Testate Amoebae Like It Hot: Species Richness Decreases Along a Subalpine-Alpine Altitudinal Gradient in Both Natural <i>Calluna vulgaris</i> Litter and Transplanted <i>Minuartia sedoides</i> Cushions

Most groups of higher organisms show a decrease in species richness toward high altitude, but the existence of such a pattern is debated for micro-eukaryotes. Existing data are scarce and mostly confounded with the diversity of habitats that also decreases with elevation. In order to disentangle these two factors, one approach is to consider only similar types of habitats occurring across an elevational gradient. We assessed the diversity and community structure of testate amoebae in two specific habitats: (1) natural <i>Calluna vulgaris</i> litter and (2) <i>Minuartia sedoides</i> cushions 7 years after their transplantation along a vertical transect from 1770 to 2430 m in the subalpine and alpine zones of the Swiss Alps. Analyses of co-variance and variance showed that testate amoeba species richness, equitability, and diversity declined with elevation and were significantly correlated to habitat type. In a redundancy analysis, the variation in the relative abundance of the testate amoeba taxa in <i>Calluna vulgaris</i> litter was equally explained by elevation and litter pH. This is the first study documenting a monotonic decrease of protist diversity in similar habitats across an elevational gradient.