ECOLOGY (Part 3 of 3)   Leave a comment

HOW DDT KILLED THE ROBINS. Dutch elm disease threatened to destroy most of the majestic elms that once flourished along residential streets. To eliminate the beetles that carry this fungus disease, many communities sprayed their elms with massive doses of DDT. The pesticide stuck to the leaves even after they fell in the autumn. Earthworms then fed on the leaves and accumulated DDT in their bodies. When spring came, robins returned to the communities to nest. They ate the earthworms and began to die in alarming numbers. Of the females that survived, some took in enough DDT to hamper the production or hatching of eggs. Robin populations were so seriously affected by DDT poisoning that the very survival of the songbird seemed in jeopardy. This experience was a vivid example of the far-ranging effects that flow from upsets in the delicate balances of nature.

Ironically, the DDT did little to prevent the spread of Dutch elm disease.

By the 1970s ecologists had accumulated considerable evidence demonstrating that the widely used pesticide DDT and its metabolites, principally DDE, altered the calcium metabolism of certain birds. The birds laid eggs with such thin shells that they were crushed during incubation. This discovery was one of many that led to the imposition of legal restraints on the use of some agricultural pesticides.

Ecologists know that the well-being of a biotic community may require the preservation of a key member-species. For example, the alligator performs a valuable service in the Florida Everglades by digging “‘gator holes.” These are ponds created by female alligators when they dig up grass and mud for their nests. During extremely dry spells, these holes often retain enough water to meet the needs of bobcats, raccoons and fish until the arrival of rainy weather.

Many birds use the holes for watering. Willow seeds take root along the edges, and fallen willow leaves later add substance to the soil. Thus, many forms of life are sustained by ‘gator holes. But poachers have been hunting the alligators almost to extinction for their valuable hides.

As a result, the number of ‘gator holes can be expected to dwindle, and various forms of Everglades wildlife may be deprived of these refuges. Such ecological findings strengthen the case for the protection of alligators.

Another ecological threat to the Everglades arose in the late 1960s, when plans were made to build a jet airport near the northern end of the national park. The airport would have wiped out part of a large swamp that furnishes the Everglades with much of its surface water. Ecologists and conservationists opposed the project, arguing that it would hamper the flow of surface water through the park and thus endanger the biota of the unique Everglades ecosystem.

Their arguments aroused public concern, and in 1970 plans for the airport were dropped.

An Ecological Mistake

Kaibab National Forest, forest in Arizona, adjoining Grand Canyon National Park; 1,780,475 acres (691,395 hectares); forest headquarters Williams, Ariz.

At times, seemingly practical conservation efforts turn out to be mistakes. Cougars, or mountain lions, and deer were once abundant in Grand Canyon National Park and Kaibab National Forest. Because the cougars preyed on the deer, hunters were allowed to shoot the cougars until only a few were left.

With their chief enemy gone, the deer of the area increased so rapidly that they consumed more forage than the Kaibab could produce. The deer stripped the forest of every leaf and twig they could reach and destroyed large areas of forage in the Grand Canyon National Park as well. The famished deer grew feeble, and many defective fawns were born. Finally, deer hunting in the Kaibab was permitted, in the hope that the size of the deer herd would drop until the range could accommodate it. In addition, the few surviving cougars were protected to allow them to multiply. They then resumed their ecological niche of keeping the herd size down and of killing those deer not vigorous enough to be good breeding stock.

The Ecological Control of Pests

Many of the insects and other pests that have plagued North America originated elsewhere. There these pests were held in check by natural enemies, and the plants and animals they infested had developed a measure of tolerance toward them. However, when they were placed in an environment free of these restraints, the pests often multiplied uncontrollably.

At first, farmers fought the pests with toxic sprays and other powerful chemicals. However, these methods were expensive, sometimes proved unsuccessful, and were often dangerous. After decades of use, some pesticides were banned. In certain instances, pesticide use gave way to an ecological approach.

Research showed that severe damage from certain pests the Mexican beetle and the European corn borer, for example is confined to crops grown on particular types of soil or under certain conditions of moisture. Changes in land use helped control some pests. Others were controlled biologically by importing parasites or predators from their native lands. This important form of pest control proved successful in limiting damage by scale insects.

By destroying birds and other animals, as well as their breeding places, people lose valuable allies in their constant war with insects. Once, when the sportsmen of Ohio supported a proposal to permit quail hunting, the farmers of the state objected. They knew that a single quail killed enough insects to make it worth at least as much to them as a dozen chickens.

