Population Biology

Birth and death rates typically are stated as numbers per 1000 population per year.

These rates are affected by the age-composition of the population; for example, a very healthy population, which, as a result, has a relatively large proportion of old people, might have a death rate similar to that of a poor population made up of predominantly younger members.

Demographers, therefore, often use measures that are free of this age-distribution influence. Two such widely used measures are the total fertility rate and the life expectancy at birth.

The total fertility rate is the number of children a woman would have during her reproductive life if she experienced the prevailing rates of fertility at each age.

High-fertility countries may have birth rates of 40 or even 50 per 1000 population (per year); corresponding levels of the Total fertility rate would be 5 to 7 children per woman.

Low-fertility countries have birth rates of 15 to 20 per 1000 and Total fertility rates of about 2.

"Replacement level" fertility (the level at which each person on average has a single successor in the next generation) corresponds to a Total fertility rate of about 2.1 under low-mortality conditions.

The life expectancy at birth is the average length of life that would be observed in a population in which the currently prevailing mortality risks at each age continued indefinitely.

Preindustrial populations were characterized by large fluctuations in mortality; long-run averages, however, would probably have shown death rates of 30 to 40 per 1000 and life expectancies of 25 to 35 years.

Under modern health conditions, death rates below 10 per 1000 and life expectancies above 70 years are common.

Another important mortality measure is the infant mortality rate.

This is the probability of death in the first year of life, usually stated as a number per 1000 births.

Many less-developed countries have infant mortality rates above 100 per 1000--that is, more than 10 percent of the children die in their first year.

In countries with effective health and educational systems, infant mortality rates are about 15 per 1000, or even lower.

WORLD POPULATION GROWTH AND DISTRIBUTION �
The United Nations (UN), an accepted authority on population levels and trends, estimates that the world population reached 6 billion in 1999, and is increasing annually by more than 77 million persons.

The rate of increase, 1.3 percent per year, has fallen below the peak rate of 2 percent per year attained by 1970.

By the late 2040s, the UN estimates, the growth rate will have fallen to about 0.34 percent annually, at which time more than 50 countries will experience negative growth.

Past and Present Growth �

Estimates of world population before 1900 are based on fragmentary data, but scholars agree that, for most of human existence, long-run average population growth approached approximately 0.002 percent per year, or 20 per million inhabitants.

According to UN estimates, the population of the world was about 300 million in the year 1 AD, and it took more than 1,600 years to reach the 600 million mark.

Growth was not steady but was marked by oscillations dictated by climate, food supply, disease, and war.

Starting in the 17th century, great advances in scientific knowledge, agriculture, industry, medicine, and social organization made possible rapid acceleration in population growth.

Inanimate energy gradually replaced human and animal labor. People slowly acquired the knowledge and means to control disease. By 1900 the world population had reached 1.65 billion, and by 1960 it stood at 3.02 billion.

Beginning about 1950, a new phase of population growth was ushered in when famine and disease could be controlled even in areas that had not yet attained a high degree of literacy or a technologically developed industrial society.

This happened as a result of the modest cost of importing the vaccines, antibiotics, insecticides, and high-yielding varieties of seeds produced since the 1950s.

With improvements in water supplies, sewage-disposal facilities, and transportation networks, agricultural yields increased, and deaths from infectious and parasitic diseases greatly declined.

Life expectancy at birth in most developing countries increased from about 35-40 years in 1950 to 66 years by 2000.

The rapid decline in deaths among people who maintained generally high fertility rates led to annual population growth that exceeded 3.1 percent in many developing nations--a rate that doubles population size in 23 years.

As of 1990, 1.2 billion people lived in the developed nations of the world, and 4.1 billion people lived in the less-developed countries.

By region, over half the world's population was in East and South Asia; China, with more than 1.2 billion inhabitants, and India, with some 880 million, were the dominant contributors.

Europe and the countries of the former USSR contained 15 percent, North and South America made up 14 percent, and Africa had 12 percent of world population.

Differences in regional growth rates are altering these percentages over time.

Africa's share of the world population is expected to more than double by the year 2025, while the population of South Asia and Latin America remains nearly constant and the other regions, including East Asia, decline appreciably in relative size.

The share of the present developed nations in world population--23 percent in 1990--is expected to fall to 17 percent by 2025.

Nine out of every ten persons who are now being added to the world's population are living in the less-developed countries.

