DDT (Dichlorodiphenyl-Trichloroacetic Acid)

Some poisonings of wildlife were directly caused by exposure to sprays of DDT. There were numerous cases of dying or dead birds being found after the spraying of DDT, for example, after its use in residential areas to kill the beetle vectors of Dutch elm disease in North America. Spray rates for this purpose were large, about 1.5-3.0 lb (1.0-1.5 kg) of DDT per tree, and resulted in residues in earthworms of 33-164 ppm. Birds that fed on DDT-laced invertebrates had intense exposures to DDT, and many were killed.

Sometimes, detailed investigations were needed to link declines of bird populations to the use of organochlorines. One such example occurred at Clear Lake, California, an important waterbody for recreation. Because of complaints about the nuisance of a great abundance of non-biting aquatic insects called midges, Clear Lake was treated in 1949 with DDD at 1 kg/ha. Prior research had shown that this dose of DDD would achieve control of the midges but would have no immediate effect on fish. Unfortunately, the unexpected happened. After another application of DDD in 1954, 100 western grebes were found dead as were many intoxicated birds. Eventually, the breeding population of these birds on Clear Lake decreased from about 2,000 to none by 1960. The catastrophic decline of grebes was linked to DDD when an analysis of the fat of dead birds found residues as large as 1,600 ppm. Fish were also heavily contaminated. The deaths of birds on Clear Lake was one of the first well documented examples of a substantial mortality of wildlife caused by organochlorine insecticides.

Damage to birds also occurred in places remote from sprayed areas. This was especially true of raptorial (that is, predatory) birds, such as falcons, eagles, and owls. These are top predators, and they food-web accumulate chlorinated hydrocarbons to high concentrations. Declines of some species began in the early 1950s, and there were extirpations of some breeding populations. Prominent examples of predatory birds that suffered population declines from exposure to DDT and other organochlorines include the bald eagle, golden eagle, peregrine falcon, prairie falcon, osprey, brown pelican, double-crested cormorant, and European sparrowhawk.

Of course, birds and other wildlife were not only exposed to DDT. Depending on circumstances, there could also be significant exposures to other chlorinated hydrocarbons, including DDD, aldrin, dieldrin, heptachlor, and PCBs. Scientists have investigated the relative importance of these chemicals in causing the declines of predatory birds. In Britain, the declines of raptors did not occur until dieldrin came into common use, and this insecticide may have been the primary cause of the damage. However, in North America DDT use was more common, and it was probably the most important cause of the bird declines there.

The damage to birds was mainly caused by the effects of chlorinated hydrocarbons on reproduction, and not by direct toxicity to adults. Demonstrated effects of these chemicals on reproduction include: (1) a decrease in clutch size (i.e., the number of eggs laid); (2) the production of a thin eggshell which might break under the incubating parent; (3) deaths of embryos, unhatched chicks, and nestlings; and (4) pathological parental behavior. All of these effects could decrease the numbers of young successfully raised. The reproductive pathology of chlorinated hydrocarbons caused bird populations to decrease because of inadequate recruitment.

This syndrome can be illustrated by the circumstances of the peregrine falcon, a charismatic predator whose decline attracted much attention and concern. Decreased reproductive success and declining populations of peregrines were first noticed in the early 1950s. In 1970, a North American census reported almost no successful reproduction by the eastern population of peregrines, while the arctic population was declining in abundance. Only a local population in the Queen Charlotte Islands of western Canada had normal breeding success and a stable population. This latter population is non-migratory, inhabiting a region where pesticides are not used and feeding largely on non-migratory seabirds. In contrast, the eastern peregrines bred where chlorinated hydrocarbon pesticides were widely used, and its prey was generally contaminated. Although the arctic peregrines breed in a region where pesticides are not used, these birds winter in sprayed areas in Central and South America where their food is contaminated, and their prey of migratory ducks on the breeding grounds is also contaminated. Large residues of DDT and other organochlorines were common in peregrine falcons (except for the Queen Charlottes). Associated with those residues were eggshells thinner than the pre-DDT condition by 15-20% and a generally impaired reproductive rate.

In 1975, another North American survey found a virtual extirpation of the eastern peregrines, while the arctic population had declined further and was clearly in trouble. By 1985 there were only 450 pairs of arctic peregrines, compared with the former abundance of 5,000-8,000. However, as with other raptors that suffered from the effects of chlorinated hydrocarbons, a recovery of peregrine populations has begun since DDT use was banned in North America and most of Europe in the early 1970s. In 1985, arctic populations were stable or increasing compared with 1975, as were some southern populations, although they remained small. This recovery has been enhanced by a captive-breeding and release program over much of the former range of the eastern population of peregrine falcons.

It is still too soon to tell for certain, but there are encouraging signs that many of the severe effects of DDT and other chlorinated hydrocarbons on wildlife are becoming less severe. Hopefully, in the future these toxic damages will not be important.