Darwin's Naturalization Conundrum revisited: The limited role of phylogenetic and trait distances as predictors of non-native plant success

Non-native species negatively impact natural biodiversity and habitats, but it's unclear which species are most likely to invade and take over. An idea is that the evolutionary or functional similarity between the non-native species and the resident, native species in a community influences the likelihood of success. Charles Darwin, himself, mused over this topic in what has come to be known as Darwin's Naturalization Conundrum. On the one hand, closely-related species may succeed because they share traits with native species that enable them to survive local environmental conditions (Preadaptation Hypothesis). On the other hand, distantly-related species may succeed because there are open niches and an absence of direct competition (Darwin's Naturalization Hypothesis). Previous research has found evidence for both hypotheses, based on surveys of species at a single point in time. However, these "snapshots" do not reveal dynamics during an invasion, as a species spreads. We tested the hypotheses using repeated surveys of plant species' abundance in 1,370 forested plots in eastern North America to see whether changes in non-native abundance was related to their phylogenetic and/or trait distances to the resident community. Plots were surveyed at least twice, with a mean of five years in between observations. We calculated the change in absolute and relative abundance of each non-native species in each plot over time, and tested their relationships to the phylogenetic and trait distances to the rest of the community using mixed-model regression. The Preadaptation Hypothesis would predict positive relationships, while Darwin's Naturalization Hypothesis would predict negative relationships. We compared both minimum and abundance-weighted mean phylogenetic distances, and tested functional traits like leaf nitrogen and specific leaf area. There was a significant, negative relationship between phylogenetic distance and abundance and a positive relationship between trait distances and abundance. But, these relationships were extremely weakly correlated, with R² values below 1%. The weakness of our models suggest that phylogenetic relatedness and trait differences are poor predictors of whether a species' abundance changes over time. While it still remains to be seen whether these measures of relatedness have greater biological significance for the initial invasion, it appears that for already established non-native species, other factors seem to be more important.