Environmental heterogeneity explains contrasting plant species richness between the South African Cape and southwestern Australia

Abstract Aim Given the importance of environmental heterogeneity as a driver of species richness through its effects on species diversification and coexistence, we asked whether the dramatic difference in species richness per unit area between two similar Mediterranean‐type biodiversity hotspots is explained by differences in environmental heterogeneity. Location The Greater Cape Floristic Region, South Africa (GCFR) and Southwest Australian Floristic Region (SWAFR). Taxon Vascular plants (tracheophytes). Methods Comparable, geospatially explicit environmental and species occurrence data were obtained for both regions and used to generate environmental heterogeneity and species richness raster layers. Heterogeneity in multiple environmental variables and species richness per unit area were compared between the two regions at a range of spatial scales. At each scale, richness was also regressed against these individual axes and against a major axis of heterogeneity, derived by principal component analysis (PCA). Results The GCFR is generally more environmentally heterogeneous and species‐rich than the SWAFR. Species richness per unit area is significantly related to the major axis of heterogeneity across both regions, the latter describing c. 38%–50% of overall heterogeneity, the slope of this relationship differing between the two regions only at the finest spatial scale. Multivariate regressions, and regressions against the first axes of the PCAs (PC1), revealed variations in the dependence of species richness on environmental heterogeneity between the two regions. Main conclusions Notwithstanding some region‐specific effects, we present evidence of a common positive relationship between floristic richness and environmental heterogeneity across the GCFR and SWAFR. This is dependent on spatial scale, being strongest at the coarsest level of sampling. The generally greater richness per unit area of the GCFR compared to the SWAFR is thus explained by the former's generally greater environmental heterogeneity and is concordant with its greater levels of floristic turnover.