The Organochlorine Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 112

5.2.5 e c o l o g i c a l e f f e c T s of f ddT
As explained in Section 5.2.3, p,p ′-DDE is much more persistent in food chains than
either p,p ′-DDT or p,p ′-DDD, and during the 1960s when DDT was still extensively
used, it was often the most abundant of the three compounds in birds and mammals
found or sampled in the field. Since the widespread banning of DDT, very little of
the pesticides has been released into the environment, and p,p ′-DDE is by far the
most abundant DDT residue found in biota. While discussing the ecological effects
of DDT and related compounds, effects on population numbers will be considered
before those on population genetics (gene frequencies).
5.2.5.1 effects on Population numbers
An early indication of the damage that OC compounds can cause in the higher lev-
els of the food chain came with a study on the East Lansing Campus of Michigan
State University in 1961 and 1962 (Bernard 1966). Over several years, leading up to
and including 1962, American robins ( Turdus migratorius ) and several other species
of birds were virtually eliminated from a 75 ha study area in the spring, follow-
ing the application of high levels of DDT (<25 lb/acre). The purpose of the exer-
cise was to control Dutch elm disease. Subsequent investigation established that all
American robins dying in this way contained more than 50 ppm of total DDT in the
brain. Comparison with experimentally poisoned birds led to the conclusion that
these levels were high enough to have caused lethal DDT poisoning. In these early
days before the development of gas chromatography, it was difficult to distinguish
between the different compounds derived from DDT, and a limitation of the study
was that deductions were based on estimates of total DDT. As has been pointed
out, the various impurities and metabolites arising from the technical material differ
considerably in their toxicity, so an estimate of total DDT residues is only of limited
usefulness when attempting to establish the cause of death. However, with the ben-
efit of hindsight, it seems clear that many birds did die of DDT poisoning following
these very high levels of application and that transfer through earthworms and other
invertebrates made a major contribution to the level of residues in the birds. It also
appeared that the effects were localized, seasonal, and transitory.
In another widely quoted earlier study, Hunt and Bischoff (1960) reported the
decline of Western Grebe ( Aechmophorus occidentalis) populations on Clear Lake,
California, following the application of rhothane (DDD) over several years. There
was evidence of a progressive buildup of p,p ′-DDD residues in sediments over the
period, and an analytical study of biota from the lake yielded the results shown in
Table 5.6.
The levels of DDD found in dying or dead grebes were high enough to suggest
acute lethal poisoning. As with the study on the American robin, there was strong
evidence for the local decline of a species occupying a high trophic level of an
ecosystem, a decline consequent upon the toxicity of a persistent OC compound
obtained via its food. At first there was a tendency to explain the very large dif-
ferences in DDD concentrations between the top and the bottom of the food chain
in terms of progressive bioaccumulation with movement up the chain. On closer
examination, however, much of this increase is explicable on the grounds of strong