A SYNOPSIS OF THE PESTICIDE PROBLEM
115
in economic production resulting from pesticide use. But these are no
substitutes for studies on total biological production. Studies on total
biological production would be in the interests of agriculture since they
would provide a better basis for understanding the long-term requirements of crop protection.
D. EFFECTS ON SUCCESSION
In the majority of cases succession of animal populations depends
on botanical succession. Therefore herbicides are more likely to affect
succession than insecticides.
Total herbicides have a similar effect to a strong fire: they may wipe
the area clean and allow a new succession to begin. The effects of more
selective herbicides are of a different nature. Sera1 development is
frequently arrested intentionally at the scrub stage. For example,
2,4,5-T is often used to prevent the invasion of grassland by bushes
and to control woody competitors of planted trees in forests. There is
likely to be an increasing use of herbicides for this purpose. Since the
scrub stage is often particularly rich in plant and animal species, increased control of scrub may have considerable repercussions on wildlife. Succession is rarely affected unintentionally because herbicidal
drift is rarely severe enough to kill shrub species. Increase in his own
population is forcing man to reclaim more natural and semi-natural
habitats, both on a small scale, e.g. hedge destruction, and on a large
scale, e.g. drainage of large marshland areas. In general man favours
vegetation which is equivalent to the early and late stages of ecological
succession - field crops and high forest. Relatively few crops represent
the intermediate scrub stage. Wild scrub has far less economic value
than wild forest. Therefore, there is likely to be an increasing pressure
on land in middle stages of succession throughout the world. The invention of herbicides (2,4,5-T, etc.) makes such reclamation easier than
hitherto. Therefore the scrub habitat is likely to be reduced yet further
as a result of the invention of herbicides. The scale of scrub reduction
is already so great that there may be significant declines of plant and
animal species dependent on it, and this could eventually affect climax
vegetation.
Few studies have been made on the effects of pesticides on sera1
development. Laird (1958) studied those of DDT on succession of
microflora and microfauna of infusions containing larvae of Anopheles
maculipennis atroparvus. He found that the pattern of succession was
similar in the cultures containing DDT and the controls, but that the
rate of change was greater in the sample containing,DDT. He showed
that this was due to the DDT killing many of the mosquito larvae which
resulted in increased bacterial activity, which in turn caused an increase
115
in economic production resulting from pesticide use. But these are no
substitutes for studies on total biological production. Studies on total
biological production would be in the interests of agriculture since they
would provide a better basis for understanding the long-term requirements of crop protection.
D. EFFECTS ON SUCCESSION
In the majority of cases succession of animal populations depends
on botanical succession. Therefore herbicides are more likely to affect
succession than insecticides.
Total herbicides have a similar effect to a strong fire: they may wipe
the area clean and allow a new succession to begin. The effects of more
selective herbicides are of a different nature. Sera1 development is
frequently arrested intentionally at the scrub stage. For example,
2,4,5-T is often used to prevent the invasion of grassland by bushes
and to control woody competitors of planted trees in forests. There is
likely to be an increasing use of herbicides for this purpose. Since the
scrub stage is often particularly rich in plant and animal species, increased control of scrub may have considerable repercussions on wildlife. Succession is rarely affected unintentionally because herbicidal
drift is rarely severe enough to kill shrub species. Increase in his own
population is forcing man to reclaim more natural and semi-natural
habitats, both on a small scale, e.g. hedge destruction, and on a large
scale, e.g. drainage of large marshland areas. In general man favours
vegetation which is equivalent to the early and late stages of ecological
succession - field crops and high forest. Relatively few crops represent
the intermediate scrub stage. Wild scrub has far less economic value
than wild forest. Therefore, there is likely to be an increasing pressure
on land in middle stages of succession throughout the world. The invention of herbicides (2,4,5-T, etc.) makes such reclamation easier than
hitherto. Therefore the scrub habitat is likely to be reduced yet further
as a result of the invention of herbicides. The scale of scrub reduction
is already so great that there may be significant declines of plant and
animal species dependent on it, and this could eventually affect climax
vegetation.
Few studies have been made on the effects of pesticides on sera1
development. Laird (1958) studied those of DDT on succession of
microflora and microfauna of infusions containing larvae of Anopheles
maculipennis atroparvus. He found that the pattern of succession was
similar in the cultures containing DDT and the controls, but that the
rate of change was greater in the sample containing,DDT. He showed
that this was due to the DDT killing many of the mosquito larvae which
resulted in increased bacterial activity, which in turn caused an increase