Earthworm Species from Diverse Ecological Groups Negatively Affect Enchytraeid Density in a Forest Ecosystem

Earthworms and enchytraeids are two very important groups of soil organisms that influence soil biology and ecology, as well as physicochemical processes occurring in the soil. The interactions within and between these major groups of soil fauna are currently among the most pressing topics in soil ecology and are still insufficiently understood. In a field mesocosm experiment, we examined the effects of the density of the following five key earthworm species in Central Europe: Dendrobaena octaedra, Lumbricus rubellus, L. terrestris, Aporrectodea caliginosa, and Allobophora chlorotica. These species were selected to represent the following three major ecological groups of lumbricids: epigeic, endogeic, and anecic. The mesocosm experiment examined the effect of these species across density gradients and at two soil profile horizons-litter and mineral soil-and at the entire soil profile within the mesocosm. This comprehensive and unique approach was used to compare the effects of earthworm density gradient on enchytraeid abundance under identical conditions in the forest soil. The results indicate that all studied species of earthworm negatively affected enchytraeid density. The strength of this effect depended on the earthworm species, earthworm density, and the level of the analyzed soil profile. Epigeic L. rubellus, endogeic Aporrectodea caliginosa, and anecic L. terrestris appeared to be the most effective. Higher earthworm density had a greater effect on the density of enchytraeids. The most significant results concerned the litter layer, where the density of enchytraeids was highest due to their natural occurrence. Our results provide new insights into the ecological relationships among key groups of soil fauna and can be a starting point for predicting changes caused by earthworms in newly occupied ecosystems.