Unraveling the drivers of plant taxonomic and phylogenetic β-diversity in a human-modified tropical dry forest

Biodiversity maintenance in human-modified landscapes largely depends on spatial variations in species composition (beta-diversity), but the impact of human disturbance on beta-diversity remains poorly understood. We examined how taxonomic and phylogenetic beta-diversity of woody plant communities in the Brazilian Caatinga dry forest respond to two emerging threats-chronic anthropogenic disturbance and water scarcity. We separately assessed diversity metrics that give a disproportionate weight to rare species, and metrics weighted by dominant species. We recorded a total of 5118 individuals from 104 species across 19 0.1-ha plots into a 21,430-ha human-modified landscape exposed to chronic disturbances and a high variation in climatic water deficit. At the landscape scale, beta-diversity was higher when considering rare species than when focusing on dominant species, especially for the phylogenetic dimension of beta-diversity. Water deficit was the primary driver of plant taxonomic and phylogenetic beta-diversity, followed by the number of cattle and inter-site isolation; however, rare species seem to depend more strongly on these factors than the dominant ones. Therefore, preserving as much forest as possible, including areas exposed to different disturbance level and climate variables, is critical to prevent the loss of rare species and maintain the compositional differentiation of biotic assemblages. Such forest cover should be maintained in a large number of forest patches scattered through the landscape to preserve biologically and functionally distinct components of this ecosystem, avoid landscape-scale biotic homogenization, and thus favor its ecological resilience.