Deconstructing Darwin's Naturalization Conundrum in the San Juan Islands using community phylogenetics and functional traits

Abstract

Aim: Darwin posed a conundrum about species invasions, postulating the importance of functional distinctiveness from the receiving native community to avoid competition, and, at the same time, the importance of shared similarity to pass environmental filters and successfully establish. Using phylogenetic distances and functional traits, we assessed this conundrum in the flora of 80 mostly uninhabited islands, where over 30% of the species are invasive. We highlight the importance of publicly available datasets to disentangle ecological processes that may drive invasion. Location: San Juan Islands archipelago, Pacific Northwest of North America. Methods: Using a supermatrix approach, we inferred a maximum-likelihood estimate of the mega-phylogeny for the vascular plants on the San Juan Islands. We gathered measurements for five ecologically relevant functional traits - seed mass, maximum height, specific leaf area, leaf size and leaf nitrogen content. We assessed phylogenetic and functional trait similarity between invasive species and the receiving native community, and tested the significance of the observed patterns against a randomly generated invading community. Results: Invasive species were more closely related (phylogenetically clustered) to their nearest native than natives were on 40 of the islands and were more clustered than any random invasive in the species pool on 22 islands. Despite phylogenetic similarity, functional traits differed between the two status groups, at least for maximum height and specific leaf area. When comparing functional differences between phylogenetically close relatives, more complex patterns emerge. Main conclusion: Only with the combination of both evolutionary history and phenotypic traits were we able to discover support for both sides of Darwin's conundrum - although invasive species have phylogenetically close native relatives, functional traits differ between the two status groups. This implies that both environmental filtering and competitive interactions may be important for invasion success in this archipelago.