Species coexistence relies on resource partitioning, which shapes biodiversity across climates, latitudes and altitudes. Resource partitioning can occur via niche size or separation in the geographic range or ecological niche. While it promotes biodiversity, the impact of different partitioning strategies on species richness remains largely unexplored. We compare two ecosystems with similar climates and ages, the species-rich Paramo of the South American Andes and the more species-poor Afroalpine in the eastern African mountains. We test whether differences in geographic range and climatic niche partitioning explain variations in species richness. Therefore, we analyzed six lineages from the Asterales and combined phylogenomic data with occurrence records to estimate metrics of size and overlap for climatic niche and geographic range. We show that the Andean species have larger climatic niches than the African species, suggesting that niche size is not explaining higher species richness in the Andes. Instead, a striking pattern for species with overlapping geographic ranges emerged: the Andean species show less climatic niche overlap than the African species. We hypothesize that this increased niche separation among geographically overlapping species in the Andes compared to the eastern African mountains contribute to the species richness difference between these tropical alpine biodiversity hotspots.