Scale dependence in the phylogenetic relatedness of alien and native taxa

Aims Darwin’s naturalization hypothesis proposes that successfully established alien species are less closely related to native species due to differences in their ecological niches. Studies have provided support both for and against this hypothesis. One reason for this is the tendency for phylogenetic clustering between aliens and natives at broad spatial scales with overdispersion at fine scales. However, little is known about how the phylogenetic relatedness of alien species alters the phylogenetic structure of the communities they invade, and at which spatial scales effects may manifest. Here, we examine if invaded understorey plant communities, i.e. containing both native and alien taxa, are phylogenetically clustered or overdispersed, how relatedness changes with spatial scale and how aliens affect phylogenetic patterns in understorey communities. Methods Field surveys were conducted in dry forest understorey communities in south-east Australia at five spatial scales (1, 20, 500, 1500 and 4500 m2). Standardized effect sizes of two metrics were used to quantify phylogenetic relatedness between communities and their alien and native subcommunities, and to examine how phylogenetic patterns change with spatial scale: (i) mean pairwise distance and (ii) mean nearest taxon distance (MNTD). Important Findings Aliens were closely related to each other, and this relatedness tended to increase with scale. Native species and the full community exhibited either no clear pattern of relatedness with increasing spatial scale or were no different from random. At intermediate spatial scales (20–500 m2), the whole community tended towards random whereas the natives were strongly overdispersed and the alien subcommunity strongly clustered. This suggests that invasion by closely related aliens shifts community phylogenetic structure from overdispersed towards random. Aliens and natives were distantly related across spatial scales, supporting Darwin’s naturalization hypothesis, but only when phylogenetic distance was quantified as MNTD. Phylogenetic dissimilarity between aliens and natives increased with spatial scale, counter to expected patterns. Our findings suggest that the strong phylogenetic clustering of aliens is driven by human-mediated introductions involving closely related taxa that can establish and spread successfully. Unexpected scale-dependent patterns of phylogenetic relatedness may result from stochastic processes such as fire and dispersal events and suggest that competition and habitat filtering do not exclusively dominate phylogenetic relationships at fine and coarse spatial scales, respectively. Distinguishing between metrics that focus on different evolutionary depths is important, as different metrics can exhibit different scale-dependent patterns.

Darwin’s naturalization hypothesis does not explain the spread of nonnative weed species naturalized in México

BackgroundDespite numerous tests of Darwin’s naturalization hypothesis (DNH) evidence for its support or rejection is still contradictory. We tested a DNH derived prediction stating that nonnative species (NNS) without native congeneric relatives (NCR) will spread to a greater number of localities than species with close relatives in the new range. This test controlled the effect of residence time (Rt) on the spread of NNS and used naturalized species beyond their lag phase to avoid the effect of stochastic events in the establishment and the lag phases that could obscure the NCR effects on NNS.MethodsWe compared the number of localities (spread) occupied by NNS with and without NCR using 13,977 herbarium records for 305 NNS of weeds. We regressed the number of localities occupied by NNSversus Rtto determine the effect of time on the spread of NNS. Then, we selected the species withRtgreater than the expected span of the lag phase, whose residuals were above and below the regression confidence limits; these NNS were classified as widespread (those occupying more localities than expected byRt) and limited-spread (those occupying fewer localities than expected). These sets were again subclassified into two groups: NNS with and without NCR at the genus level. The number of NNS with and without NCR was compared usingχ2tests and Spearman correlations between the residuals and the number of relatives. Then, we grouped the NNS using 34 biological attributes and five usages to identify the groups’ possible associations with spread and to test DNH. To identify species groups, we performed a nonmetric multidimensional scaling (NMDS) analysis and evaluated the influences of the number of relatives, localities, herbarium specimens,Rt, and residuals of regression. The Spearman correlation and the Mann–WhitneyUtest were used to determine if the DNH prediction was met. Additionally, we used the clustering objects on subsets of attributes (COSA) method to identify possible syndromes (sets of biological attributes and usages) associated to four groups of NNS useful to test DNH (those with and without NCR and those in more and fewer localities than expected byRt).ResultsResidence time explained 33% of the variation in localities occupied by nonnative trees and shrubs and 46% of the variation for herbs and subshrubs. The residuals of the regression for NNS were not associated with the number or presence of NCR. In each of the NMDS groups, the number of localities occupied by NNS with and without NCR did not significantly differ. The COSA analysis detected that only NNS with NCR in more and fewer localities than expected share biological attributes and usages, but they differ in their relative importance.DiscussionOur results suggest that DNH does not explain the spread of naturalized species in a highly heterogeneous country. Thus, the presence of NCR is not a useful characteristic in risk analyses for naturalized NNS.

Different effects of invader–native phylogenetic relatedness on invasion success and impact: a meta-analysis of Darwin's naturalization hypothesis

Darwin's naturalization hypothesis (DNH), which predicts that alien species more distantly related to native communities are more likely to naturalize, has received much recent attention. The mixed findings from empirical studies that have tested DNH, however, seem to defy generalizations. Using meta-analysis to synthesize results of existing studies, we show that the predictive power of DNH depends on both the invasion stage and the spatial scale of the studies. Alien species more closely related to natives tended to be less successful at the local scale, supporting DNH; invasion success, however, was unaffected by alien–native relatedness at the regional scale. On the other hand, alien species with stronger impacts on native communities tended to be more closely related to natives at the local scale, but less closely related to natives at the regional scale. These patterns are generally consistent across different ecosystems, taxa and investigation methods. Our results revealed the different effects of invader–native relatedness on invader success and impact, suggesting the operation of different mechanisms across invasion stages and spatial scales.