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N. W. MOORE
and flooding which kill many or few animals according to severity, but
on the other hand leave plants relatively unaffected. Systemic insecticides, like schradan, whose effects are limited to animals that suck plant
juices, have no counterpart among “natural” factors. Specific chemicals
resemble specific diseases only in the sense that they are specificthere is no equivalent of intraspecific infection.
Persistent pesticides with their attendant phenomena of dispersal,
biological concentration and concentration in food chains, share many
features with radioactive materials (Odum, 1959; Templeton, 1965). A
detailed comparison between the two types of environmental contamination would be valuable but is not attempted here. Like persistent
pesticides, radioactive particles become widely distributed over the surface of the earth, they have widely different effects on different organisms (Higgins, 1951), and they become concentrated a t different rates
in different species of plants and animals. However, radioactive isotopes
are not taken up differentially - their concentration rate depends first
on the normal uptake of the element concerned and secondly on the
proportion of the element in the environment which is radioactive.
The concentration factors of radioisotopes in food chains vary considerably but are comparable to those of persistent insecticides. Pendleton and Hanson (1958) show that plants in an aquatic environment
accumulate I3’Cs 500 times as much as plants grown in soils, and that
animals feeding on aquatic plants accordingly become proportionately
more contaminated. The radioactive burdens of predators, unlike
organochlorine insecticide residues of predators, do not greatly exceed
those of prey species. The pattern of concentration factors of radioisotopes with a relatively short half life (e.g. 32P) differs from that of
persistent insecticides in that predators carry smaller radioactive burdens than herbivores because there is not time for them to accumulate
larger amounts than their prey. Some radioactive substances differ
from pesticides in that they occur naturally and are merely increased
by man, whereas most pesticides do not occur naturally. There are other
important differences: some radioactive materials remain active very
much longer than pesticides and the rate of their decay is regular and
predictable, whereas the decay of pesticides depends on a number of
factors and is not constant. In this connection it should be noted that
the term “half life” is sometimes used to describe the likely duration of
a pesticide in soils, but it is misleading since it is not equivalent to the
half life of a radioactive substance.
B. P E S T I C I D E S A S P A R T O F A D E V E L O P I N G S Y S T E M
One of the aims of this paper is to demonstrate that the ecological
effect of a pesticide is not the same as the toxicological effect writ large.
N. W. MOORE
and flooding which kill many or few animals according to severity, but
on the other hand leave plants relatively unaffected. Systemic insecticides, like schradan, whose effects are limited to animals that suck plant
juices, have no counterpart among “natural” factors. Specific chemicals
resemble specific diseases only in the sense that they are specificthere is no equivalent of intraspecific infection.
Persistent pesticides with their attendant phenomena of dispersal,
biological concentration and concentration in food chains, share many
features with radioactive materials (Odum, 1959; Templeton, 1965). A
detailed comparison between the two types of environmental contamination would be valuable but is not attempted here. Like persistent
pesticides, radioactive particles become widely distributed over the surface of the earth, they have widely different effects on different organisms (Higgins, 1951), and they become concentrated a t different rates
in different species of plants and animals. However, radioactive isotopes
are not taken up differentially - their concentration rate depends first
on the normal uptake of the element concerned and secondly on the
proportion of the element in the environment which is radioactive.
The concentration factors of radioisotopes in food chains vary considerably but are comparable to those of persistent insecticides. Pendleton and Hanson (1958) show that plants in an aquatic environment
accumulate I3’Cs 500 times as much as plants grown in soils, and that
animals feeding on aquatic plants accordingly become proportionately
more contaminated. The radioactive burdens of predators, unlike
organochlorine insecticide residues of predators, do not greatly exceed
those of prey species. The pattern of concentration factors of radioisotopes with a relatively short half life (e.g. 32P) differs from that of
persistent insecticides in that predators carry smaller radioactive burdens than herbivores because there is not time for them to accumulate
larger amounts than their prey. Some radioactive substances differ
from pesticides in that they occur naturally and are merely increased
by man, whereas most pesticides do not occur naturally. There are other
important differences: some radioactive materials remain active very
much longer than pesticides and the rate of their decay is regular and
predictable, whereas the decay of pesticides depends on a number of
factors and is not constant. In this connection it should be noted that
the term “half life” is sometimes used to describe the likely duration of
a pesticide in soils, but it is misleading since it is not equivalent to the
half life of a radioactive substance.
B. P E S T I C I D E S A S P A R T O F A D E V E L O P I N G S Y S T E M
One of the aims of this paper is to demonstrate that the ecological
effect of a pesticide is not the same as the toxicological effect writ large.