Description:
<p>Aim: One hypothesis of Darwin's naturalization conundrum, the pre-adaptation hypothesis, proposed that exotics that are more closely related to species in native communities might be more successful invaders because close relatives share similar traits and might be favoured in similar environments. We test this prediction using regional angiosperm assemblages that have been heavily invaded by exotic species and relating patterns of phylogenetic relatedness to potential environmental drivers--temperature and precipitation. Location: California, USA. Methods: Using a massive, dated phylogeny of angiosperm plants, we assessed differences in phylogenetic structure between native, naturalized exotic and full floras of 35 bioregions in California. We quantified phylogenetic relatedness using the net relatedness index (NRI) and the nearest taxon index (NTI) at varying phylogenetic extent. We measured changes in phylogenetic structure using a metric (phylogenetic relatedness anomaly), which is calculated as the phylogenetic relatedness of the full flora of a bioregion minus that of the native flora of the bioregion. We assessed correlations between phylogenetic structure measures and temperature or precipitation. Results: In general, introduction of exotic species decreased phylogenetic clustering when considering NRI but increased phylogenetic clustering when considering NTI. However, patterns of phylogenetic relatedness for some families were opposite to those for other families (e.g., NRI anomalies for Amaranthaceae, Caryophyllaceae, Fabaceae and Poaceae were opposite to those for Brassicaceae, Asteraceae and all angiosperms combined). Native communities were more phylogenetically clustered in dry regions, while exotic phylogenetic structure was less clearly related to climate. Main conclusions: Exotic species tend to come from clades that are poorly represented but not completely absent from the native flora. Within families, NTI patterns tended to be fairly similar between native and exotic species, suggesting shared responses to the environment, particularly temperature and precipitation, in these two groups.</p>