Earthworm δ13C and δ15N analyses suggest that putative functional classifications of earthworms are site-specific and may also indicate habitat diversity

Natural abundances of the stable isotope pairs 13C/12C and 15N/14N (δ13C and δ15N) were measured from earthworms sampled from six sites with contrasting habitats (deciduous and coniferous woodland, arable and permanent pasture). Knowledge about the function of earthworms is important to the understanding of their ecology. The hypothesis, that endogeic (primarily soil and organic matter feeders) and epigeic (surface litter feeders, ingesting little or no soil) earthworms would be isotopically distinct and that isotopic values for anecic (surface litter and soil feeders) earthworms would fall between the other two groups based on their feeding strategies, was rejected. Earthworm δ13C and δ15N values from six sites indicated that classifying earthworms into the functional groups epigeic, anecic and endogeic is site-dependent. In contrast, δ values clearly separated earthworms into humic formers and humic feeders. Average 13C-enrichment (3.9‰) between earthworm and putative dietary source (vegetation) across all sites was larger than the typically reported enrichment (1‰) between a single trophic level suggesting that earthworms, as expected, derive nutrition from a number of sources, not just living vegetation. Enrichments of 13C and 15N in earthworms, relative to diet, could be developed as a tool for assessing habitat diversity.