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eliminated by living organisms. As will be discussed later, biomarker assays are
already making an important contribution to the recognition and quantification of
sublethal effects in ecotoxicology (see Chapter 4, Section 4.7).
In ecotoxicology, the primary concern is about effects seen at the level of popu-
lation or above, and these can be the consequence of the indirect as well as the
direct action of pollutants. Herbicides, for example, can indirectly cause the decline
of animal populations by reducing or eliminating the plants they feed on. A well-
documented example of this on agricultural land is the decline of insect populations
and the grey partridges that feed on them, due to the removal of key weed species by
herbicides (see Chapter 13). Thus, the toxicity of pollutants to plants can be critical
in determining the fate of animal populations. When interpreting ecotoxicity data
during the course of environmental risk assessment, it is very important to have an
ecological perspective.
Toxicity is the outcome of interaction between a chemical and a living organism.
The toxicity of any chemical depends on its own properties and on the operation of
certain physiological and biochemical processes within the animal or plant that is
exposed to it. These processes are the subject of the present chapter. They can oper-
ate in different ways and at different rates in different species—the main reasons for
the selective toxicity of chemicals between species. On the same grounds, chemi-
cals show selective toxicity (henceforward simply “selectivity”) between groups of
organisms (e.g., animals versus plants and invertebrates versus vertebrates) and also
between sexes, strains, and age groups of the same species.
The concept of selectivity is a fundamental one in ecotoxicology. When consider-
ing the effects that a pollutant may have in the natural environment, one of the first
questions is which of the exposed species/life stages will be most sensitive to it.
Usually this is not known, because only a small number of species can ever be used
for toxicity testing in the laboratory in comparison with a very large number at risk
in the field. As with the assessment of risks of chemicals to humans, environmental
risk assessment depends upon the interpretation of toxicity data obtained with surro-
gate species. The problem comes in extrapolating between species. In ecotoxicology,
such extrapolations are particularly difficult because the surrogate species is seldom
closely related to the species of environmental concern. Predicting toxicity to preda-
tory birds from toxicity data obtained with feral pigeons (
Columba livia
) or Japanese
quail (
Coturnix coturnix japonica
) is not a straightforward matter. The great diver-
sity of wild animals and plants, and the striking differences between groups and
species in their susceptibility to toxic chemicals cannot be overemphasized. For this
reason, large safety factors are often used when estimating environmental toxicity
from the very sparse ecotoxicity data.
Understanding the mechanistic basis of selectivity can improve confidence in mak-
ing interspecies comparisons in risk assessment. Knowing more about the operation
of the processes that determine toxicity in different species can give some insight
into the question of how comparable different species are, when interpreting toxicity
data. The presence of the same sights of action, or of similar levels of key detoxifying
enzymes, may strengthen confidence when extrapolating from one species to another
in the interpretation of toxicity data. Conversely, large differences in these factors
between species discourage the use of one species as a surrogate for another.
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