A SYNOPSIS OF THE PESTICIDE PROBLEM
109
requirements. There are many examples where it is known that plant
species have increased as a result of herbicide use. For example Yemm
and Willis (1962) showed that treatments of maleic hydrazide caused
the progressive replacement of the tufted grasses Arrhenathrum elatius
and Dactylis glomerata by the rhizomatous species Festuca rubra and
Poa pratensis. The widespread use cjf MCPA and 2,4-D in crops has
caused a great increase in weed species which previously were relatively
rare. If effects on competition are extensive they may lead to the virtual
substitution of one species for another. For example Aitken and Trapido
(1961) have shown that when the mosquito Anopheles labranchiae was
greatly reduced by DDT spraying in the anti-malarial campaign in
Sardinia, 1947-48, four other species of Anopheles greatly increased.
An explanation of the substitution was available in the case of Anopheles
hispaniola: A . labranchiae rested in houses much more than did A .
hispaniola and houses were the habitat most thoroughly sprayed.
Further, when water was sprayed the larvae of A . hispaniola were
found to dive and avoid the surface for much longer periods than were
the larvae of A . labranchiae, and so they had less contact with DDT.
Most of the cases of substitution of species have been discovered
among invertebrates. Perhaps it is less likely to occur in animals with
more highly developed nervous systems, since the latter may be able
to adapt more quickly to new situations. The carrion crow has probably increased during the period when the sparrow-hawk and kestrel
have been virtually eliminated from Eastern England (Prestt, 1965).
It is possible that this is a case of substitution caused by pesticides;
one in which a highly adaptable species of oninivorous habits has filled
the niche of species with more selective habits and which are, perhaps,
more sensitive to organochlorine insecticide poisoning.
The effects of pesticides on ecological “buffering” should be noted.
The effects of predators are often modified and reduced in severity
because, when they begin to cause a severe reduction in one species,
they turn their attention to another and so reduce pressure on the first.
This system of buffering is almost certainly the cause of the greater
stability of complex ecosystems. If a pesticide reduces the numbers of
an alternative prey it may cause a predator to feed proportionally more
upon another species which hitherto it had scarcely affected. For
example, Edwards et al. (1964) showed that an application of TDE led
to a great reduction of culicids, and as a result the trout in the experimental pond fed on crustacea more than before. If the crustacea were
the competitors of the culicids the immediate effect of the spray may
have been advantageous to the crustacea by removing competitors, but
by indirectly causing the trout to feed more extensively upon them the
eventual result may have been, theoretically at least, a reduction in
109
requirements. There are many examples where it is known that plant
species have increased as a result of herbicide use. For example Yemm
and Willis (1962) showed that treatments of maleic hydrazide caused
the progressive replacement of the tufted grasses Arrhenathrum elatius
and Dactylis glomerata by the rhizomatous species Festuca rubra and
Poa pratensis. The widespread use cjf MCPA and 2,4-D in crops has
caused a great increase in weed species which previously were relatively
rare. If effects on competition are extensive they may lead to the virtual
substitution of one species for another. For example Aitken and Trapido
(1961) have shown that when the mosquito Anopheles labranchiae was
greatly reduced by DDT spraying in the anti-malarial campaign in
Sardinia, 1947-48, four other species of Anopheles greatly increased.
An explanation of the substitution was available in the case of Anopheles
hispaniola: A . labranchiae rested in houses much more than did A .
hispaniola and houses were the habitat most thoroughly sprayed.
Further, when water was sprayed the larvae of A . hispaniola were
found to dive and avoid the surface for much longer periods than were
the larvae of A . labranchiae, and so they had less contact with DDT.
Most of the cases of substitution of species have been discovered
among invertebrates. Perhaps it is less likely to occur in animals with
more highly developed nervous systems, since the latter may be able
to adapt more quickly to new situations. The carrion crow has probably increased during the period when the sparrow-hawk and kestrel
have been virtually eliminated from Eastern England (Prestt, 1965).
It is possible that this is a case of substitution caused by pesticides;
one in which a highly adaptable species of oninivorous habits has filled
the niche of species with more selective habits and which are, perhaps,
more sensitive to organochlorine insecticide poisoning.
The effects of pesticides on ecological “buffering” should be noted.
The effects of predators are often modified and reduced in severity
because, when they begin to cause a severe reduction in one species,
they turn their attention to another and so reduce pressure on the first.
This system of buffering is almost certainly the cause of the greater
stability of complex ecosystems. If a pesticide reduces the numbers of
an alternative prey it may cause a predator to feed proportionally more
upon another species which hitherto it had scarcely affected. For
example, Edwards et al. (1964) showed that an application of TDE led
to a great reduction of culicids, and as a result the trout in the experimental pond fed on crustacea more than before. If the crustacea were
the competitors of the culicids the immediate effect of the spray may
have been advantageous to the crustacea by removing competitors, but
by indirectly causing the trout to feed more extensively upon them the
eventual result may have been, theoretically at least, a reduction in