Agriculture Reference
In-Depth Information
poisoning of vertebrate predators. It is therefore important not only to know what the earthworm
body burdens of any particular toxic chemicals are, but also to understand the various pathways of
metabolism and detoxification.
The use of organisms as indicators of toxicity implies that the organisms can indicate the
presence or absence of a particular chemical or environmental factor or condition. The presence
or absence of a particular species can tell us about the environment. Thus, the use of earthworms
as indicators of environmental contamination is a topic of considerable interest but of limited
practicality. Their uses as such indicators are limited because a variety of environmental influences
could be responsible for the observed condition of the earthworm population, making the estab-
lishment of causal relationships between the earthworm population densities and the degree of
pollution very difficult.
The term
has also been used loosely to refer to biological monitors. Biological
monitors (over and above their possible role as indicators) are usually available in abundance
throughout areas of study (Martin and Coughtrey 1982). They respond to changes in the degree of
pollution and may retain the pollutant progressively during the exposure period. Earthworms have
been used extensively in environmental monitoring, especially as biological monitors of heavy
metal and organochlorine insecticide pollution because they bioconcentrate these chemicals. A clear
relationship has been demonstrated between the concentrations of some metals in earthworms and
those in the surrounding soils. Concentrations of lead and cadmium in earthworms are related
closely to those in soil, making earthworms good monitors for at least these two metals; data on
others are mostly lacking or do not show such a clear relationship. The interpretation of results,
however, should take interspecific variations into account as well as soil characteristics and other
physical factors.
The use of earthworms as biological monitors to determine the accumulated concentrations of
pollutants in their tissue as an integrative estimate of the degrees of pollution is an attractive idea,
but in practice it is fraught with difficulties (Morgan et al. 1992). Closely related earthworm species
differ in the ways that they accumulate pollutants, and factors such as age, size, season, and diet
influence rates of accumulation. Furthermore, the possibility of genetic adaptation to selection
pressures complicates matters further. The use of slope-intercept plots (Morgan et al. 1992) can,
under certain circumstances, provide insight into the bioavailability of pollutants. To use earthworms
effectively as biomonitoring tools, it is important to assess the body burdens of any particular toxic
chemical and to have data on the various pathways of concentration metabolism and detoxification.
Chemical analyses over a wide range of doses and chemicals are an expensive undertaking, but
when the vertebrate toxicity of a chemical is relatively high, such residue analyses should be
undertaken, especially if the long-term persistence and the potential for bioaccumulation of a
pollutant is high.
Measurement of levels of pollutants in organisms indicates how much is present at a particular
moment in time but does not indicate dynamic fluxes or rates of metabolic degradation of the
pollutant. Considerable research has been undertaken on the uptake of trace contaminants into
earthworms, but no standard protocols exist for measuring bioaccumulation in terrestrial ecosystems
(Phillips 1993). More research along the lines of the modeling done by Connell and Markwell
(1990) for bioaccumulation of lipophilic compounds into earthworms is needed.
indicator
RISK ASSESSMENT USING EARTHWORMS
The objective of ecotoxicological risk assessment is to use all available toxicological and ecological
information to estimate the possibility and probability that some undesired ecological event will
occur (Wilson and Crouch 1987). These events or ecological end points (as opposed to toxicity
end points) are not confined to specific taxa, are clouded with uncertainties, and require an
ecosystem approach. The final estimated effect at the ecosystem level is usually expressed as a
probability, and the identification of critical ecological end points is of prime importance (Bartell