Urban Concentration �
As a country develops from primarily an agricultural to an industrial economy, large-scale migration of rural residents to towns and cities takes place.

During this process, the growth rate of urban areas is typically double the pace of overall population increase.

Some 29 percent of the world population was living in urban areas in 1950; this figure was 43 percent in 1990, and is projected to rise to about 50 percent by the year 2000.

Urbanization eventually leads to a severe decline in the number of people living in the countryside, with negative population growth rates in rural areas.

Rapid growth of overall population has deferred this event in most less-developed countries, but it is projected to occur in the early decades of the 21st century.

Most migrants to the cities can be assumed to have bettered themselves in comparison to their former standard of living, despite the serious problems of overcrowding, substandard housing, and inadequate municipal services that characterize life for many arrivals to urban centers.

Dealing with these conditions, especially in very large cities, presents massive difficulties for the governments of less-developed countries.

Population Projections �Most of the potential parents of the next two decades have already been born.

Population projections over this interval can, therefore, be made with reasonable confidence, barring catastrophic changes.

Beyond two decades, however, uncertainties about demographic magnitudes and other characteristics of human societies build up rapidly, making any projections somewhat speculative.

The UN medium projections issued in 1990 show the world population increasing from 6 billion in 2000 to 7.8 billion in 2025 and 8.9 billion in 2050.

"High" and "low" projections for 2025 are 9.1 billion and 7.9 billion respectively.

The average world birth rate is projected to decline from the 1990 level of 26 per 1000 to 22 per 1000 at the end of the century and to 17 per 1000 in 2025 (corresponding to a fall in Total fertility rate from 3.3 in 1990 to 2.3 in 2025).

Because of the expanding share of the population at high-mortality ages, the average world death rate is expected to decline only slightly; from 9 (per 1000) in 1990 to 8 in 2025.

Average world life expectancy, however, is projected to rise from 65 years in 1990 to 73 years in 2025.

Wide variations in population growth will undoubtedly persist.

In the developed world, population growth will continue to be very low and in some nations will even decline.

Western Europe as a whole is projected to have a declining population after 2000.

U.S. Census Bureau projections, assuming middle fertility and mortality levels and net immigration averaging 880,000 per year, show U.S. population increasing from 249 million in 1990 to 334 million in 2025 and 383 million in 2050.

Thereafter, growth would be virtually zero.

The UN expects the less-developed countries to have steadily falling rates of population growth.

For the less-developed world as a whole, the 1990 growth rate of 2.0 percent per year is projected to be cut in half by 2025.

Africa will remain the region with the highest growth rate. In 1990 this rate was 3.1 percent; in 2025 it is projected to be about 2.2 percent.

Africa's population would almost triple, from 682 million in 1990 to 1.58 billion in 2025, and then continue growing at a rate that would almost double the population size in another 35 years.

POPULATION POLICIES �
Population Policy in the U.S. �The early immigrants to North America found a vast continent with a relatively small indigenous population. Overcrowding was incomprehensible because of the expanse of land to the west.

In the mid-20th century, as the rest of the world awakened to the potential crisis brought on by unchecked population growth, the U.S. government examined the possible impact of overpopulation in the nation.

The President's Commission on Population Growth and the American Future began a two-year study in 1970. Submitted to President Richard M. Nixon in 1972, it welcomed the prospect of zero population growth in the U.S., but did not propose that the government take strong measures to attain it.

The commission did, however, advocate education on family planning and widely available access to contraception and abortion services. Primarily because of this, the president rejected the commission's recommendations.

Since then, U.S. fertility has fallen below replacement level. This is due in part to the implicit policies that, taken together, make bearing and raising children very costly to parents.

Future policy concerns may reflect worry over population aging and the demographic aspects of funding social security.

In addition, the conflicting interests involved in determining numbers and characteristics of migrants is likely to keep immigration policy on the political agenda.

Population Policies in Developed Nations �European countries did not address the issue of a national population policy until the 20th century.

Subsidies were granted to expanding families by such disparate nations as the United Kingdom, Sweden, and the USSR.

The Italian Fascists in the 1920s and the National Socialists (Nazis) in Germany during the 1930s made population growth an essential part of their doctrines.

Japan, with an economy comparable to those of the European nations, was the first developed country in modern times to initiate a birth-control program.

In 1948 the Japanese government formally instituted a policy using both contraception and abortion to limit family size.

