Response of temperate anecic earthworm individual biomass to species interactions

Earthworms contribute to a wide range of ecosystem services provided by the soil. Nevertheless, synecology of these organisms is still not properly elucidated especially in terms of species interactions. The aim of the present study was to determine the effects of anecic earthworm species interactions on their individual biomass. These effects were measured using three epi-anecic species, Lumbricus rubellus rubellus (Hoffmeister, 1843), Lumbricus centralis (Bouché, 1972), Lumbricus terrestris (Linné 1758), and three strict-anecic species, Aporrectodea caliginosa meridionalis (Bouché, 1972), Aporrectodea nocturna (Evans, 1946), Aporrectodea giardi (Savigny, 1826). Twenty-one pairs of individuals were established following five assemblages: monospecific pairwise assemblages of epi- and strict-anecic earthworms (2 × 3 treatments), bispecific pairwise assemblages within epi- and within strict-anecic earthworms (2 × 3 treatments) and bispecific pairwise assemblages with one epi- and one strict-anecic earthworm (3 × 3 treatments). Treatments were maintained in mesocosms for 30 days under controlled conditions (food provided at the soil surface at the beginning of the experiment) and changes in the earthworm individual biomass were measured. Strict-anecic earthworms in monospecific or bispecific assemblages maintained their initial biomass. In contrast, epi-anecic earthworms exhibited an increase of 12.4% and 23.7% of their biomass in monospecific and bispecific assemblages, respectively. In bispecific assemblages combining one epi- and one strict-anecic earthworm, epi-anecic earthworms solely gained biomass leading to a total increase of a 6.9%. Surprisingly, the biomass' changes were not homogenous within the two sub-categories as the six earthworm species exhibited species-specific responses. The greatest increases in individual biomass were recorded for epi-anecic earthworms in the bispecific assemblages. This study provides further evidence for the distinction between the two anecic sub-categories, as it indicates that species interactions is positive only for epi-anecic earthworm biomass.