Agriculture Reference
In-Depth Information
by the badger population was estimated as less than 5% of the total earthworm biomass available,
and the overall effect on the earthworm population was deemed negligible. The red fox showed
similar foraging behavior in relation to
L. terrestris
and can have a catch rate of up to 10 earthworms
per minute (Macdonald 1983).
Invertebrate predators of earthworms include centipedes (Chilopoda) and ground beetles (Car-
abidae). On the basis of an exclusion experiment, Judas (1989) concluded that birds, shrews, and
rodents did not affect earthworm abundance in a German beechwood, but chilopods and carabids,
at the population densities recorded for the study area, could potentially have a significant impact
on the earthworm population.
Evidence available does not suggest that predation has a major long-term influence on earthworm
population dynamics in most habitats, although short-term population reductions may sometimes
occur following periods of intensive feeding by birds such as golden plover. A notable exception is
the case of the New Zealand flatworm (
Arthurdendyus triangulatus),
which has recently become
established in Ireland and Scotland and other areas in northwestern Europe, where it is potentially
capable of causing severe reductions in earthworm populations and bringing about major changes in
community structure (Blackshaw and Stewart 1992; Blackshaw 1995; Boag and Yeates 2001).
PARASITISM AND DISEASE
A wide variety of parasitic and pathogenic organisms have been recorded from earthworms (Lee
1985; Edwards and Bohlen 1996); these include bacteria, fungi, protozoans, rotifers, platyhelminths,
nematodes, mites, and dipterous larvae. Many are parasites of earthworm predators, and in some
cases, earthworms are secondary hosts in which the parasites complete part of their life cycle. The
effects of many of these organisms on their earthworm hosts are not well understood and are
generally not considered significant, but some are known to be harmful. Larvae of the cluster fly
(
Pollenia rudis
) parasitize and kill lumbricids in Europe and North America (Walton 1928 as cited
by Lee 1985); Kingston (1989) attributed
,
at least partially, high summer mortality in
A. caliginosa
and
L. rubellus
to larvae of
Calliphora dispar
in irrigated pastures in Tasmania. Earthworms in
nonirrigated land were not affected similarly because the earthworm population was isolated from
the parasite by being quiescent 12 to 20 cm below the soil surface.
An anoetid mite,
Histiosoma murchiei
, was reported to parasitize cocoons of
A. chlorotica
and
to a lesser extent
Eiseniella tetraedra
in Michigan (Oliver 1962); the mites feed on and destroy
the developing earthworms. Anoetid mites are common in wet habitats, and infection rates in
A.
chlorotica
cocoons were consistently around 40% in a heavily forested area subject to spring floods.
This parasite has also been reported from a peaty, poorly drained pasture in the Str¾dam nature
reserve, Denmark (Gjelstrup and Hendriksen 1991), where about 20% of the cocoons of
A. calig-
inosa
and about 7% of those of
L. terrestris
were parasitized.
The potential impact of
Bacillus thuringiensis
(Bt) toxin released into the soil by genetically
modified plants is a cause for concern. Preliminary studies with Bt maize have not indicated adverse
effects on populations of
Lumbricus terrestris
or other selected beneficial soil organisms (Saxena
and Stotzky 2001); however, further studies are needed to evaluate the effects on a wider range of
taxa and on soil biodiversity generally.
LAND MANAGEMENT
Human activities can drastically alter the soil environment and influence earthworm populations
directly by physical disturbances and indirectly by altering the physicochemical environment and
the food supply. Some of the main ways in which activities such as mining, deforestation and
afforestation, grassland management, arable cropping, pesticide use, and water management may
influence earthworms are reviewed briefly.