European pronatalist policies were conspicuously unsuccessful in the 1930s, and their milder variations over the past few decades (in, for example, France and many Eastern European nations) have apparently done little to slow a continuing fertility decline. Government control of migration is more straightforward. Short-term migration tied to labor demands (guest workers) has been a common practice in Western Europe, allowing the various nations the flexibility to curtail migration during economic recessions.

Population Policies in the Third World �In 1952 India took the lead among developing nations in adopting an official policy to slow its population growth.

India's stated purpose was to facilitate social and economic development by reducing the burden of a young and rapidly growing population.

Surveys to ascertain contraceptive knowledge, attitude, and practice showed a high proportion of couples wishing no more children. Few, however, practiced efficient contraception.

Family-planning programs were seen as a way to satisfy a desire for contraception by a large segment of the population and also to confer health benefits from spacing and limiting births.

Asia's lowered growth rate can be attributed mainly to the stringent population policies of China.

Although it has a huge population, China has successfully reduced both fertility and mortality. The government has recently been advocating one-child families to lower the nation's growth rate from a current estimate of 14 per 1000 annually to close to zero by the year 2000.

By 1979 more than 90 percent of the population in developing countries lived under governments that, in principle at least, supported access to contraceptives by their citizens, based on considerations of health and the right to choose to have children and to space them at desired intervals.

Recent evidence indicates that progress toward the objectives of lowered fertility and national growth is being achieved in many nations, in part by government support for family-planning programs.

Pollution

Pollution is, contamination of the earth's environment with materials that interfere with human health, the quality of life, or the natural functioning of ecosystems (living organisms and their physical surroundings).

Although some environmental pollution is a result of natural causes like volcanic eruptions, most is caused by human activities.

There are two main categories of polluting materials, or pollutants.

Biodegradable pollutants are materials, such as sewage, that rapidly decompose by natural processes. These pollutants become a problem when added to the environment faster than they can decompose.

Nondegradable pollutants are materials that either do not decompose or decompose slowly in the natural environment. Once contamination occurs, it is difficult or impossible to remove these pollutants from the environment.

Nondegradable compounds like dichlorodiphenyltrichloroethane (DDT), dioxins, polychlorinated biphenyls (PCBs), and radioactive materials can reach dangerous levels of accumulation as they are passed up the food chain into the bodies of progressively larger animals.

For example, molecules of toxic compounds may collect on the surface of aquatic plants without doing much damage to the plants. A small fish that grazes on these plants accumulates a high concentration of the toxin. Larger fish or other carnivores that eat the small fish will accumulate even greater, and possibly life-threatening, concentrations of the compound. This process is known as bioaccumulation.

IMPACTS OF POLLUTION �

Because humans are at the top of the food chain, they are particularly vulnerable to the effects of nondegradable pollutants.

This was clearly illustrated in the 1950s and 1960s when residents living near Minamata Bay, Japan, developed nervous disorders, tremors, and paralysis in a mysterious epidemic.

Over 400 people died before authorities discovered that a local industry had released mercury into Minamata Bay.

This highly toxic element accumulated in the bodies of local fish and eventually in the bodies of people who consumed the fish.

More recently research has revealed that many chemical pollutants, such as DDT and PCBs, mimic sex hormones and interfere with the human body's reproductive and developmental functions. These substances are known as endocrine disrupters.

Pollution also has a dramatic effect on natural resources.

Ecosystems such as forests, wetlands, coral reefs, and rivers perform many important services for the earth's environment. They enhance water and air quality, provide habitat for plants and animals, and provide food and medicines.

Any or all of these ecosystem functions may be impaired or destroyed by pollution.

Moreover, because of the complex relationships among the many types of organisms and ecosystems, environmental contamination may have far-reaching consequences that are not immediately obvious or that are difficult to predict.

For instance, scientists can only speculate on some of the potential impacts of the depletion of the ozone layer, the protective layer in the atmosphere that shields the earth from the sun's harmful ultraviolet rays.

Another major effect of pollution is the tremendous cost of pollution cleanup and prevention.

The global effort to control emissions of carbon dioxide, a gas produced from the combustion of fossil fuels such as coal or oil, or of other organic materials like wood, is one such example.

The cost of maintaining annual national carbon dioxide emissions at 1990 levels is estimated to be 2 percent of the gross domestic product for developed countries.

Expenditures to reduce pollution in the United States in 1993 totaled $109 billion, including $105.4 billion on reduction, $1.9 billion on regulation, and $1.7 billion on research and development.

