I14
N . \V. MOC)lCI<
over large areas. Nxtinction is only likely when pesticides are applied
to isolated communities, for example on oceanic islands. Where extinction occurs the pattern of diversity will be permanently affected, but
in most cases the ecosystem is likely to return to a pattern similar to
that which occurred before the application of the pesticide.
Few studies have been made which could indicate the long-term
effects of pesticides on diversity or other features of ecosysteins,
whethcr the pesticides were applied on one occasion or as repeated
treatments. The study of the effects of TDE and other pesticides on
the fauna of Clear Lakc by Hunt and his colleagues is exceptional.
Catastrophic dcclincs in bird populations caused by spraying forest
with orgnnol)hosphorus insecticides appear to be made good by the
following year (e.g. Schneicler, 1966). When DD’I’ was applicd over a
pcriod of four ycars in a forest in Maryland, U.S.A., there was a decline
of 26% in the bird population; but a significant decline was only observed in three of the twelve common species present (Robbins et al.,
1951). Since birds are known rapidly to recolonize vacant habitat, the
true effects of pesticide treatment can only be discovered by marking
all individuals in the experimental population and recording their fate.
Studies on the effects of pesticides on aquatic invertebrates show a
much slower return to normal (see, for example, Ide, 1956 and Hitchcock, 1960). This is not surprising in view of three special characteristics
of aquatic animals. First they are more likely than terrestrial animals
to concentrate pesticides from their medium. Secondly they are often
unable to escape from the sprayed area, and thirdly the rate of recolonization tends to be slower owing to the slower dispersal rates of
most aquatic species. It can be concluded that in general pesticides are
more likely to reducc species diversity in aquatic ecosystems than in
terrestrial ones.
c. E F F E C T S O N P R O D U C T I O N
Menhinick’s data (see Table V) suggest that pesticide treatment has
considerable effects on the number of individuals and on the biomass of
the animals at different trophic levels. He concludes that there has
been a decrease in the numbers of large organisms and a compensating
increase in the numbers of small tolerant, organisms. He also noted a
shift, in trophic structure in fibvour of lower trophic levels. He suggests
that increased decomposition rates could result from chemical treatments. Much work will have to be done before generalizations can be
made, but it is clear from this work that effects on diversity, population
density and biomass can vary between trophic levels. Such effects are
bound to affect production.
A number of studies have I J W l l made (c.g. LiLverton, 1!164) on changes
N . \V. MOC)lCI<
over large areas. Nxtinction is only likely when pesticides are applied
to isolated communities, for example on oceanic islands. Where extinction occurs the pattern of diversity will be permanently affected, but
in most cases the ecosystem is likely to return to a pattern similar to
that which occurred before the application of the pesticide.
Few studies have been made which could indicate the long-term
effects of pesticides on diversity or other features of ecosysteins,
whethcr the pesticides were applied on one occasion or as repeated
treatments. The study of the effects of TDE and other pesticides on
the fauna of Clear Lakc by Hunt and his colleagues is exceptional.
Catastrophic dcclincs in bird populations caused by spraying forest
with orgnnol)hosphorus insecticides appear to be made good by the
following year (e.g. Schneicler, 1966). When DD’I’ was applicd over a
pcriod of four ycars in a forest in Maryland, U.S.A., there was a decline
of 26% in the bird population; but a significant decline was only observed in three of the twelve common species present (Robbins et al.,
1951). Since birds are known rapidly to recolonize vacant habitat, the
true effects of pesticide treatment can only be discovered by marking
all individuals in the experimental population and recording their fate.
Studies on the effects of pesticides on aquatic invertebrates show a
much slower return to normal (see, for example, Ide, 1956 and Hitchcock, 1960). This is not surprising in view of three special characteristics
of aquatic animals. First they are more likely than terrestrial animals
to concentrate pesticides from their medium. Secondly they are often
unable to escape from the sprayed area, and thirdly the rate of recolonization tends to be slower owing to the slower dispersal rates of
most aquatic species. It can be concluded that in general pesticides are
more likely to reducc species diversity in aquatic ecosystems than in
terrestrial ones.
c. E F F E C T S O N P R O D U C T I O N
Menhinick’s data (see Table V) suggest that pesticide treatment has
considerable effects on the number of individuals and on the biomass of
the animals at different trophic levels. He concludes that there has
been a decrease in the numbers of large organisms and a compensating
increase in the numbers of small tolerant, organisms. He also noted a
shift, in trophic structure in fibvour of lower trophic levels. He suggests
that increased decomposition rates could result from chemical treatments. Much work will have to be done before generalizations can be
made, but it is clear from this work that effects on diversity, population
density and biomass can vary between trophic levels. Such effects are
bound to affect production.
A number of studies have I J W l l made (c.g. LiLverton, 1!164) on changes