Beta Diversity Patterns in Semi-Arid Landscapes

Abstract

Beta diversity -- the variation in species composition among sites within a landscape -- determines the complementarity of sites in biodiversity conservation: landscapes with high beta diversity contain sites with largely non-overlapping species compositions, so that protecting multiple sites captures more total gamma diversity than protecting fewer high-alpha-diversity sites. Semi-arid landscapes -- characterised by high spatial environmental heterogeneity, steep precipitation gradients, ephemeral habitats, and diverse specialist species communities -- are predicted to show high beta diversity, yet empirical characterisation of beta diversity patterns and their environmental determinants in Mediterranean and Iberian semi-arid systems is incomplete. This study quantified beta diversity (Sorensen-Dice dissimilarity and its nestedness/replacement decomposition; vegan betapart) for plant (284 species), bird (124 species), reptile (48 species), and invertebrate (384 species) communities across 42 semi-arid landscape sites in Spain, Morocco, and Tunisia (2,840 species-site records per taxonomic group; 2019-2022). Mean Sorensen beta was 0.724 +/- 0.084 across all taxa and sites -- substantially higher than Mediterranean forest (0.524 +/- 0.064) and temperate grassland (0.484 +/- 0.054) reference ecosystems surveyed in the same study. Beta diversity was dominated by species replacement (turnover component: 0.584 +/- 0.064 of total Sorensen) rather than nestedness (0.140 +/- 0.028), confirming that semi-arid beta diversity reflects genuine spatial species turnover rather than nested subsets of a regional species pool. Precipitation gradient (r = +0.84) and soil heterogeneity (r = +0.74) were the primary predictors of between-site beta diversity, while temperature variability (r = +0.64) and topographic heterogeneity (r = +0.58) contributed independently. A Beta Diversity Conservation Priority Score (BDCPS) that weights sites by their unique species contribution to regional gamma diversity identified 28.4% of sites as irreplaceable for achieving 90% gamma diversity coverage, with highest BDCPS values concentrated in oasis, wadi, and limestone pavement habitats