In addition to its effects on the economy, health, and natural resources, pollution has social implications.

Research has shown that low-income populations and minorities do not receive the same protection from environmental contamination as do higher-income communities.

Toxic waste incinerators, chemical plants, and solid waste dumps are often located in low-income communities because of a lack of organized, informed community involvement in municipal decision-making processes.

TYPES OF POLLUTION �

Pollution exists in many forms and affects many different aspects of the earth's environment. Point-source pollution comes from specific, localized, and identifiable sources, such as sewage pipelines or industrial smokestacks.

Nonpoint-source pollution comes from dispersed or uncontained sources, such as contaminated water runoff from urban areas or automobile emissions.

The effects of these pollutants may be immediate or delayed.

Primary effects of pollution occur immediately after contamination occurs, such as the death of marine plants and wildlife after an oil spill at sea.

Secondary effects may be delayed or may persist in the environment into the future, perhaps going unnoticed for many years.

DDT, a nondegradable compound, seldom poisons birds immediately, but gradually accumulates in their bodies. Birds with high concentrations of this pesticide lay thin-shelled eggs that fail to hatch or produce deformed offspring.

These secondary effects, publicized by Rachel Carson in her 1962 book, Silent Spring, threatened the survival of species such as the bald eagle and peregrine falcon, and aroused public concern over the hidden effects of nondegradable chemical compounds.

Air Pollution �

Human contamination of the earth's atmosphere can take many forms and has existed since humans first began to use fire for agriculture, heating, and cooking.

During the Industrial Revolution of the 18th and 19th centuries, however, air pollution became a major problem. As early as 1661 British author and founding member of the British Royal Society John Evelyn reported of London in his treatise Fumifugium, "�the weary Traveller, at many Miles distance, sooner smells, than sees the City to which he repairs. This is that pernicious Smoake which fullyes all her Glory, superinducing a sooty Crust or Furr upon all that it lights�"

Urban air pollution is commonly known as smog. The dark London smog that Evelyn wrote of is generally a smoky mixture of carbon monoxide and organic compounds from incomplete combustion (burning) of fossil fuels such as coal, and sulfur dioxide from impurities in the fuels.

As the smog ages and reacts with oxygen, organic and sulfuric acids condense as droplets, increasing the haze.

Smog developed into a major health hazard by the 20th century. In 1948, 19 people died and thousands were sickened by smog in the small U.S. steel mill town of Donora, Pennsylvania. In 1952, 2000 Londoners died of its effects.

A second type of smog, photochemical smog, began reducing air quality over large cities like Los Angeles in the 1930s. This smog is caused by combustion in car, truck, and airplane engines, which produce nitrogen oxides and release hydrocarbons from unburned fuels.

Sunlight causes the nitrogen oxides and hydrocarbons to combine and turn oxygen into ozone, a chemical agent that attacks rubber, injures plants, and irritates lungs. The hydrocarbons are oxidized into materials that condense and form a visible, pungent haze.

Eventually most pollutants are washed out of the air by rain, snow, fog, or mist, but only after traveling large distances, sometimes across continents.

As pollutants build up in the atmosphere, sulfur and nitrogen oxides are converted into acids that mix with rain. This acid rain falls in lakes and on forests, where it can lead to the death of fish and plants, and damage entire ecosystems.

Eventually the contaminated lakes and forests may become lifeless. Regions that are downwind of heavily industrialized areas, such as Europe and the eastern United States and Canada, are the hardest hit by acid rain.

Acid rain can also affect human health and man-made objects; it is slowly dissolving historic stone statues and building facades in London, Athens, and Rome.

One of the greatest challenges caused by air pollution is global warming, an increase in the earth's temperature due to the buildup of atmospheric gases such as carbon dioxide.

With the heavy use of fossil fuels in the 20th century, atmospheric concentrations of carbon dioxide have risen dramatically. Carbon dioxide and other gases, known as greenhouse gases, reduce the escape of heat from the planet without blocking radiation coming from the sun.

Because of this greenhouse effect, average global temperatures are expected to rise 1.8� to 6.3� F by the year 2100.

Although this trend appears to be a small change, the increase would make the earth warmer than it has been in the last 125,000 years, possibly changing climate patterns, affecting crop production, disrupting wildlife distributions, and raising the sea level.

Air pollution can also damage the upper atmospheric region known as the stratosphere.

