Unraveling the inverse latitudinal gradient: Environmental and geographic influences on plant diversity in South American coastal lomas

AbstractWater availability is widely recognized for its importance in shaping plant diversity gradients globally. However, even in arid ecosystems such as drylands, a remarkable diversity of plant species can persist. Yet, the environmental, topographic, and geographic drivers that have structured plant diversity gradients within drylands over evolutionary time, and their relative importance, are poorly understood, particularly for understudied ecosystems such as coastal lomas. Here, we evaluated the geographic distribution and drivers of taxonomic and phylogenetic diversity of plants across the coastal lomas of South America, which are isolated fog-fed vegetation oases within the Peruvian and Chilean coastal desert. We collated the most comprehensive data set to date of 72 coastal lomas, and used generalised linear models to assess latitudinal diversity gradients, and evaluated the relative importance of environmental, topographic, and geographic drivers of plant taxonomic and phylogenetic diversity. Notably, we found that both plant species richness and standardized mean nearest taxon distance (MNTD) decreased linearly towards the Equator, while standardized phylogenetic diversity (PD) and mean phylogenetic distance (MPD) exhibited non-linear relationships with latitude, peaking in the hyper-arid core of the Atacama Desert. Our results suggest that current climate, environmental heterogeneity, and geographical factors are the primary drivers of plant diversity. Plant species richness increased with cloud cover, slope, and area, and decreased with soil pH, while standardized PD increased with aridity, elevation, and the human footprint and decreased with distance to the coast. Standardized MNTD increased with elevation, and decreased with increasing area and temperature; standardized MPD increased with slope, and decreased with increasing aridity and distance to the coast. Our results suggest that environmental filtering is not the only macroecological process acting on plant diversity across coastal lomas, and has often been counterbalanced at different points in evolutionary time by factors associated with environmental heterogeneity, possibly reflecting the presence of climate refugia and diverse habitats that promote species coexistence. Also, our results offer empirical evidence supporting an inverse latitudinal gradient in plant diversity across coastal lomas, which emerges as a consequence of multiple factors that influence water availability, both directly and indirectly. Over evolutionary time, these factors may have significantly contributed to shaping the current structure and composition of coastal lomas.