Endangered Species

An example of such a species is the American whooping crane (Grus americana), whose numbers were down to only 15 birds in 1941. Even after intense conservation efforts, there were still only 30 birds in 1963. The number in the wild has increased in recent years, and today there are more than 450 wild and captive birds.

Poisoning of the Environment

Another threat to wildlife has increased since the middle of the 20th century—the poisoning of the environment. As the human population has grown, vast amounts of wild land have been cleared for agriculture, thus depriving wildlife of habitat. New agricultural techniques to make the land more productive have included the widespread use of pesticides.

Herbicides that kill weeds and insecticides that destroy insects are dumped on the land in huge quantities. Such chemicals are beneficial in increasing crop yields and controlling disease-carrying insects. However, many of the poisons undergo chemical breakdown very slowly, allowing them to accumulate in the soil and wash into streams, lakes, and oceans.

Once they enter water, the chemicals are eaten or absorbed by microscopic organisms. As these tiny animals and plants are eaten by larger animals, the poisons accumulate in the bodies of the animals in increasingly high concentrations. Thus, the largest animals—those at the top of the so-called food chain—suffer the greatest damage because they take in the highest concentrations of poisons. For example, contaminated algae may be eaten by a small crustacean, which in turn is eaten by a small fish. The small fish may then be eaten by a larger fish, and the larger fish eaten by a grizzly bear. This progression makes up a food chain. The bear at the top of the chain receives the most poison.

The insecticide DDT moves along the food chain in this manner. Beginning in 1946, DDT was widely used as an effective weapon against agricultural pests. But DDT does not break down easily in nature, causing it to remain in the environment for many years.

DDT can cause physiological and genetic changes in animals and humans. For example, it disrupts the calcium-producing mechanism in birds. This means that their eggs are laid with an inadequate, thin, flaky shell that usually breaks before the embryos reach maturity. As a result, populations of the American bald eagle (Haliaeëtus leucocephalus), the osprey (Pandion haliaëtus), the peregrine falcon (Falco peregrinus), the brown pelican (Pelecanus occidentalis), and the Bermuda petrel (Pterodroma cahow) declined sharply. DDT also reaches humans—as analysis of the milk of some nursing mothers shows.