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elimination of the forest floor litter, reductions in carbon storage in soil, changed carbon:nitrogen
ratios, and reductions in fine-root biomass in the upper mineral soil (Bohlen et al. 2003a; Fisk et
al. 2003). The impacts of earthworm invasions on soil carbon dynamics in previously cultivated
forest patches were less, emphasizing the importance of the site history as a determinant of
earthworm effects of invasions on soil processes (Bohlen et al. 2003b). Interestingly, soil phosphorus
fractions were affected differentially by different earthworm species. The anecic species
L. terrestris
increased the total phosphorus in surface soils by transporting parent material from deeper soil
layers, and the epi-endogeic species
increased the extent of exchangeable phosphorus
and phosphorus leaching by consuming organically enriched surface materials and producing
phosphorus-enriched castings (Surez et al. 2003). Such results demonstrate the difficulty in making
generalizations about effects of earthworm invasions. Further research is needed under a wider
L. rubellus
range of climatic, edaphic, and land-use conditions and a larger number of invasive earthworm
species. Models of earthworm effects on soil process dynamics (e.g., Chertov and Komarov 1997)
may be useful in the search for broad-scale patterns of earthworm invasions.
Exotic earthworm invasions also have been reported in areas inhabited by native earthworms,
but their impacts on ecosystem processes appear to depend on the previous disturbance history of
the site, earthworm invasion pressure, and the degree to which the native earthworm assemblage
is intact (
Figure 5.1
). In highly disturbed soils, native earthworm populations are often reduced,
and invasions by the typical anthropochore species usually have significant effects on soil processes.
Observations have been made worldwide in agricultural and pastoral ecosystems, where introduced
European lumbricids have modified soil structure, rates of organic matter decomposition, nutrient
dynamics, and in some cases plant productivity, for example, in reclaimed polders in the Netherlands
(Hoogerkamp et al. 1983); grasslands in New Zealand and California (Stockdill 1982; Winsome
2003); and cropping systems in Australia and the United States (Parmelee et al. 1990; Baker 1998).
Similar impacts of invasive earthworms have also been reported in at least moderately disturbed
natural areas where native earthworm species are still present but probably not at natural abundances,
for example, in California chaparral (Graham and Wood 1991) and tropical forests (Fragoso et al.
1995; Liu and Zou 2002).
Where native and exotic earthworm species coexist, the magnitude of their effects on soil
processes may be determined by the relative abundance of the various species of earthworms,
ecological strategies, and environmental fitness of the dominant species. James (1991) suggested
that native earthworms were usually better adapted to local soil and climatic conditions and hence
could maintain longer periods of activity and have greater effects on nutrient dynamics in tallgrass
prairie soils than could invading European lumbricids. Lachnicht et al. (2002) reported that native
Estherilla
coexisted by partitioning the soil volume physically
in microcosms containing forest floor and mineral soil from tabonuco forests in Puerto Rico.
Interactions between the two species reduced the impact of
spp. and the exotic
P. corethrurus
P. corethrurus
on carbon and nitrogen
mineralization rates.
The dynamics and impacts of earthworm invasions in undisturbed ecosystems in which soil,
vegetation, and indigenous earthworm assemblages are intact have not been well studied. In such
instances, there may even be some degree of biotic resistance to earthworm invasions; hence, the
impacts on ecosystem processes in these situations may be different from those in areas that were
previously devoid of earthworms. This possibility needs further research.
WHAT CAN BE DONE ABOUT EXOTIC
EARTHWORM INVASIONS?
Many exotic earthworm species are now naturalized in areas beyond their place of origin, notable
examples being European lumbricids in temperate regions worldwide and
throughout
the tropics (Reynolds 1994; Fragoso et al. 1999). Although humans and their disturbed habitats are
P. corethrurus