Excessive production of chlorine-containing compounds such as chlorofluorocarbons (CFCs) (compounds used in refrigerators, air conditioners, and in the manufacture of polystyrene products) has depleted the stratospheric ozone layer, creating a hole above Antarctica that lasts for several weeks each year.

As a result, exposure to the sun's harmful rays has damaged aquatic and terrestrial wildlife and threatens human health in high-latitude regions of the northern and southern hemispheres.

Water Pollution �
The demand for freshwater rises continuously as the world's population grows.

From 1940 to 1990, withdrawals of fresh water from rivers, lakes, reservoirs, and other sources has increased fourfold.

Of the water consumed each year, 69 percent is used for agriculture, 23 percent for industry, and 8 percent for domestic uses.

Sewage, industrial wastes, and agricultural chemicals such as fertilizers and pesticides are the main causes of water pollution. In 1995, the U.S. Environmental Protection Agency (EPA) reported that about 37 percent of the country's lakes and estuaries, and 36 percent of its rivers, are too polluted for basic uses such as fishing or swimming, during all or part of the year.

In developing nations, over 95 percent of urban sewage is discharged untreated into rivers and bays, creating a major human health hazard.

Water runoff, a nonpoint source of pollution, carries fertilizing chemicals such as phosphates and nitrates from agricultural fields and yards into lakes, streams, and rivers. These combine with the phosphates and nitrates from sewage to speed the growth of algae, a type of aquatic plant.

The water body may then become choked with decaying algae, which severely depletes the oxygen supply.

This process, called eutrophication, can cause the death of fish and other aquatic life.

Erosion, the wearing away of topsoil by wind and rain, also contributes to water pollution.

Soil and silt (a fine sediment) washed from logged hillsides, plowed fields, or construction sites, can clog waterways and kill aquatic vegetation.

Even small amounts of silt can eliminate desirable fish species. For example, when logging removes the protective plant cover from hillsides, rain may wash soil and silt into streams, covering the gravel beds that trout or salmon use for spawning.

The marine fisheries supported by ocean ecosystems are an essential source of protein, particularly for people in developing countries; approximately 950 million people worldwide consume fish as their primary source of protein.

Yet pollution in coastal bays, estuaries, and wetlands threatens fish stocks already depleted by overfishing. In 1989, 260,000 barrels of oil was spilled from the oil tanker Exxon Valdez into Alaska's Prince William Sound, a pristine and rich fishing ground.

In 1992 there were 8790 reported spills in and around U.S. waters, involving 5.7 million liters of oil.

Soil Pollution �

Soil is a mixture of mineral, plant, and animal materials that forms during a long process that may take thousands of years.

Unhealthy soil management methods have seriously degraded soil quality, caused soil pollution, and enhanced erosion.

Treating the soil with chemical fertilizers, pesticides, and fungicides interferes with the natural processes occurring within the soil and destroys useful organisms such as bacteria, fungi, and other microorganisms.

For instance, strawberry farmers in California fumigate the soil with methyl bromide to destroy organisms that may harm young strawberry plants. This process indiscriminately kills even beneficial microorganisms and leaves the soil sterile and dependent upon fertilizer to support plant growth.

This results in heavy fertilizer use and increases polluted runoff into lakes and streams.

Improper irrigation practices in areas with poorly drained soil may result in salt deposits that inhibit plant growth and may lead to crop failure.

In 2000 BC, the ancient Sumerian cities of the southern Tigris-Euphrates Valley in Mesopotamia depended on thriving agriculture. By 1500 BC, these cities had collapsed largely because of crop failure due to high soil salinity.

The same soil pollution problem exists today in the Indus Valley in Pakistan, the Nile Valley in Egypt, and the Imperial Valley in California.

Solid Waste �
Solid wastes are unwanted solid materials such as garbage, paper, plastics and other synthetic materials, metals, and wood.

Billions of tons of solid waste are thrown out annually. The United States alone produces about 200 million metric tons of municipal solid waste each year

.

A typical American generates an average of four pounds of solid waste each day.

Cities in economically developed countries produce far more solid waste per capita than those in developing countries.

For instance, Washington D.C. produces five times the solid waste, per person, of Quito, Ecuador.

Moreover, waste from developed countries typically contains a high percentage of synthetic materials that take longer to decompose than the primarily biodegradable waste materials of developing countries.

Areas where wastes are buried, called landfills, are the cheapest and most common disposal method for solid wastes worldwide. But landfills quickly become overfilled and may contaminate air, soil, and water.

