Increases in species richness cause decreases in phylogenetic diversity in Mediterranean and Temperate plant assemblages

The relationship between taxonomic and phylogenetic diversities remains underexplored. Our goal was to determine whether a causal link exists between species richness and phylogenetic diversity and to determine the direction of this relationship. We aimed to evaluate whether species richness determines the phylogenetic diversity in realized assemblages (the taxonomic determinant hypothesis) or, conversely, whether phylogenetic diversity determines the species richness (the phylogenetic determinant hypothesis). We also hypothesized that this causal framework could shift under different climates. We sampled over 1700 plant assemblages in grasslands and shrublands across three bioclimatic regions in Navarra, Spain, along a sharp and long climate gradient of 160 km. Using non-recursive structural equation modeling, we found that species richness causes phylogenetic diversity, and that this relationship remains consistently negative and is unaffected by climate differences among regions. Specifically, greater plant richness leads to reduced standardized phylogenetic diversity. This means that the incorporation of new species into assemblages involves adding closely related species in phylogenetic terms, regardless of the bioclimatic region. Despite the pronounced environmental differences between Mediterranean and Temperate vegetation types, an increase in species richness caused a decrease in phylogenetic diversity.