Phylogenetic relatedness of plant species co-occurring with an invasive alien plant species varies with elevation

Darwin's naturalization conundrum posits that the alien species either succeed in the introduced region because they are phylogenetically related to the native species and thus tend to have niches similar to those of native species, or they are phylogenetically dissimilar to native species and thus occupy unfilled niches. This conundrum has received a lot of attention, but the findings regarding these two hypotheses have been contradictory. To reconcile the two seemingly contradictory hypotheses, we assessed the phylogenetic relationship of a highly widespread and invasive species, Anthemis cotula L. (focal species), separately with the native species and all its co-occurring species (including native and non-native species) along an elevation gradient. Our results show that the abundance of A. cotula declined continuously with an increase in elevation and with an increase in the number of both all co-occurring and native species only. The phylogenetic distance between the focal species and the native species based on weighted mean pairwise distance (MPD_aw) declined with elevation as well as native species richness. Our study also revealed that soil nutrients strongly influence the abundance of A. cotula; however, their effect on the phylogenetic distance between the focal and other species is negligible. These findings suggest a strong role of microecological factors and spatial heterogeneity in the abundance distributions of invasive species and community assembly. Thus, it could be concluded that the phylogenetic relationship of A. cotula with co-occurring all or only native species varies with elevation and species richness, adding another layer of complexity to the resolution of Darwin's naturalization conundrum.