Agriculture Reference
In-Depth Information
that is easy to rear and genetically homogenous (Bouch 1992) can be fulfilled by using individuals
of this species. Heimbach (1984) was of the opinion that the sensitivity of
E. fetida andrei
is
sufficiently comparable with that of
in spite of the ecological differences
between these species. Heimbach (1992) obtained good correlations between median lethal con-
centration (LC
Lumbricus terrestris
) values from the artificial soil test (using this species) and a standardized field
test. Callahan et al. (1994) suggested that
50
may be representative of four other earthworm
species based on the concentration-response relationships for 62 chemicals and after applying the
Weibull function. However, all these species are, to a large extent, representatives of the same
ecological group, and extrapolation to earthworm species from other groups may be less successful.
As reviewed by Edwards and Coulson (1992), in 42 pesticide comparisons by various
researchers involving two to five earthworm species, there was no consistent toxicity relationship
among species. However, it must be kept in mind that the toxicity tests were seldom conducted
under identical experimental conditions. A factor of 10 was suggested to bring
E. fetida
in line
with the most sensitive species (Heimbach 1992), an approach that allows the continued use of
E. fetida
E. fetida
combines sensitivity, economic
importance, and ecological relevance, it can be seen as the selected earthworm species for routine
toxicity testing. Those who criticize
in many toxicity tests. In the sense that
E. fetida
as not representative of most other soil-dwelling
species, although generally valid, should keep in mind that a three-tier approach is customary
in risk assessment. The first tier is merely an indicator in the procedure, and the existing guidelines
are suitable for most risk assessments required for legislative purposes (C. Kula 1998). In chronic
toxicity tests as well as bioaccumulation studies, the ideal would be to include a representative
of each ecological type. The three main ecological types of earthworms (Bouch 1972) are the
endogeic, epigeic, and anecic species.
Sublethal effects on the growth and reproduction of earthworms have been measured in the
laboratory (Lofs-Holmin 1980; N.A. Martin 1986; Van Gestel et al. 1989; Helling et al. 2000),
but lack of sufficient comparative data on sublethal effects has made comparisons based on
species susceptibility impossible. Reproduction toxicity tests on
E. fetida
(Van Gestel et al. 1989)
have led the way in showing that this species could also be considered a standard laboratory
earthworm species for studying sublethal effects. Edwards and Coulson (1992) advocated a
program of cooperative research to compare the susceptibility of different species under identical
conditions at different laboratories. This has not yet materialized fully, although the database is
growing steadily. Edwards (2002) also advocates the use of soil microcosms and terrestrial model
ecosystems (TMEs) to do more holistic integrated studies that are more field relevant than single-
species tests.
There is clearly a need to distinguish between the purposes of the various earthworm toxicity
tests. Although
E. fetida
may be suitable for some tests, and there are also sufficient grounds to
extrapolate to certain other species and soils, it can never serve as the sole representative species
of the soil environment for assessing ecological risks. A much broader-based approach, although
less practical for regulatory purposes, is required if an in-depth understanding of the effects of
chemicals on soil ecosystems is required. In this context, the earthworm has only a limited
contributory role to play. Earthworm researchers in the field of ecotoxicology should therefore not
limit themselves to the requirements proposed for regulatory purposes but should follow a more
holistic approach in recognizing that earthworms are but one component of a very complex soil
environment, of which there is only a very limited understanding. Because
E. fetida
is a useful
organism for toxicity testing does not necessarily make it a useful biological indicator species in
the ecological sense or a more useful biomonitoring species.
E. fetida
ACUTE TOXICITY TESTING WITH EARTHWORMS
Acute toxicity studies are conducted to ascertain the total adverse biological effects caused during
a finite period of time following the administration of single, normally large, doses of a chemical.
