Consequences of anecic earthworm removal over 18 months for earthworm assemblages and nutrient cycling in a grassland

Earthworms are recognised widely for playing important roles in soil functioning, but few studies have attempted to assess the effects of separate functional groups under natural field conditions. We investigated the effects of selective removal of large anecic earthworms (primarily Lumbricus terrestris) over 18 months on earthworm assemblages, earthworm trophic ecology, and plant nutrient uptake in a temperate grassland. We used unenclosed field plots to simulate selective predation of large anecic individuals by alien flatworms and isotopically enriched plant material (13C and 15N) to trace nutrients. Though surface addition of plant material to plots increased the abundance and biomass of total and anecic earthworms, compared to control plots, earthworm composition was different and more variable where anecics had been removed. Most notably, in treatments receiving litter, abundance and biomass of the litter-feeding epi-anecic Lumbricus festivus and epigeic Satchellius mammalis were significantly greater where anecics had been removed. Addition of labelled plant material enriched individuals from all species in 13C and 15N, especially in litter-feeding epigeics. Similar abundances but altered isotopic compositions suggest that the removal of anecics influenced the feeding activities of other earthworm species. In particular, the soil-feeding endogeic Aporrectodea caliginosa was less enriched where anecics had been removed, suggesting that this species benefits from anecic surface foraging activity. Individual L. terrestris tended to be less enriched isotopically in the removal treatment, probably reflecting re-colonisation from outside litter addition plots. There was no effect of anecic removal on 15N uptake into above-ground biomass of each of three plant functional groups, though there was a trend of greater enrichment in removal plots. Taken together, these findings provide novel evidence, from a real field setting, that low-level reduction of anecic earthworm populations (experimental removal of 4 large individuals per 1 m2 plot over 18 months) can affect other earthworm species in terms of their abundance and trophic relations.