Agriculture Reference
In-Depth Information
by researchers. This is because there are many conflicting interests requiring wise compromises.
Good decisions on the protection of soil biodiversity and sustainable use of soils will be taken only
if a thorough appreciation of the role of earthworms and their interactions with other organisms in
soil exists.
CONCLUSIONS
Earthworms play an important role as engineers in many soils, in which they contribute to the
complex processes of organic matter decomposition and affect aeration, water transport, and soil
structure. Their protection from polluting chemicals is therefore important. The use of earthworms
for ecotoxicological evaluation has made considerable progress in the years since the first Interna-
tional Workshop on Earthworm Ecotoxicology was held in Sheffield in 1991. Two other international
workshops, in the Netherlands in 1997 (Sheppard et al. 1998) and Denmark in 2001, made
comprehensive recommendations to researchers and regulators. There is clearly a need to distinguish
between the purposes and use of the various earthworm toxicity tests. Although
may be
suitable for some toxicity tests and there are sufficient grounds to extrapolate such data to certain
other species and soils, it can never serve as the sole representative species of the soil environment
for the purpose of assessing ecological risks.
Acute toxicity tests with earthworms are useful, but they are designed to identify very toxic
chemicals that have immediate effects on earthworms and not to determine acceptable environmen-
tal concentrations. Such tests, however, can serve as a qualitative screen for detection of other
toxicological effects and give a first estimate of the NOEC level for continuous exposure to the
toxicant. Such tests do not necessarily absolve any chemical from further toxicity testing but do
assist in practice in setting priorities for further testing of sublethal effects. The OECD standard
artificial soil test was not developed to enable the direct extrapolation of toxicity results into field
situations.
Earthworm behavior is not often used in ecotoxicological studies because it is not fully
understood. Some field studies reported that earthworms can migrate over large distances in
response to environmental changes, although this is still open to debate; other studies demonstrated
avoidance behavior of earthworms when exposed to highly contaminated soils. The Denmark
workshop (2001) recommended that more attention be given to study earthworm behavior and
interpret avoidance responses and surface migrations.
The use of sensitive biomarkers in earthworm ecotoxicology represents a fairly new approach
that may yield promising results if a suitable battery of biomarkers can be employed to predict
toxic risks from pollutants at higher levels of organization. An important development is the
application of modern molecular techniques to identify biomarkers in earthworms. Heavy metal
responsive genetic indexes (Strzenbaum et al. 1998) may in the future improve existing toxicity
testing protocols. A number of molecular genetic techniques have already been utilized to identify
those genes responsive to certain types of chemical exposure. The value of biomarkers in risk
assessment has received increasing attention because of problems experienced when trying to relate
environmental concentrations of toxicants with their bioavailable fraction. The bioavailability and
toxicity of a chemical may differ considerably in laboratory tests compared with those observed
in the field. Biomarkers, on the other hand, respond only to the dose entering the animalÔs body,
and the idea of using biomarkers for more accurate quantification of contaminant exposure is
gaining support.
To obtain a balanced judgment on the potential toxicity and environmental hazard of a chemical,
a suitable databank of results from both laboratory and field experiments is needed. Because of
their labor demands and cost, complete field tests are rare, and results remain difficult to interpret.
The rates of recovery of earthworm populations, after exposure, are important in field tests.
Microcosm studies can contribute to a more field-relevant understanding of toxic effects of chem-
icals on earthworms. The use of earthworms as biological monitors to determine accumulations of
E. fetida
