Let earthworms be functional - Definition of new functional groups based on their bioturbation behavior

Abstract

The three main ecological categories of earthworms (anecic, endogeic, epigeic) are often used as proxies for functional groups. This is troublesome since they were not designed for this purpose and thus the relevance of such a use was never tested nor proven. How earthworms influence the different soil functions is tightly linked to their bioturbating behavior and thus they are considered as physical ecosystem engineers. Here, we characterized the different facets of this behavior (burrow creation, production of casts below and above ground, and litter consumption) for 50 species or subspecies of European earthworms under standardized laboratory conditions. Using PCA and K-means methods, we defined six new functional groups of earthworms, i.e. intense tunnelers, burrowers, shallow bioturbators, deep bioturbators, litter dwellers and intermediates. Intense tunnelers and burrowers build continuous and vertically oriented burrows, the first however burrow more and consume less litter. Deep bioturbators live deeply in the soil, make very discontinuous burrows and consume no litter. They could be seen as a novel group but they can be linked to the hypo-endogeics ecological categories defined 50 years ago. Litter-dwellers and shallow bioturbators can be respectively linked to the epidegeics and endogeics ecological categories. However, the new groups only partially overlap with the traditional ecological categories. We assume that their use, based on quantitative behavioral assessments, will improve the study of the relationships between earthworm communities and the associated provision of ecosystem services by minimizing the overlooked variability within the three main ecological categories regarding their behavior.