Agriculture Reference
In-Depth Information
(1998) reported resistance to invasion by exotic earthworm species in Australian forests inhabited
by native species.
On the other hand, we have observed that the species
which inhabits the
riparian zone was absent from a Skunk River, Iowa, riverbank habitat near public access points,
and that the Asian species
Diplocardia riparia
(used as fish bait) was abundant. However, after
prolonged inundation during the summer floods of 1993, populations of
Amynthas hupeiensis
A. hupeiensis
were appar-
ently wiped out, and populations of
recovered. In this example, the native species was
better adapted to the natural disturbance regime of the riparian zone, but under current conditions
of flood control by reservoirs upstream (decreased disturbance frequency or intensity), the exotic
species can dominate.
A combination of factors is probably involved in the outcome of earthworm invasions when a
native earthworm fauna is already present (Hendrix and Bohlen 2002). Habitat destruction or
disturbance is almost always cited as a precursor to earthworm invasions, but it is difficult to say
whether it is a necessary prerequisite. Appropriate experiments have not been done to determine
the outcome of earthworm introductions to areas of primary vegetation with an intact native
earthworm community. Parallel experimental introductions could be made to primary vegetation
soils where earthworms have been removed and in sites with populations of native earthworms
intact. A related question is whether the rate of earthworm expansion into a new area is slower
(but greater than zero) when native species are present than when they are absent.
The reduction or elimination of native earthworm species is another factor or prerequisite to
invasion, but it is difficult to say if it is a necessary factor for successful earthworm invasions; most
examples suggest that it certainly helps (Kalisz and Wood 1995). The only factor clearly necessary
for successful establishment is the arrival of the exotic species, without which there can be no
invasion.
D. riparia
HOW DO INVASIVE EARTHWORMS INTERACT WITH
OTHER ORGANISMS?
Hawaii, which had no earthworms prior to human colonization, is now occupied by a variety of
exotic earthworm species (Nakamura 1990). In a massive invasional meltdown (Simberloff and
von Holle 1999), the invasive nitrogen-fixing tree
is increasing the amounts of nitrogen
cycling in the ecosystems and increasing earthworm populations by a factor between 2 and 10,
depending on site (Aplet 1990). In addition, feral hogs, when foraging for earthworms, are ripping
up the forest floor, creating more open sites for germination of the seeds of an exotic flora. The
best that can be said of the Hawaiian situation is that it creates an opportunity for humans to hunt
feral pigs.
There are several recorded examples of invasive earthworms altering the composition of soil
microbial and faunal communities. For example, McLean and Parkinson (2000a) reported that
populations of
Myrica faya
in pine-forest floors in Alberta significantly altered microfaunal abun-
dance and diversity in the organic horizon and in the mineral soil, increasing the number of the
faster-growing microinvertebrate taxa. Similarly, soil microarthropod communities were changed
by the presence of
D. octaedra
P. elongate
in pastures in Martinique (Loranger et al. 1998) and by that of
D.
octeadra
in pine forests in Alberta (McLean and Parkinson 2000b). In both instances, there were
positive and negative correlations between earthworm populations and measures of microarthropod
diversity and abundance; these effects were attributed to changes in physical structure of the soil
and organic layers.
In Minnesota, perturbation by introduced earthworms was responsible for the reduction of
populations of the endangered fern
(Gundale 2002), and interactions between
effects of earthworms and deer browsing dramatically reduced the forest-floor vegetation (Hale et
al. 2000). We have observed a large gallery-forming beetle larva in the thick, forest organic horizons
Botrychium mormo
