A SYNOPSIS O F THE PESTICIDE PROBLEM
113
have been due to factors other than pesticides. However, the results
are consistent with other studies, e.g. of Edwards et al. (1964), and
there is little doubt that the pesticide treatments caused the differences
observed. The results are also in accordance with what is known about
the effects of other forms of pollution. They are also in accordance with
probability: most ecosystems consist of species which range from the
very abundant to the very rare. If a pesticide had deleterious effects
on all the species present it would be likely to exterminate some of the
rarer species and so reduce diversity. Predators are necessarily rarer
than their prey and hence are always present in relatively small numbers in any ecosystem. Therefore it can be stated as a general law that
any deleterious density-independent factor is likely to have a particularly severe effect on predators. In the case of ecosystems affected by
organochlorine insecticides, the effect on predators is likely to be enhanced because they receive higher doses of organochlorine insecticides
than do other species. Therefore a differential decline in predators is
likely to occur both because they are relatively rare and because they
are at the end of food chains. These facts are of course interrelated.
It was shown above that pesticides have differential effects on species,
therefore superimposed on the patterns outlined above is the pattern
determined by differences in the susceptibility to pesticides. Menhinick
(1962) recorded a greater decline in large herbivores compared with
small ones in the chemically treated areas. This could have arisen
because the larger animals had lower population densities but other
explanations are possible, for example, a slower rate of recovery because of low reproductive rate. Differential toxicity may run parallel
to taxonomic groups and feeding habit categories. For example, many
crustacea, insect larvae and spiders appear to be more susceptible to
most insecticides than are molluscs; worms and adult insects are intermediate. Algae and flowering plants are more susceptible than bacteria
to herbicides. The innate “resistance” or adaptability of bacteria to
pesticides is a fact of enormous practical importance; if it were not so,
pesticides would greatly reduce the fertility of farmland.
In general, pesticides reduce diversity and this is likely to produce
less stable systems (Elton, 1958). The extent to which such a decrease
in diversity persists will depend on the rate at which the sprayed area
can be recolonized, and this depends on the size of the sprayed area
and the extent to which neighbouring ones have been or are being
treated. It is probable that organochlorine insecticides have caused the
extinction of certain birds of prey over large areas in which their
original population density was relatively low (e.g. Ratcliffe, 1963).
However, I do not know of any case where pesticide treatment has
caused the total extinction of dense populations of non-pest species
113
have been due to factors other than pesticides. However, the results
are consistent with other studies, e.g. of Edwards et al. (1964), and
there is little doubt that the pesticide treatments caused the differences
observed. The results are also in accordance with what is known about
the effects of other forms of pollution. They are also in accordance with
probability: most ecosystems consist of species which range from the
very abundant to the very rare. If a pesticide had deleterious effects
on all the species present it would be likely to exterminate some of the
rarer species and so reduce diversity. Predators are necessarily rarer
than their prey and hence are always present in relatively small numbers in any ecosystem. Therefore it can be stated as a general law that
any deleterious density-independent factor is likely to have a particularly severe effect on predators. In the case of ecosystems affected by
organochlorine insecticides, the effect on predators is likely to be enhanced because they receive higher doses of organochlorine insecticides
than do other species. Therefore a differential decline in predators is
likely to occur both because they are relatively rare and because they
are at the end of food chains. These facts are of course interrelated.
It was shown above that pesticides have differential effects on species,
therefore superimposed on the patterns outlined above is the pattern
determined by differences in the susceptibility to pesticides. Menhinick
(1962) recorded a greater decline in large herbivores compared with
small ones in the chemically treated areas. This could have arisen
because the larger animals had lower population densities but other
explanations are possible, for example, a slower rate of recovery because of low reproductive rate. Differential toxicity may run parallel
to taxonomic groups and feeding habit categories. For example, many
crustacea, insect larvae and spiders appear to be more susceptible to
most insecticides than are molluscs; worms and adult insects are intermediate. Algae and flowering plants are more susceptible than bacteria
to herbicides. The innate “resistance” or adaptability of bacteria to
pesticides is a fact of enormous practical importance; if it were not so,
pesticides would greatly reduce the fertility of farmland.
In general, pesticides reduce diversity and this is likely to produce
less stable systems (Elton, 1958). The extent to which such a decrease
in diversity persists will depend on the rate at which the sprayed area
can be recolonized, and this depends on the size of the sprayed area
and the extent to which neighbouring ones have been or are being
treated. It is probable that organochlorine insecticides have caused the
extinction of certain birds of prey over large areas in which their
original population density was relatively low (e.g. Ratcliffe, 1963).
However, I do not know of any case where pesticide treatment has
caused the total extinction of dense populations of non-pest species