Spatial organization of earthworm assemblages in pastures of northwestern France

Studying spatial patterns of soil engineers has become an important issue that may contribute to a better understanding of soil functioning. We investigated the spatial patterns of earthworm species assemblages in a recently settled and an old temperate pasture. Earthworms were sampled following a spatially explicit sampling design. Data were analysed using Spatial Analysis by Distance IndicEs methods to describe the main characteristics of earthworm spatial patterns: aggregation index, patch and/or gap number and size, and the frequencies of species association or dissociation. Ten lumbricid species composed the earthworm assemblages in both pastures, some of them displaying a spatial distribution characterized by clusters with areas of patches and gaps. By comparing aggregation indices between the pastures, an increase of the spatial organization level in the community was observed in the old pasture. We also observed differences between the pastures in the number of aggregated species, cluster characteristics, ratio between spatial association/dissociation and spatial overlap. We also found some significant relationships between species pairs that were already described in the literature, enabling us to discuss the possible nature of interactions. Our results suggest that earthworm spatial distribution and community assembly are likely driven by interspecific interactions at the local scale.