South China Botanical Garden Reveals a New Mechanism of Earthworm Invasion

Lumbricidae and Pheretimoid were the two most popular invasive earthworm groups in this world. It is notable that although the introduced Lumbricidae have distributed widely in northern China, the abundance declined significantly in southern China where many native earthworm of Pheretimoid dominated. This distribution pattern of earthworm may result from the combination effect of climate, habitat and species trait of earthworm. However, it is difficult to quantify the contributions of these factors separately through conventional approaches of invasion ecology; hence it is difficult to know to what extent the performance of invaders reflects the invasiveness or resistance of earthworm species.

A "third habitat" approach was proposed to deal with this puzzle by Dr ZHANG Weixin, a researcher in South China Botanical Garden (SCBG), under the direction of Professor FU Shenglei in SCBG and Professor Paul F. Hendrix in Odum School of Ecology at the University of Georgia, USA. That is to examine the interaction between Lumbricidae and Pheretimoid at an unfamiliar "third habitat" in which both earthworms were exotic. Theoretically, there was no long term co-evolution of species-habitat and thus the chance was equal for each species to encounter a suitable or unsuitable environment in such "third habitat".

In this context, the biological traits of earthworms will determine the fate of earthworm invasion. Interspecific interaction between two popular invasive earthworm species, i.e., one species of European Lumbricidae (Lumbricus rubellus) and one species of Asian Pheretimoid (Amynthas agrestis), was examined, and the contribution of species invasiveness and habitat invisibility on earthworm invasion were separated through the "third habitat" approach. The diet characteristics of earthworms were found to play key role on the species invasiveness, and Asian Pheretimoid outcompeted European Lumbricidae through its impact on soil biota (probably G+ bacteria) which was found to be involved in the litter-feeding process of earthworms. More concretely, on one hand, Pheretimoid preferred soil (including soil biota) and Lumbricidae preferred fallen litter; on the other hand, Pheretimoid was ready to adapt its dietary but Lumbricidae failed to digest as much litter as in normal situation when soil biota was depleted. It was the fact that the feeding process of Pheretimoid disrupted the essential relationship between Lumbricidae and soil microorganisms and consequently hindered litter consumption and ruined the life of Lumbricidae in the temperate northern American forest.

This study revealed a new mechanism of earthworm invasion, presented additional approaches in invasion ecology, and contributed to a greater understanding of the importance of interactions between multiple invading species. Importantly, it also provides some new dimensions to explore the underlying mechanisms of the strong invasive resistance of Asian Pheretimoid in natural forests of southern China. This work has been published online in Ecology, the leading international journal in its field (http://www.esajournals.org/doi/abs/10.1890/09-0979).