Past climate and species ecology drive nested species richness patterns along an east-west axis in the Himalaya
Although global species richness patterns appear consistent across taxa and continents, patterns are elusive at smaller spatial scales. At regional/subcontinental scales, climatic, environmental and taxon-specific contingencies are likely to interact to modify general richness patterns. We develop a biogeographical paradigm for the Himalayan range as representative of regions at similar spatial scales, and where historical climate fluctuations might interact with species ecology to drive species richness patterns.
Himalayan range, Asia
We obtained a cell × species presence–absence matrix for babblers and murid rodents in 1° latitude × 1° longitude cells in the Himalayan range. We investigated nestedness in species richness patterns in these taxa along a distance gradient from the species-rich eastern towards the relatively depauperate west. We also investigated the relationship between species autecology and westward extent along the Himalaya. Climate data were obtained from published sources.
Himalayan babbler and murid assemblages are nested along an east–west axis, with assemblages in westward cells tending to be subsets of assemblages immediately to the east. Distance westward from the eastern Himalaya was related positively to altitudinal mobility of babbler assemblages, while body size increased with distance westward for murid assemblages.
The eastern Himalaya, which was not glaciated over during glacial maxima, was a potential refugium for babbler and murid species. Following glacial retreat, species could have recolonized the Himalaya westwards to different extents based on ecological traits (size, altitudinal migration) determining ability to deal with the more seasonal west. This produces both (1) a nested species richness pattern, and (2) correlations between ‘filtering’ autecological traits and distance. Such patterns should be replicated in other regions with historical climatic refugia; investigating nestedness along distance gradients from refugia would be a powerful tool in mapping biogeographical history, especially in separating historical effects from currently proposed energy-productivity relationships.