Increases in Species Richness Lead to Decreases in Phylogenetic
Diversity in Mediterranean Species Assemblages
Abstract
The relationship between taxonomic and phylogenetic diversity remains
underexplored. Our goal was to determine whether a causal link exists
between species richness and phylogenetic diversity. We wanted to
evaluate whether species richness determines the phylogenetic diversity
in realized assemblages (taxonomic determinant hypothesis) or
phylogenetic diversity determines the species richness (phylogenetic
determinant hypothesis). We also hypothesize that this causal framework
could shift in different bioclimatic regions. We sampled over 1700 plant
assemblages in grasslands and shrublands across three bioclimatic
regions in Navarra, Spain. Using non-recursive structural equation
modelling, we found that species richness influences phylogenetic
diversity, and that this causal relationship remains consistently
negative and is unaffected by climate differences among regions.
Specifically, greater plant richness leads to increased phylogenetic
convergence, resulting in reduced 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.