108
N. W. MOORE
on herbivores living on plants now reduced by herbicides must
have been affected. For example, D. J. Cross and T. R. E. Southwood
(pers. comm.) have shown that young partridges, which are dependent
on insect foods in the first few weeks of their lives, find less food in
sprayed crops than in unsprayed. Accordingly they have to use more
energy in finding the necessary food, and under wet conditions this
involves a proportionally higher loss of energy through cooling. As a
result partridges are likely to do less well where herbicides are used extensively. The decline in many partridge populations in Britain recorded
in recent years may be partly due to the increase in the use of herbicides.
Extremely few studies have been made of this type of effect, and it
may be much more important to non-pest species than is generally
believed. Similarly the extensive use of insecticides must have greatly
reduced the food of certain species, especially of insectivorou.s species
with narrow food ranges. There is little evidence that pesticides reduce
total production (see p. 114), therefore species with very wide food
tolerances are less likely to be affected by pesticide applications on their
food supply - they merely feed more extensively on those species
which are unaffected or which have increased as a result of pesticide use.
E. REDUCTION O F H A B I T A T
Plants, in addition to providing food, are important to animals and
other plants because they provide shelter. Pesticides, by destroying a
habitat, may have a direct effect on numbers. Herbicides are the main
agents of this sort. The simplification of crop ecosystems lessens the
range of microhabitat for many species. The use of 2,4-D and 2,4,5-T
to control scrub by roads and in woods reduces the essential habitat of
almost all British land birds, which, because they are survivors of the
original forest fauna, are still dependent on trees and bushes. Again
very little work has been done to demonstrate the floristic and faunal
changes resulting from habitat changes due to herbicides. But it is safe
to assume that many species are being affected in this way.
However, it should be noted that herbicide treatments rarely destroy
a habitat completely: they alter it. For example, scrub may be changed
to grassland, or an aquatic flora consisting of submerged flowering
plants may be changed to one consisting entirely of green algae. The
new habitat may be more rather than less suitable for certain species.
Again the loss is more likely to be one of diversity than of biomass
(see p. 111).
F. R E M O V A L O F A C O M P E T I T O R
Many plants and animals appear to be limited in their numbers by
competition with other species which make use of the same basic
N. W. MOORE
on herbivores living on plants now reduced by herbicides must
have been affected. For example, D. J. Cross and T. R. E. Southwood
(pers. comm.) have shown that young partridges, which are dependent
on insect foods in the first few weeks of their lives, find less food in
sprayed crops than in unsprayed. Accordingly they have to use more
energy in finding the necessary food, and under wet conditions this
involves a proportionally higher loss of energy through cooling. As a
result partridges are likely to do less well where herbicides are used extensively. The decline in many partridge populations in Britain recorded
in recent years may be partly due to the increase in the use of herbicides.
Extremely few studies have been made of this type of effect, and it
may be much more important to non-pest species than is generally
believed. Similarly the extensive use of insecticides must have greatly
reduced the food of certain species, especially of insectivorou.s species
with narrow food ranges. There is little evidence that pesticides reduce
total production (see p. 114), therefore species with very wide food
tolerances are less likely to be affected by pesticide applications on their
food supply - they merely feed more extensively on those species
which are unaffected or which have increased as a result of pesticide use.
E. REDUCTION O F H A B I T A T
Plants, in addition to providing food, are important to animals and
other plants because they provide shelter. Pesticides, by destroying a
habitat, may have a direct effect on numbers. Herbicides are the main
agents of this sort. The simplification of crop ecosystems lessens the
range of microhabitat for many species. The use of 2,4-D and 2,4,5-T
to control scrub by roads and in woods reduces the essential habitat of
almost all British land birds, which, because they are survivors of the
original forest fauna, are still dependent on trees and bushes. Again
very little work has been done to demonstrate the floristic and faunal
changes resulting from habitat changes due to herbicides. But it is safe
to assume that many species are being affected in this way.
However, it should be noted that herbicide treatments rarely destroy
a habitat completely: they alter it. For example, scrub may be changed
to grassland, or an aquatic flora consisting of submerged flowering
plants may be changed to one consisting entirely of green algae. The
new habitat may be more rather than less suitable for certain species.
Again the loss is more likely to be one of diversity than of biomass
(see p. 111).
F. R E M O V A L O F A C O M P E T I T O R
Many plants and animals appear to be limited in their numbers by
competition with other species which make use of the same basic