Agriculture Reference
In-Depth Information
indicators of contamination, and for regulatory toxicity testing, similar to developments in the
aquatic field.
Earthworms play an important role as ÑengineersÒ in many soils, in which they contribute to
the complex processes of organic matter decomposition and affect aeration, water transport, and
soil structure. Van Hook (1974) suggested that earthworms could serve as useful biological indi-
cators of contamination because of the fairly consistent relationships among the concentrations of
certain contaminants in earthworms. They can also play a valuable role as
sentinels for
assessing biological risks from toxic components in terrestrial environments (Morgan 1992). As
earthworms form an important component of the soil biota, especially because of their role in soil
fertility, their protection is also important.
Because earthworms are common in many soils, they are vulnerable to chemical and physical
impacts on soils. Although earthworms are by no means the only important soil-dwelling inverte-
brates, they are selected for assessment of environmental risk for various reasons. They are widely
distributed and beneficial in promoting soil fertility and serve as food for a variety of animals. Not
only are they general representatives of the soil fauna, but also they are practical to breed and use
in both laboratory and field toxicity tests. They are convenient to handle because of their relatively
large size and can reach a relatively high biomass in some soils. Many species are suitable for
captive breeding, which makes them readily available. Their behavior brings uncontaminated
individuals into close contact with the soil, and they have relatively short life cycles, allowing long-
term studies of successive generations. They are therefore very suitable for genetic studies of
adaptive responses and could, for example, be used to evaluate the genetic erosion hypothesis by
quantifying genetic variation in toxicant-stressed populations (Van Straalen and Timmermans 2002).
New trends in environmental toxicity testing have not rendered earthworm testing obsolete.
The use of aquatic toxicity tests for predicting the toxicity of soil samples (Glli et al. 1994) was
a useful development that was also followed by the pore-water concept (Van Gestel 1997). This
method extracts an aqueous leachate from the soil, which is then bioassayed aquatically. However,
it does not replace the need for actual tests with soil-dwelling organisms because compounds that
are not sufficiently water soluble or adsorb very strongly on soil particles cannot be evaluated by
aquatic bioassays.
The use of earthworms for ecotoxicological evaluation has undergone considerable progress since
the first International Workshop on Earthworm Ecotoxicology was held in Sheffield in 1991. Two
further international workshops, in the Netherlands in 1997 (Sheppard et al. 1998) and Denmark in
2001, have made comprehensive recommendations to researchers and regulators; these are discussed,
but not repeated in detail here. Various new approaches have been adopted to refine and simplify
procedures, which could lead to more ecologically relevant information because ecotoxicological test
methods for soils are relatively underdeveloped (Van Straalen and Van Gestel 1993). However, various
problems remain, and although some have been addressed by adopting compromises, there is an
urgent need to develop acceptable approaches to field testing and the use of biomarkers. Some of
these new approaches became evident at the Seventh International Symposium on Earthworm Ecology
(ISEE), which took place in Cardiff, Wales, in 2002. This chapter attempts to discuss the latest
approaches to the various problems associated with using earthworms in toxicity testing and to explain
new routes of investigation, with some emphasis on the contributions to the International Workshops
in situ
on Earthworm Ecotoxicology (see also
Chapter 17
this volume).
THE TEST ORGANISMS
Single-species tests have often been criticized severely for their lack of realism in terms of their
modes of exposure and for difficulties in extrapolating such results to field conditions (Edwards
2002). There have been doubts expressed about using
as a ÑtypicalÒ earthworm in
single-species toxicity testing. However, in acute toxicity testing for regulatory purposes, the
position of this species seems to be well established. The basic requirement of finding a species
Eisenia fetida
