Environmental drivers of earthworm communities along an altitudinal gradient in the French Alps

Abstract

The study of elevational diversity gradients is a central topic in biodiversity research. In this study, we tested for the effect of climate, resource quality and habitat heterogeneity on earthworm communities along an altitudinal gradient and around the treeline in the French Alps. Earthworm communities and environmental properties (i.e. climate, soil properties and vegetation structure and composition) were sampled in six altitudinal stages from 1400 to 2400 m. Results were analysed through multi-table factorial analyses and structural equation modelling. We found average density, biomass and species richness in the range of what is usually reported in comparable ecosystems. We found no monotonic decrease in species richness along the altitudinal gradient, which we explain by the species pool being dominated by taxa with high environmental tolerance and dispersal capacities. Instead, we highlighted the ecotone associated with the treeline as the primary driving factor of earthworm communities: at 1800-2000m altitude, communities were more abundant and diverse, and had a greater variability in body mass. This result was largely explained by the structure and composition of the vegetation, whereas soil and climate appeared to have only indirect effects. Therefore, the treeline effect on earthworm communities can be explained both by the effect of environmental heterogeneity and of trophic resource quality which increases at the ecotone level.