Midpoint attractor models resolve the mid-elevation peak in Himalayan plant species richness

The midpoint attractor models (MPA) of species richness integrate a unimodal environmental favourability gradient and neutral effects forced by geometric constraints and thus extend ecologically neutral mid-domain model. However, both alternative MPA algorithms assume that underlying environmental favourability peaks within the modeling domain. Here, we used elevational distribution data for 1054 plant species occurring in NW Himalaya to explore species richness gradients and MPA performance in species groups defined by biogeography, taxonomy and life form. MPA models achieved an excellent fit, but the two MPA algorithms produced contrasting estimates of midpoint attractor location, especially for species groups with richness originating in lowlands. Therefore, we propose a modification of the MPA model accounting for the environmental favourability peak outside the study domain to reflect these situations. Biogeographic origin was more decisive for midpoint attractor location than taxonomic or life-form classification, indicating relatively low climatic niche conservatism in plants.