Soil moisture enhances the vibrations induced by moving earthworms, especially of detritivores

Earthworms (Lumbricidae) are moisture-loving soil-dwellers that can be categorized into two feeding guilds with distinct behaviours: detritivores and geophages. Detritivores feed on litter fragments and geophages on smaller litter particles mixed with soil. When earthworms move within and across soil layers to forage, soil characteristics might influence the scale of these movements and their detectability to predators. To investigate the extent to which the detection of earthworm movements and the vibrations they thereby generate is affected by soil moisture, we used an innovative method to detect and quantify vibrations of moving earthworms emitted through the soil. We designed a mesocosm experiment in which we recorded, with a geophone, the movement-induced vibrations of detritivores and geophages in drier and wetter soils. Earthworm vibrations were detected best at 200 Hz and were detected significantly more in wetter than in drier soils. Moreover, in wetter soils, vibrations induced by detritivores were detected more frequently and at higher intensities than those of geophages. These findings suggest differences in activity levels between detritivores and geophages, as a function of soil moisture content. The observed activity patterns are likely to have food web consequences because earthworm consumers relying on mechanoreceptors to detect prey might detect detritivores more often, specifically in wetter soils.