Niche conservatism limits the distribution of Medicago 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.