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McLean and Parkinson 1997, 2000a,b); and in North American natural grasslands (Callaham and
Blair 1999; Callaham et al. 2001).
In Minnesota, the invasion by exotic earthworms consisted of several species arriving in
successive invasion fronts. First came the epigeic litter-dwelling species, followed by polyhumic
endogeic species and then slower-moving anecic and endogeic species. This phenomenon of
successive waves of invasion by earthworms depends on the presence of diverse ecological types
of earthworm, so it is not necessarily going to apply to all places. However, it demonstrates that
an earthworm invasion is not a simple monolithic process, and that it may require several years to
determine the outcome of an invasion of a single site. The species content of invasive waves are
in accordance with what we might predict given the life histories, ecology, and reproductive
potentials of the various ecological categories of earthworms.
On the other hand, sometimes the results of the initial invasive wave, such as that of epigeic
species (
), have proven different in places like Minnesota (Gundale 2002;
Hale et al. 2000) and Alberta (Scheu and Parkinson 1994; McLean and Parkinson 1997). In the
former area, little impact of the invasion on soil horizons was noted, whereas in the latter, there
were significant effects on the soil strata. In Minnesota, the main effects of invasive earthworms
on soil organic horizons occurred after the arrival of
Dendrobaena octaedra
Lumbricus rubellus
in the second wave of
invasion. In some New England forests, the arrival of
is now causing total destruction
of the organic horizon in spite of centuries of opportunity for European earthworms to do the same.
When earthworms arrive in a forest with soils with thick organic horizons but previously devoid
of earthworms, there is a large and readily available supply of food. Years later, the amount of
organic matter per square meter has probably diminished significantly or, if not, it has been
transformed profoundly and mixed with the mineral soil. In either case, negative consequences
would be expected for the early invading species that took advantage of the mass of surface organic
matter, with more favorable conditions occurring for endogeic species. Anecic species might be
less affected, except as small juveniles, because the larger earthworms can pull surface organic
material into their burrows. Nevertheless, populations may be limited by food availability at some
point because the entire litter fall is usually consumed annually.
Such long-term changes in the total food supply available should mean that populations of
some earthworm species would go through peaks and declines during the course of the invasion
and then reach something approximating a steady state or equilibrium, because the earthworms
have to live off current food income rather than the organic capital accrued prior to their arrival.
A. hilgendorfi
HOW DO INVASIVE EARTHWORMS INTERACT WITH
NATIVE EARTHWORMS?
When exotic earthworm species arrive at a site already occupied by a native earthworm fauna, there
could be several other things occurring. We have noted the replacement of native earthworm species
by exotic species in Illinois, California, and South Africa, but this was mainly in urban and
agricultural areas, so it is a different situation from similar replacements in undisturbed habitats or
in less-disturbed systems such as second-growth forests, where earthworm invasions are unlikely
(Kalisz and Dotson 1989).
In some disturbed habitats, native and exotic species coexist (Fragoso et al. 1995, 1999;
Callaham and Blair 1999; Bhadauria et al. 2000); in others, the exotic species dominate. Stebbings
(1962) suggested that exotic earthworm species may be out-competing the native species on some
forested sites in the central United States, but Kalisz and Wood (1995) suggested that replacements
might not occur in at least some minimal area of undisturbed habitats. Lavelle and Pashanasi
(personal communication) reported that
did not invade adjacent primary Peruvian
forests from agricultural land, where it was well established. The primary forest had an endemic
earthworm fauna that was absent from the land that was converted to agriculture. Dalby et al.
P. corethrurus
