Assembly rules within earthworm communities in North-Western France—A regional analysis

This paper addresses the factors that constrain the assembly of earthworm communities in temperate land use systems. An occurrence matrix containing 44 localities and 20 species was build from data sets of different studies carried out in Haute Normandie (France). The whole data set included species lists from the three main landscape units and from a range of land use types from unmanaged forests and grasslands to intensive pastures and annual crops. We used species pool, multivariate (correspondence analysis) and null model analyse to explore how species occurring in a given community are sorted from the regional species pool. Special attention was paid to identifying non-randomness in co-occurrence and morpho-ecological patterns and to describing community patterns in response to habitat constraints. Twenty species of Lumbricidae were identified in the total species pool. We found no significant effect of landscape units on species richness and density, while land use type had a significant impact on both variables, with the lowest values in crops and forests, and the highest values in grasslands. Correspondence analysis highlighted how broad land use types, soil gradients and land use intensity influence patterns of community composition and ecological coherence. Non-random patterns in guild proportion were found in a few situations, with differences between grasslands and forests. This shows that environmental filters select species at specific scales according to particular ecological traits, resulting in a convergence of these traits between species of a given community. Local species richness never exceeded nine species and species co-occurrence was significantly lower than expected by chance and much lower than described in the literature for other taxa. Although no significant patterns of limiting similarity were observed at the scale of our study, we conclude that earthworm communities are also highly structured by competition. Our study thus agrees with the idea that both habitat and competitive constraints operate simultaneously to determine how community assembly takes place.