Niche conservatism limits the distribution of <i>Medicago</i> in the tropics

The increase in species number from poles to the equator is one of the most fundamental patterns in ecology. Although several hypotheses have been proposed, there is a lack of consensus on the mechanisms underlying this pattern. While most hypotheses provide plausible explanation for high tropical diversity of tropical clades, it is unclear if similar mechanisms drive the diversity of extra-tropical clades. Here, we investigated the environmental drivers influencing the diversity pattern of a Mediterranean plant genus Medicago and dissect their effects across continents and biomes. We compiled a comprehensive dataset on the distribution of all Medicago species and mapped their distribution at the spatial resolution of 100 × 100 km2. We used generalized linear models to quantify the relative effects of environmental factors on the richness patterns of Medicago and its two life forms. Next, using geographically weighted regression, we explored the variation in richness–environment relationship across latitude. We found that Quaternary climate change and environmental energy were important explanatory variables, and their effects were consistent at global, continental and biome scales. However, we found contrasting effect of environmental energy in temperate and tropical regions, with species richness increasing with energy in the temperate and decreasing in the tropics. We also found strong affinity of Medicago species to its ancestral Mediterranean climatic niche. Our results suggest that the spatial variation in Medicago richness patterns may be strongly influenced by heat tolerance as opposed to the tropical clades that depend upon cold tolerance. This dependency on heat tolerance can be attributed to strong niche conservatism exhibited by Medicago species. Our findings provide new insights into the richness-energy hypothesis and suggest that high environmental energy may not necessarily yield high species diversity but may also lower species diversity, especially of extra-tropical clades towards the tropics.