Ask Mr. Smarty Plants

You are correct that there is concern about the ecological impact of exotic earthworm species in North America. Eisenia fetida is an import from Europe. Nick Proulx of the Minnesota Department of Natural Resources discusses the problem in Ecological Risk Assessment of Non-indigenous Earthworm Species. The major concern for exotic earthworms is for recently glaciated regions of the United States where native earthworms are lacking. In these northern deciduous forests the earthworms feed on the leaf litter and the concern is that these exotic earthworms cycle the leaf litter so quickly that it affects the soil's structure. This can result in far-ranging effects on many native plants and animals. For unglaciated regions of North America where there are native earthworm species, the main concern is that the exotic species will out compete the native species. One study in Kentucky showed native species occurring in undisturbed forest areas and exotic species occurring in disturbed areas. The bottom line is that in the far northern deciduous forests there is great concern over the presence of exotic species of earthworms because of their effect on the soils; while in the unglaciated southern regions, although there is some concern, the impact of the exotic species on soil processes is of minimal concern.

The study on the effects of exotic invasive earthworm species is ongoing—see the 2006 article Invasion of Exotic Earthworms into Ecosystems Inhabited by Native Earthworms by P. F. Hendrix et al. in Biological Invasions, Volume 8, pp. 1267-1800. The paper concludes (given limited data) that exotic earthworms do invade areas occupied by native species and this can occur even if there is no obvious disturbance. Sometimes the natives dominate and sometimes it is the exotics. In cases where the exotics are predominant, it may be because that they are able to use resources that the natives can't use. It is still uncertain as to how serious a threat the exotic species are to native species in the unglaciated regions.

The habitat and available food will define where exotic species can live. Eisenia fetida lives in surface soil and its food is leaf litter, microbes, and soil with high organic content. Hendrix and Bohlen (Hendrix, P. F. and P. J. Bohlen. 2002. Exotic Earthworm Invasions in North America: Ecological and Policy Implications. BioScience V. 52, no. 9, pp. 801-811) say: "Earthworms species from northern latitudes (e.g., European lumbricids and some Asian megascolecids) are poor colonizers in tropical or subtropical climates (except in localized temperate situations), and vice versa. For example, despite continued and widespread introduction throughout the United States, Eisenia fetida, the lumbricid 'manure worm' commonly used in vermicomposting, is not often found in natural habitats in the southern United States." So, unless your worm bin is situated in an area with lots of leaf litter and/or soil high in organic matter, any escapees from the bin should not be a serious problem. You can read more about the biology of Eisenia fetida The Science of Composting (from Cornell University).