In some 3,000 locally organized Resource Conservation Districts ecological principles are being used to guide land use and community maintenance practices. These districts encompass the federal lands of the United States and more than 95 percent of its privately owned farmlands.

GOALS OF ECOLOGY

Throughout the world man-made communities have been replacing the communities of nature. However, the principles that govern the life of natural communities must be observed if these man-made communities are to thrive. People must think less about conquering nature and more about learning to work with nature.

In addition, each person must realize his interdependence with the rest of nature, including his fellow human beings. To safeguard life on Earth, people must learn to control and adjust the balances in nature that are altered by their activities.

Maintenance of the Environment

Climate cannot be changed except sporadically by cloud seeding, inadvertently by pollution, and on a small scale by making windbreaks or greenhouses. However, human activities can be successfully adapted to the prevailing climatic patterns. Plants and animals, for example, should be raised in the climates best suited to them, and particular attention should be paid to the cold and dry years rather than to average years or exceptionally productive years. In the United States the serious dust storms of the 1930s occurred because land that was ploughed in wet years to grow wheat blew away in dry years. Much of that land should have been kept as range land

Soil is a measure of an environment’s capacity to support life. It forms very slowly but can be lost quickly as much as an inch in a rainstorm. Wise land use ensures its retention and improvement.

For agricultural purposes, land is used principally as timber land, range land, or crop land Timber land and range land are natural communities. Crop land is formed when what was originally timber land or range land is cultivated. To ensure the best possible use of land, it is classified according to its ability to sustain the production of timber, pasture, or crops.

Water, like soil, is a measure of the abundance of life. Usable water depends on the amount and retention of rainfall. An excessive run-off of rainwater, however, may result from human activities for example, the building of roads and drainage ditches; the construction of extensive parking areas and shopping centres; the unwise harvesting of timber; year-round grazing of ranges; and the cultivation of easily eroded lands. Excessive run-off may cause floods. It may also lead to drought, which can occur when too little water is stored underground. Moreover, run-off strips soil from the land. This is deposited in reservoirs, ship channels, and other bodies of water. These silt-laden bodies must then be either dredged or abandoned. Water movements in and out of the soil must be controlled in such a way as to minimize damage and maximize benefits.

The Conservation of Natural Communities

Community, in biology, a group of organisms living together in a particular environment.

The communities of plants and animals established by humans usually consist of only a few varieties, often managed in a way that harms the environment. By contrast, natural communities usually enhance the environment and still yield many products and sources of pleasure to people.

Land once cultivated but now lying idle should be restored to the natural communities that formerly occupied it. In addition, people should use the findings of ecology to improve their artificial communities such as fields, gardens, orchards, and pastures. For example, few man-made agents for the control of pests can outperform the wide variety of insect-eating birds.

The Curtailment of Waste

Modern machines and weapons and the harmful wastes of technology can be used to destroy the environment. At the same time, the wise use of machinery can also enable humans to conserve their surroundings. Just as negotiation rather than warfare can be employed to resolve international disputes, no doubt the means can be devised to curtail the destructive wastes of factories and vehicles. True, ever-growing demands for goods and services, nurtured by increasing human populations and rising expectations, are placing more and more pressure on the environment. An understanding of the causes and consequences of environmental deterioration, however, may bring about a change in the goals that people pursue and the means they use to achieve these goals.

Increases in human material possessions have been accompanied by a potentially dangerous worsening of the natural environment. A central function of ecology is to study human interactions with the natural environment in order to modify them favourably.

Assisted by E.J. Dyksterhuis, Professor of Range Ecology, Texas A & M University.

BIBLIOGRAPHY FOR ECOLOGY

Books for Children

Jaspersohn, William. How the Forest Grew (Greenwillow, 1980).

Pringle, Laurence. City and Suburb: Exploring an Ecosystem (Macmillan, 1975).

Sabin, Francene. Ecosystems and Food Chains (Troll, 1985).

Selsam, M.E. How Animals Live Together, rev. ed. (Morrow, 1979).

Books for Young Adults

Billington, E.T. Understanding Ecology, rev. ed. (Warne, 1971).

Pringle, Laurence. Lives at Stake: The Science and Politics of Environmental Health (Macmillan, 1980).

Sharpe, G.W. Interpreting the Environment, 2nd ed. (Wiley, 1982).

Sharpe, G.W. and others. Introduction to Forestry, 4th ed. (McGraw, 1976).

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