Unsustainable cattle load in alpine pastures alters the diversity and the composition of lichen functional groups for nitrogen requirement

Despite their considerable relevance as biodiversity hotspots, alpine ecosystems are presently threatened by several anthropogenic disturbances. Among these, pasture-derived nitrogen deposition is expected to significantly alter community composition and diversity of many sensitive organisms, such as lichens. We analysed patterns of γ-diversity components in epilithic lichen communities of pasturelands in the Maritime Alps (north-west Italy). Our aims were: (1) to estimate the shift in functional groups for nitrogen tolerance along a gradient of increasing cattle load; (2) to establish the cattle critical load. High cattle load reduced the species replacement and significantly increased the similarity in the oligotrophic component of the lichen communities. The oligotrophic component was the most sensitive functional group to nitrogen. Based on the relative abundance of oligotrophs we set the cattle critical load of the area at 0.12 grazing animals per hectare.