Recent increase in earthworm abundance in Mid-Atlantic deciduous forests

Background/Question/Methods

Over 90 % of the forests in the Mid-Atlantic region are secondary forests at different successional stages. Soil macrofauna in these forests is often dominated by non native European and Asian earthworms, but native species can also be present. In 1999 we conducted a survey on the earthworm fauna at the Smithsonian Environmental Research Center (SERC), Maryland, USA. We found significantly higher earthworm abundance in young (70 yr) than in old (150 yr) forests. Species composition was also different. In addition smaller, uncut forest patches have not been invaded by earthworms. The objective of this study was to assess if this pattern holds ten years later. Between 2006 and 2009 we conducted another assessment, which included resampling the original plots and adding new ones. Earthworms were sampled using formalin extraction from three randomly selected 50 cm X 50 cm quadrats at each site. In 2009 samples were taken in spring, summer and fall.

Results/Conclusions

Among the eight species recorded, only Eisenoides loennbergi is native, whereas others are cosmopolitan with European origin. Species compositions in old forests differ from that in young ones by the presence of E. loennbergi and the absence of Aporrectodea rosea, Allobophora chlorotica, and Octolasion lacteum. A recently reported invasive species, Lumbricus friendi, is recorded from all sites.

Earthworm densities were significantly lower in old forests than in young ones in 2006, but this pattern was less evident in samples taken after that. In 2009, earthworm densities were lowest in the summer with the lowest ratio of mature to total individuals and the possible absence of Aporrectodea caliginosa. In the present sampling we recorded higher densities, ranging from 75 to 305 and from 96 to 467 individuals per square meter in old and young forests respectively, than ten years ago, and the difference between young and old forests is diminishing. The uncut sites are still earthworm-free. We conclude that the earthworm fauna in the SERC forests is stable in terms of community composition but dynamic in terms of abundance.