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lethal dose in a single meal if they were feeding on cattle that had died within two
days of receiving of the drug. So, once again, a chlorinated compound that shows a
fair degree of persistence in vertebrates has been implicated in lethal poisoning and
population decline in the field. Speaking more widely, there continue to be cases of
gross misuse of pesticides and other organic pollutants in some parts of the world,
including on humans who use them. In India, for example, there are still reports of
organophosphorous poisoning of spray operators working in crops such as cotton. We
know little of the ecological consequences of the misuse of such pesticides.
The following sections will attempt to look ahead to likely future problems
with organic pollution, to probable changes in the ways in which it is studied and
monitored, and in the tests and strategies used for environmental risk assessment of
organic chemicals.
17.2 tHe adoPtIon of more ecoLogIcaLLy
reLeVant PractIceS In ecotoxIcIty teStIng
Currently, the environmental risk assessment of chemicals for registration purposes
depends on the comparison of two things: (1) An estimate (sometimes a measure)
of the environmental concentration of a chemical, and (2) an estimate of the envi-
ronmental toxicity of this chemical. Environmental concentration is difficult to esti-
mate, especially for mobile species in terrestrial ecosystems. In the present example,
the estimation of environmental toxicity is expressed as a concentration, which may
be a No Observable Effect Concentration (NOEC) or an LC
50
for the most sensitive
organism found in a series of ecotoxicity tests. Very seldom is the species used in
the toxicity test one of those most at risk in the natural environment. Nearly always a
surrogate is used, and there is the immediate question of species differences in sus-
ceptibility, which are largely unknown. In the case of birds, for example, two or three
species are used in ecotoxicity testing, whereas some 8700 species exist in nature.
For further discussion of these issues, see Chapter 6 in Walker et al. (2000), Calow
(1993), and Walker (1998b). Because of the high levels of uncertainty involved, the
estimate of environmental toxicity is divided by a large safety factor, commonly
1000. Following these computations, there is perceived to be a risk if the estimate of
environmental concentration (1) exceeds the estimate of environmental toxicity (2).
The limitations of this approach are not difficult to appreciate (see, for example,
Kapustka, Williams, and Fairbrother 1996). It is based on the approach to risk assess-
ment used in human toxicology and has been regarded as the best that can be done
with existing resources. It is concerned with estimating the likelihood that there will
be a toxic effect upon a sensitive species following the release of a chemical into the
environment. With the very large safety factors that are used, it may well seriously
overestimate the risks presented by some chemicals. More fundamentally, it does
not address the basic issue of effects upon populations, communities, or ecosystems.
Small toxic effects may be of no significance when it comes to possible harmful
effects at these higher levels of biological organization, where population numbers
are often controlled by density-dependent factors (Chapter 4). Also, it does not deal
with the question of indirect effects. As mentioned earlier (Chapter 14), standard
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