaders may have traits pre-adapted to the introduced range, allowing them to successfully establish and spread. The project tests these hypotheses in two tribes of the sunflower family, Asteraceae, in two Mediterranean climate regions. Mediterranean climates represent both one of the most diverse, species-rich regions as well as the most imperiled, and are among the most invaded biomes of the world. The sunflower family comprise the majority of invasive species in these regions, and patterns of relatedness between native and non-native species in the thistle tribe Cardueae and the daisy tribe Astereae were specifically examined in the California Floristic Province and Italian Peninsula, respectively, using rigorous molecular phylogenetic approaches. Evolutionary patterns within these highly supported clades show that not only are introduced taxa more closely related to natives more likely to be invasive than those more distantly related, but these invasive species are also evolutionarily closer to native flora than expected by chance, contrary to the predictions of Darwin’s Naturalization Hypothesis. To elucidate the mechanisms behind these patterns, we examined the environmental preferences of these species using ecological niche modeling methods, and compared niche similarity between species with their evolutionary relatedness. In both systems, invasive species were found to have higher degrees of niche overlap with native species than non-invasive introduced species do, and pairwise niche distance was significantly correlated with evolutionary time, indicating the presence of phylogenetic niche conservatism. Hence closely related species in the two groups tend to share similar ecological preferences, partially explaining why introduced species closely related to native species are more likely to become invasive. Another major factor that determines the spread and distribution of taxa is the ability to reach environmentally suitable areas, and it has been suggested that dispersal limitation may have an equal or greater influence on plant distributions than niche preferences. By examining the dispersal traits of over 4000 fruits from thistle species in California, we were able to show that invasive thistles have superior dispersal capabilities compared to non-invasive introduced species. Furthermore, these dispersal capabilities were phylogenetically conserved, indicating that closely related species share similar dispersal abilities. Indeed, as successful dispersal strategies may vary with habitat and represent adaptions to a species’ particular niche, at least part of the success of invasive thistle closely related to natives may be attributed to superior, pre-adapted dispersal traits. Together with shared niche preferences, these results explain why close relatives make bad neighbors in the Asteraceae. In the face of widespread anthropogenic threats to native plant communities worldwide, these approaches may prove a fruitful means for furthering our understanding of biological invasions and developing predictive frameworks for screening potential invasive taxa. The project results also are meaningful in a political sense in addition to their scientific value, as legislation and programs regarding biological invasions are generally state-specific. Therefore results of the study were directly communicated to land/resource managers such as officials of the California Department of Food and Agriculture and members of the California Invasive Plant Council to facilitate efforts to manage invasive weeds and conserve biodiversity. In addition, the results of the project were communicated not only through scientific journals, but a plethora of conferences, seminars and talks, open to the public as well as the scientific community, and have also been featured online on University websites. The project supported the empowerment and education of underrepresented minorities, offering invaluable research opportunities to students from Pakistan, Iran, Taiwan, and Vietnam throughout its duration. It also supported the Students Engaging, Exploring & Discovering Science program and the Young Scholars Program at the University of California Davis, introducing young minds to the biological sciences and offering hands on research experience, an indispensable part of a good scientific education. Furthermore, the project allowed the Co-PI Park to augment the participation of underrepresented minorities in ecological sciences at the professional level. While Koreans are no longer considered an underrepresented minority in the sciences in the US, Korean-Americans, and Asian-Americans in general are sorely underrepresented in field sciences and environmental leadership roles.
