Agriculture Reference
In-Depth Information
with no earthworms at the University of Michigan Biological Station. Adjacent earthworm-popu-
lated areas seem to have no place for this beetle to live. If earthworm introductions were the only
thing that changed, there would be little cause for concern, but many interactions and ecosystem
processes can be altered profoundly, sometimes with clear negative effects (Hendrix and Bohlen
2002).
EFFECTS OF EXOTIC EARTHWORMS INVASIONS ON
ECOSYSTEM PROCESSES
In addition to issues concerning the population and community dynamics of exotic species of
earthworms, there are questions about the impacts of earthworm invasions on ecosystem processes.
Of particular importance are soil processes mediated by biological activities (e.g., litter decompo-
sition, nutrient mineralization), which may be susceptible to the intensified earthworm activities
characteristic of earthworm invasions.
Although exotic earthworm invasions have been reported for over a century, quantitative studies
of their effects on soils have been much more recent. In North America, invasions by European
lumbricids and Asian megascolecids have been reported north of Pleistocene glacial margins in
Canada and the United States, that is, in areas previously devoid of earthworms. One early study
by Buntley and Papendick (1960) described a large area in eastern South Dakota (approximately
3400 km
) where the soil profile, of a chernozem on Pleistocene deposits, had been disrupted by
earthworms. Soil horizon boundaries were obliterated, and the physical structure of the soil con-
sisted of casts and filled earthworm burrows to a depth of 90 cm. Organic carbon and nitrogen,
CaCO
2
, and clay distributions were also changed because of vertical translocation of these sub-
stances. The earthworm species involved was not indicated, but the area was probably previously
devoid of native earthworms. However, Gates (1967) reported that the species in question was
3
L.
terrestris
, which could have been introduced less than 100 years earlier with the first European
settlements in the area. He cited several examples of early settlers transporting fruit trees and other
horticultural materials during the westward migrations of the 19th century. In Idaho, there was even
an attempt to establish exotic earthworms into soils for use as fishing bait because no indigenous
species of earthworms were found in the region.
The time required for the extensive alteration of soils observed in South Dakota was probably
less than 100 years. More recent studies in New Brunswick and Minnesota demonstrated that only
a few years are needed for transformations of mor horizons into mull horizons in forests that are
invaded by several lumbricid species (Langmaid 1964; Alban and Berry 1994). Current research
in Minnesota (Hale et al. 2000) and Alberta (Migge et al. 2003) suggest that these lumbricid
invasions tend to proceed across the landscape in sequential waves, as noted in the section on
mechanisms of invasion. In the Minnesota instance, it began with
D. octaedra
, which consumed
the F and H layers; followed by
L. rubellus
and
Aporrectodea
spp., which mixed the organic and
mineral horizons; and finally by
which drew recent litter into its burrows and produced
middens and casts on the soil surface. The final result of this multistage invasion was a compacted,
bare mineral soil surface covered only by recent litterfall. Impacts of the loss of the O-horizon on
plant and animal populations in the forest floor have been reported (e.g., Maerz et al. 2001; Gunndale
2002).
Intensive experimental studies of
L. terrestris,
invasions in aspen and
pine forests in the Rocky Mountains of Alberta showed significant changes in structures of microbial
communities, decreases in microbial biomass, and increases in nutrient leaching rates in the O-
horizon in addition to physical disruptions of the soil profile. Declines in the total organic matter
content were also noted in response to the highest earthworm population densities (Scheu and
Parkinson 1994; McLean and Parkinson 1997, 2000a,b). Similarly, studies of undisturbed hard-
wood-forest patches in New York that were invaded by a variety of lumbricids revealed the
D. octaedra
and
Octolasion lacteum
