Gunnera tinctoria invasions increase, not decrease, earthworm abundance and diversity

Invasive plants often modify soil biotic communities through changes in soil physicochemical characteristics or the amount and/or quality of litter inputs. We assessed the impacts of Gunnera tinctoria invasions on soil and the earthworm community, on Achill Island, Co. Mayo, Ireland. We compared replicated (n = 5) areas invaded by G. tinctoria with uninvaded semi-natural grasslands, as well as with areas subjected to mechanical removal or herbicide treatment. Modifications in physiochemical properties included lower soil temperatures and higher soil pH during the summer in invaded areas, yet little effect on C and N stocks, or soil moisture. Marked differences in litter were observed, however, with invaded areas having c. 20-fold higher (above-ground) litter input than uninvaded ones, as well as lower C:N ratio (17 vs. 29). This was associated with a significantly higher overall abundance and biomass of earthworms in invaded plots (375 individuals m–2, 115 g biomass m–2), compared to the uninvaded control (130 individuals m–2, 45 g biomass m–2), with removal treatments having intermediate values. Earthworm communities comprised 10 species, typical for Irish grasslands, dominated by the common endogeic species Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea rosea. Both earthworm species richness and Shannon diversity were significantly higher in invaded areas, but only in spring samples. Based on this new information, plant invaders may increase the abundance and diversity of earthworms, mainly due to much larger litter inputs, increased soil pH and possibly lower soil temperatures in the summer typical of Irish grasslands.