Plant extinctions and introductions lead to phylogenetic and taxonomic homogenization of the European flora

Abstract

Human activities have altered the composition of biotas through two fundamental processes: native extinctions and alien introductions. Both processes affect the taxonomic (i.e., species identity) and phylogenetic (i.e., species evolutionary history) structure of species assemblages. However, it is not known what the relative magnitude of these effects is at large spatial scales. Here we analyze the large-scale effects of plant extinctions and introductions on taxonomic and phylogenetic diversity of floras across Europe, using data from 23 regions. Considering both native losses and alien additions in concert reveals that plant invasions since AD 1500 exceeded extinctions, resulting in (i) increased taxonomic diversity (i.e., species richness) but decreased phylogenetic diversity within European regions, and (ii) increased taxonomic and phylogenetic similarity among European regions. Those extinct species were phylogenetically and taxonomically unique and typical of individual regions, and extinctions usually were not continent-wide and therefore led to differentiation. By contrast, because introduced alien species tended to be closely related to native species, the floristic differentiation due to species extinction was lessened by taxonomic and phylogenetic homogenization effects. This was especially due to species that are alien to a region but native to other parts of Europe. As a result, floras of many European regions have partly lost and will continue to lose their uniqueness. The results suggest that biodiversity needs to be assessed in terms of both species taxonomic and phylogenetic identity, but the latter is rarely used as a metric of the biodiversity dynamics.

Fig. 1.

Changes in β_tax vs. changes in β_phyl among European regions (n = 253 pairwise comparisons). Current native flora = current natives species, excluding extinct species; current total flora = current natives plus alien species; original flora = native species including extinct species. Red areas indicate significant effect of both measurements (Fisher's paired comparison design test; P < 0.001). (A) Combined effect: extinctions of native species and invasions of alien species lead to taxonomic and phylogenetic homogenization [β_{tax (current total flora)} < β_{tax (original flora)} and β_{phyl (current total flora)} < β_{phyl (original flora)}]. (B) Extinct effect: extinctions of native species lead to taxonomic and phylo-genetic differentiation [β_{tax (current native flora)} > β_{tax (original flora)} and β_{phyl (current native flora)} > β_{phyl (original flora)}]. (C) Alien effect: invasions of alien species lead to taxonomic and phylogenetic homogenization [β_{tax (current total flora)} < β_{tax (current native flora)} and β_{phyl (current total flora)} < β_{phyl (current native flora)}].