Life on a tropical planet: niche conservatism and the global diversity gradient

Aim The niche conservatism hypothesis proposes that species distribution patterns are, by and large, governed by ancestral climatic affinities. Here, we test this hypothesis by combining information on current diversity gradients among lineages, lineage initiation dates, and palaeoclimatic reconstructions. Location World-wide. Methods We test the niche conservatism hypothesis by comparing slopes of latitudinal diversity gradients among terrestrial and aquatic lineages derived from 343 studies from around the world. The prediction is that clades originating during warm periods should be very species rich in tropical regions, exhibiting a steeper richness gradient from lower to higher latitudes than clades originating during cold periods, which are expected to exhibit shallower latitudinal species richness gradients. Results Latitudinal gradients for clades that originated in warm climates are steeper and with a strong tropical affinity, whereas organisms originating in colder periods exhibit a shallower diversity gradient or no tropical affinity. Conclusions For a broad variety of plants and animals of both marine and terrestrial realms our results are consistent with the idea that higher diversities have arisen among tropical clades because the earth has been predominantly tropical throughout most of its history. Most clades radiated in tropical climates, with subsequent climate changes causing a retraction in distributions. Our study implies that global climate change by itself, even when developing over tens of millions of years, could have shaped the large-scale patterns of diversity prevailing on earth today.