Glaciation as an historical filter of below-ground biodiversity
The latitudinal gradient in species richness is one of the most studied biodiversity patterns. Here we explore a north–south gradient in earthworm diversity, and evaluate the importance of current and historical filters in shaping the distribution of present-day below-ground species richness.
Using high resolution data on earthworm distributions across France, we document the latitudinal gradients in alpha (α), beta (β) and gamma (γ) diversity. We relate these gradients to species' traits, taxonomic aggregation and co-occurrence patterns, and correlate them with the present climate and the history of glaciation in Europe.
We found that γ-diversity decreases from south to north whereas α-diversity increases along the same latitudinal gradient. Communities in formerly glaciated regions are composed of smaller, more mobile species and show trait and taxonomical aggregation. In more southerly populations, which did not experience glaciation, earthworm species are larger, have smaller geographical ranges, and communities demonstrate a decrease in species co-occurrence resulting in lower local species richness.
We show that species richness gradients can present different – sometimes opposite – latitudinal trends depending upon the scale of the analysis. This scale dependence sheds new light on the underlying causes of global biodiversity gradients. The opposing latitudinal trends of the different components of diversity suggest that recolonization following glaciations during the Pleistocene acted as an environmental filter, and that competitive exclusion may be a more dominant ecological force in these former refugial areas. Overall our results show that past climate changes have left a deep footprint on present-day earthworm diversity patterns, from community to macroecological scales, and that different mechanisms of earthworm community assembly may predominate at different latitudes.