Environmental and geographic predictors of seed plant diversity across South American terrestrial coastal lomas

Understanding the predictors of plant diversity gradients remains a major challenge in biogeography, particularly in understudied arid ecosystems where water availability is highly constrained. Coastal lomas—isolated, fog-fed vegetation oases distributed across the Peruvian and Chilean coastal deserts of South America–provide a unique system for examining how climate, topography, and geography shape plant diversity under extreme aridity. Here, we evaluated the geographic variation and predictors of the taxonomic and phylogenetic diversity of seed plants in these environments. We compiled the most comprehensive data set to date of 71 coastal lomas and used generalized linear models to assess latitudinal diversity gradients. We also evaluated the relative importance of environmental, topographic, and geographic predictors of plant taxonomic and phylogenetic diversity. We found that mean phylogenetic distance (MPD) exhibited non-linear relationships with latitude, and standardized phylogenetic diversity (PD) showed a weak variation with latitude, with both peaking in the hyper-arid core of the Atacama Desert. In contrast, standardized mean nearest taxon distance (MNTD) and species richness (adjusted by area) showed no significant association with latitude. Our results suggest that current climate, environmental heterogeneity, and geographical factors are the primary predictors of seed plant diversity in these regions. Plant species richness (adjusted by area) increased with aridity and cloud cover, while standardized PD decreased with aridity and distance to the coast while increasing with human footprint and northness. Standardized MNTD also increased with human footprint and northness, and decreased with increasing aridity and temperature. Standardized MPD increased with slope, temperature, northness, and aridity, and decreased with cloud cover, isothermality, and distance to the coast. These results suggest that environmental filtering is not the only macroecological process acting on plant diversity across coastal lomas and is often counterbalanced at different points in evolutionary time by factors associated with environmental heterogeneity, possibly reflecting the presence of climate refugia and habitats that promote species coexistence. Over evolutionary time, these factors may have significantly contributed to shaping the current composition of coastal lomas. Plant diversity did not follow a simple latitudinal gradient across coastal lomas. Aridity predominantly explains variation in diversity facets, underscoring the importance of water availability in shaping coastal lomas. Evolutionary diversity reached its peak unexpectedly in the hyper-arid Atacama Desert. Plant communities were clustered into distinct assemblages that correspond to geographic barriers along the coastal desert. Coastal lomas reflect multiple ecological and evolutionary processes rather than environmental filtering alone.