Endemic fish promote ecological structure in a tropical biodiversity hotspot

Endemic species enrich biodiversity hotspots, but how do they contribute to biodiversity structure at macroecological scales? Here, we argue that classifying endemic species using a framework defined by the complementary axes of taxonomic and functional diversity is key to revealing how these patterns underpin community convergence and divergence. These processes are known to configure communities to be compositionally more similar or dissimilar, respectively. Using the endemic freshwater fish communities of India's Western Ghats Escarpment (WGE), one of the 'hottest' spots of global biodiversity, as a test case, we find that geographically widespread, trait-distinct endemics are disproportionately present in the west-flowing basins of the WGE where they promote overall convergence (i.e. both taxonomic and functional). In contrast, among east-flowing basins, a lower-than-expected occurrence of the same category of species supports taxonomic divergence and functional convergence. We attribute this heterogeneity to western-flowing basins having higher (i) ecosystem productivity that supports trait-distinctiveness, and (ii) temporary lateral connectivity that facilitates fish dispersal. Our study demonstrates how different dimensions of diversity interact to produce ecological structure, thereby underlining their role in resilience. Thus, this framework has application in conservation and policy and can guide global efforts to protect endemic biodiversity in hotspots.