The Phylogenetic Structure of Biological Invasions: A Test of Darwin's Naturalization Hypothesis in the Asteraceae

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. Darwin's Naturalization Hypothesis predicts that invaders less related to native flora are more likely to be successful than those that are closely related to natives. This study tests this hypothesis 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 Asteraceae 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. The ecological niche similarity between taxa and their evolutionary relatedness were compared to elucidate the mechanisms behind these patterns. 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. Furthermore, invasive thistles in California displayed superior dispersal capabilities compared to non-invasive introduced species, and these capabilities exhibited a phylogenetic signal as well. These results suggest that close relatives make bad neighbors in the Asteraceae due to the presence of pre-adaptive niche preferences and dispersal traits. 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.