A SYNOPSIS O F THE PESTICIDE PROBLEM
111
the pesticide are described in terms of total effect - on changing the
characteristics of the whole ecosystem on which the pesticide impinges.
Ecosystems can be described and so compared in quantitative terms.
Measurements of species diversity, supported by knowledge of feeding
habits, provide patterns showing the relative proportions of species in
different trophic levels. If information about species is extended to
cover information on populations and biomass, a further step can be
taken: patterns of energy exchange can be assessed. Eventually it
should be possible to assess the effects of pesticides in terms of production as well as diversity. Ecosystems tend to develop in time until a
relatively stable state is reached: by affecting diversity and production
a pesticide may also affect succession.
R. EFFECTS O N IIIVERSITY
Few studies have been made to determine the effects of pesticides on
species diversity as such. Menhinick (1 962) compared the animal populations of soil and litter of mown grassland in areas treated with fifteen
insecticides and fungicides and those in unsprayed areas. The sprayed
areas contained fifty-three tax&, the unsprayed eight-two. Phenoxyacetic acid herbicides applied to pastures rich in species result in a
decline in species because they kill off most of the dicotyledonous species
which make up a large proportion of the total (see Yemm and Willis,
1962). It is reasonable to suppose that the loss of many plant species
results in a corresponding loss of animal species. Yet herbicidal treatment does not necessarily result in a decline in species. For if a herbicide
is used to kill a dominant species which is growing in a virtually pure
stand, the result may be an increase since the destruction of the dominant allows colonization by a wide range of new species. For example,
hawthorn scrub a t a site in Kent was given a basal bark treatment with
a 2,4-D/2,4,5-T mixture. Before treatment the scrub was so dense that
there was virtually no ground flora beneath it. As a result of chemical
treatment, the canopy was broken and light reached the ground flora
which was quickly colonized by nettles, sow thistles and other species
(Davis, Moore and Way, unpublished). In general, however, pesticides
are applied t o complex ecosystems and so it can be assumed that they
normally cause a decrease in diversity. The decline of diversity is not
random - some groups are affected more than others. The differences
between the sprayed and unsprayed areas referred to above (Menhinick,
1962) were as shown in Table V, columns 1 and 2. He concluded that
there was a decrease in the number of taxa and that the decline was
most notable in the case of large organisms, especially in predaceous
species. Unfortunately no records were made of the pre-spray situation
on the treated areas so the differences observed could, theoretically,
111
the pesticide are described in terms of total effect - on changing the
characteristics of the whole ecosystem on which the pesticide impinges.
Ecosystems can be described and so compared in quantitative terms.
Measurements of species diversity, supported by knowledge of feeding
habits, provide patterns showing the relative proportions of species in
different trophic levels. If information about species is extended to
cover information on populations and biomass, a further step can be
taken: patterns of energy exchange can be assessed. Eventually it
should be possible to assess the effects of pesticides in terms of production as well as diversity. Ecosystems tend to develop in time until a
relatively stable state is reached: by affecting diversity and production
a pesticide may also affect succession.
R. EFFECTS O N IIIVERSITY
Few studies have been made to determine the effects of pesticides on
species diversity as such. Menhinick (1 962) compared the animal populations of soil and litter of mown grassland in areas treated with fifteen
insecticides and fungicides and those in unsprayed areas. The sprayed
areas contained fifty-three tax&, the unsprayed eight-two. Phenoxyacetic acid herbicides applied to pastures rich in species result in a
decline in species because they kill off most of the dicotyledonous species
which make up a large proportion of the total (see Yemm and Willis,
1962). It is reasonable to suppose that the loss of many plant species
results in a corresponding loss of animal species. Yet herbicidal treatment does not necessarily result in a decline in species. For if a herbicide
is used to kill a dominant species which is growing in a virtually pure
stand, the result may be an increase since the destruction of the dominant allows colonization by a wide range of new species. For example,
hawthorn scrub a t a site in Kent was given a basal bark treatment with
a 2,4-D/2,4,5-T mixture. Before treatment the scrub was so dense that
there was virtually no ground flora beneath it. As a result of chemical
treatment, the canopy was broken and light reached the ground flora
which was quickly colonized by nettles, sow thistles and other species
(Davis, Moore and Way, unpublished). In general, however, pesticides
are applied t o complex ecosystems and so it can be assumed that they
normally cause a decrease in diversity. The decline of diversity is not
random - some groups are affected more than others. The differences
between the sprayed and unsprayed areas referred to above (Menhinick,
1962) were as shown in Table V, columns 1 and 2. He concluded that
there was a decrease in the number of taxa and that the decline was
most notable in the case of large organisms, especially in predaceous
species. Unfortunately no records were made of the pre-spray situation
on the treated areas so the differences observed could, theoretically,