Agriculture Reference
In-Depth Information
INITIAL
STATE
INTERMEDIATE
STATE
CURRENT
STATE
Disturbance
Exotic Invasion
Native
Earthworms
Eliminated
Exotic
Earthworms
Exclusively
A
Severe
Successful
Successful
1
Native
Earthworms
Diminished
"Pristine" System
(Native Earthworms
Exclusively)
Native and Exotic
Earthworms
Coexisting
Moderate
B
2
Successful?
Successful?
1
2
Native
Earthworms
Exclusively
Native
Earthworms
Exclusively
C
Minimal
Unsuccessful
FIGURE 5.1
Hypothesized pathways of exotic earthworm invasions in ecosystems inhabited by native earth-
worms. Pathway A is the extreme case leading to exclusively exotic assemblages, as often observed with
anthropochorous earthworms in agricultural soils; the same outcome may occur under less-severe disturbance
but perhaps with more aggressive exotic invaders via pathway B-1. Pathways B-2 and C-1 lead to the sometimes
observed co-occurrence of native and exotic species through varying levels of habitat disturbance and invasion
intensity; pathway C-1 suggests direct competitive displacement of native species by exotic species. Whether
co-occurrence is a stable condition or whether native species or exotic species maintain dominance is an
interesting long-term question, and hence the question marks are shown for successful invasion on these
pathways. Pathway C-2 suggests that native earthworm assemblages under minimally disturbed, native con-
ditions are resistant to invasion by exotic species. The alternative is best represented by pathway C-1, by
which forest fragmentation, for example, may foster exotic invasions without direct habitat disturbance.
(Modified from Hendrix et al. 2004.)
the cause, these peregrine earthworm distributions must still be considered expansions of range for
these species, albeit at very rapid rates in ecological and geological time. The important issues now
are how, at local-to-regional scales, the spread of invasive species into areas where they are not
wanted (e.g., remote or sensitive ecosystems) can be impeded and how, at national-to-continental
scales, introduction of new invasive earthworm species can be prevented.
The management of earthworm populations, whether invasive or not, has received considerable
attention in the context of agriculture and organic waste management, and there are numerous cases
of successful outcomes of such management, for example, enhanced plant productivity or acceler-
ated decomposition of organic wastes (Lee 1995; Edwards 1998; Lavelle et al. 1999). Indeed, there
probably would be no incentive to prevent or regulate earthworm introductions for such purposes.
The management of earthworm populations in the context of mitigation of invasions has only
recently emerged as a topic in need of development after it was brought to light by some of the
reports noted here of adverse effects in forested ecosystems.
There are at least two approaches to the management of invasive exotic earthworms. The first
is essentially to provide no management at all, allowing invasions to run their course and assuming
that, over time, invaded ecosystems will reach a new ÑequilibriumÒ under the influence of the newly
reassembled biotic community. This approach accepts any changes in soil characteristics, bio-
geochemical cycling rates, and above- and belowground biotic communities that are likely to occur.
In fact, this approach is in effect by default, in many areas where earthworm invasions are in
progress and where the changes are witnessed as they occur, as in some of the experimental studies
described here. Additional chronosequence studies, which compare sites at various stages of earth-
worm invasion from the beginning to equilibrium, might reveal a long-term progression in such
changes and whether the ultimate outcomes are acceptable in particular regions. For example,
