Agriculture Reference
In-Depth Information
forests in central to northern European countries may represent the very long-term postglacial end
points of lumbricid invasions; similar situations might occur in tropical ecosystems (e.g., Fragoso
et al. 1999). Such studies should help create predictive models of earthworm invasion dynamics,
which would be useful in developing improved management strategies.
A second approach to managing earthworm invasions is human intervention. It is a well-known
tenet in invasion biology that it is much easier to prevent an invasion than to stop one in progress.
Therefore, the first step is the one most often used by governments, that is, interception and
quarantine at the borders. As reviewed by Hendrix and Bohlen (2002), some countries have specific
regulations concerning the importation of earthworms; others do not. Canada, for example, allows
earthworm imports only from the Netherlands (
) and the United States (i.e., only species
that are known already to occur in Canada). This represents a Ñclean-listÒ or Ñguilty-until-proven-
innocentÒ approach to invasive species control (Reichard and Hamilton 1997; Mack et al. 2000).
The converse or Ñinnocent-until-proven-guiltyÒ appears to be the default approach followed by
many nations, including the United States, where current regulations regarding earthworm imports
are based on the Federal Plant Pest Act, under which the U.S. Department of Agriculture Animal
and Plant Health Inspection Service (APHIS) controls imports of soils that might carry pathogens.
In the absence of any pathogens, there are no specific considerations of earthworms as invasive
organisms, although this situation may be changing as APHIS develops new guidelines (Hendrix
and Bohlen 2002).
There is a rich literature base on the invasion biology of plants and insects of economic
importance (Simberloff 1989; Mack et al. 2000), but invasions by soil invertebrates have not been
well studied (Ehrenfeld and Scott 2001). There are some precedents in the Formosan termite, fire
ant, and Japanese beetle invasions in the United States and terrestrial flatworm invasions in Europe
and Australia; we may at least learn what not to do from these case studies. However, invasions
by more cryptic and less-mobile earthworms appear to be qualitatively and quantitatively different
from those by most other invertebrates and may be more similar to plant invasions than to those
of other animals (di Castri 1991). Invasions by terrestrial planarians that attack earthworms (Boag
and Yeates 2001) may be the best model for understanding and controlling earthworm invasions
because of similarities between these groups with respect to ecology, life history, and modes of
transport.
Control or mitigation of earthworm invasions after they occur has received very little attention,
probably because it is a daunting proposition. As anyone knows who has sampled earthworm
populations in the field, removing them from soil is often very destructive to soils and almost never
L. terrestris
100% effective, even in small plots. Removal of invasive earthworms over large areas is probably
not feasible. Therefore, the best approach may be containment of exotic earthworm populations to
areas where they already exist or, at least, a reduction of rates of dispersal from such areas into
surrounding ecosystems in which the earthworms may have adverse effects. Physical barriers, such
as buffer zones of unsuitable habitat around agricultural sites, might impede earthworm migration.
Also, simple practices, such as not dumping fishing bait on stream banks or cleaning horse hooves
or off-road equipment tires before transporting them into remote areas, might reduce the likelihood
of earthworm invasions. In addition to experimental studies on the effectiveness of preventive
measures, public awareness campaigns, e.g., those by conservation and outreach groups such as
Minnesota Worm Watch (
http://www.nrri.umn.edu/worms
) could help reduce the flow of propagules
of earthworms as well as those of other exotic species. Ultimately, regulations at state, provincial,
or regional levels may be needed to prevent the transport of exotic earthworms into remote or
particularly sensitive areas.
Finally, as discussed by Hendrix and Bohlen (2002), more basic knowledge is needed in terms
of the natural history and ecology of invasive earthworms, both in their native habitats and in
ecosystems in which they have invaded and had significant impacts. Which factors control earth-
worm populations under natural local conditions? Which characteristics of the organisms and of
the habitats have contributed to successful invasions and to earthworm invasion failures? How
