Agriculture Reference
In-Depth Information
of living in mineral soil. Oligohumic species are those that feed mostly on greatly modified organic
matter in the soil and are virtually always unpigmented.
Most
species maintain deep burrows from which they emerge to feed on surface organic
matter. They usually are large bodied, have dark anterior pigmentation, and have lower reproductive
rates.
From this quick overview of earthworm ecological categories, it might be guessed that the
epigeic species are those most likely to be invasive, but this is not entirely true because some of
their habitat requirements can be restrictive. Many invasive species are endogeic, and these are
primarily of the poly- or mesohumic types. Some anecic species, such as
anecic
, are also
invasive. The reproductive potential of a species also plays some role in its potential to be invasive,
although it probably is less important than its ecological relations, which are intimately related to
the life history strategy of the particular species (r vs. K selection, for example).
A more obviously important factor in the ability of earthworms to colonize new habitats
successfully is the nature of the reproductive system. Although most species of earthworms are
obligately outcrossing hermaphrodites, some use other options. Some species may self-fertilize,
and others are parthenogenetic. In either of these cases, reproduction is or can be uniparental as
opposed to biparental. A single uniparentally reproducing earthworm can found a new population,
and it is clear that this has been important in establishing earthworms in new areas. For instance,
the invasive
L. terrestris
Octolasion
species are parthenogenetic, as are many of the invasive
Amynthas
and
Microscolex
. Uniparental reproduction, although not necessary
or sufficient to achieve successful invasiveness, is certainly a useful property for this purpose.
species and probably
P. corethrurus
MECHANISMS OF EARTHWORM INVASION AND THE
DYNAMICS OF INVADING POPULATIONS
We mentioned some of the mechanisms of earthworm invasions, especially those directly involving
human activity and the transportation of earthworms to new locations. In this section, we review
the available knowledge of earthworm invasion dynamics in earthworm-free habitats (typically,
higher latitudes subject to recent glaciation). There are still some unresolved questions about
situations in which an indigenous earthworm fauna is still present on a site where exotic species
have been introduced.
Invasions of agroecosystems by exotic earthworms are virtually the general rule, particularly
in North America, because native species capable of tolerating a less-buffered soil environment
and the frequent soil disturbances are uncommon. It is fair to say that the diversity of agriculture-
tolerant earthworms is to the total diversity of earthworms as the diversity of cultivated plants is
to the total diversity of plants. If it is not quite true yet, it seems likely that soon there will be two
or three major earthworm groups operating in temperate, subtropical, and tropical climate zones.
For this reason, and because the vast majority of earthworm research on agroecosystems has dealt
with exotic species, readers are referred to reviews of earthworm ecology in agroecosystems
(Edwards and Bohlen 1996; Baker 1998; Hendrix 1998; Lavelle et al. 1999). Here, we confine
discussion to earthworms in nonagricultural land.
In the temperate deciduous and mixed forests of North America, several research groups
have recorded profound changes in the structure of soils and the forest floor following the
introduction of exotic species of earthworms. The first publication on this subject was by Nielsen
and Hole (1964), who noted the conversion of podzols to a mixed mineral-humus soil in New
Brunswick, Canada. These authors saw nearly total destruction of forest floor litter, leaving only
the most recent leaf fall on the ground, in several locations in Michigan and Iowa. Similar
situations have been reported or are under investigation in Minnesota (Alban and Berry 1994;
Hale et al. 2000); New Jersey (Kourtev et al. 1999); Rhode Island, Pennsylvania, and New York
(Steinberg et al. 1996; Burtelow et al. 1998); Alberta, Canada (Scheu and Parkinson 1994;