Incineration, or burning, of waste reduces the volume of solid waste, but produces dense ashen wastes (some of which become airborne) that often contain dangerous concentrations of hazardous materials such as heavy metals and toxic compounds.

Composting, using natural biological processes to speed the decomposition of organic wastes, is an effective strategy for dealing with organic garbage and produces a material that can be used as a natural fertilizer.

Recycling, extracting and reusing certain waste materials, has become an important part of municipal solid waste strategies in developed countries. According to the EPA, over one-fifth of the municipal solid waste produced in the United States is now recycled or composted.

Recycling also plays a significant, informal role in solid waste management for many Asian countries, such as India, where organized waste-pickers comb streets and dumps for items such as plastics, which they use or resell.

Expanding recycling programs worldwide can help reduce solid waste pollution, but the key to solving severe solid waste problems lies in reducing the amount of waste generated.

Waste prevention, or source reduction, such as altering the way products are designed or manufactured to make them easier to reuse, reduces the high costs associated with environmental pollution.

Hazardous Waste �
Hazardous wastes are solid, liquid, or gas wastes that may be deadly or harmful to people or the environment and tend to be persistent or nondegradable in nature.

Such wastes include toxic chemicals and flammable or radioactive substances, including industrial wastes from chemical plants or nuclear reactors, agricultural wastes such as pesticides and fertilizers, medical wastes, and household hazardous wastes such as toxic paints and solvents.

About 400 million metric tons of hazardous wastes are generated each year. The United States alone produces 240 million metric tons--70 percent from the chemical industry.

The use, storage, transportation, and disposal of these substances pose serious environmental and health risks.

Even brief exposure to some of these materials can cause cancer, birth defects, nervous system disorders, and death.

Large-scale releases of hazardous materials may cause thousands of deaths and contaminate air, water, and soil for many years.

The world's worst nuclear reactor accident took place near Chernobyl', Ukraine, in 1986 . The accident killed at least 31 people, forced the evacuation of over 100,000 more, and sent a plume of radioactive material into the atmosphere that contaminated areas as far away as Norway and the United Kingdom.

Until the Minamata Bay contamination was discovered in Japan in the 1960s and 70s, most hazardous wastes were legally dumped in solid waste landfills, buried, or dumped into lakes, rivers, and oceans.

Legal regulations now restrict how such materials may be used or disposed, but such laws are difficult to enforce and often contested by industry.

It is not uncommon for industrial firms in developed countries to pay poorer countries to accept shipments of solid and hazardous wastes, a practice that has become known as the waste trade.

Moreover, cleaning up the careless dumping of the mid-20th century is costing billions of dollars and progressing very slowly, if at all.

The United States has an estimated 10,000 abandoned hazardous waste dumps that need immediate action. Cleaning them up could take 50 years and cost $100 billion.

Hazardous wastes of particular concern are the radioactive wastes from the nuclear power and weapons industries. To date there is no safe method for permanent disposal of old fuel elements from nuclear reactors.

Most are kept in storage facilities at the original reactor sites where they were generated. With the end of the Cold War, nuclear warheads that are decommissioned, or no longer in use, also pose storage and disposal problems.

Noise Pollution �
Unwanted sound, or noise, such as that produced by airplanes, traffic, or industrial machinery, is considered a form of pollution.

Noise pollution is at its worst in densely populated areas. It can cause hearing loss, stress, high blood pressure, sleep loss, distraction, and lost productivity.

Sounds are produced by objects that vibrate at a rate that the ear can detect. This rate is called frequency and is measured in hertz, or vibrations per second. Most humans can hear sounds between 20 and 20,000 hertz, while dogs can hear high-pitched sounds up to 50,000 hertz.

While high-frequency sounds tend to be more hazardous and more annoying to hearing than low-frequency sounds, most noise pollution damage is related to the intensity of the sound, or the amount of energy it has.

Measured in decibels, noise intensity can range from zero, the quietest sound the human ear can detect, to over 160 decibels.

Conversation takes place at around 40 decibels, a subway train is about 80 decibels, and a rock concert is from 80 to 100 decibels. The intensity of a nearby jet taking off is about 110 decibels. The threshold for pain, tissue damage, and potential hearing loss in humans is 120 decibels. Long-lasting, high-intensity sounds are the most damaging to hearing and produce the most stress in humans.

Solutions to noise pollution include adding insulation and sound-proofing to doors, walls, and ceilings; using ear protection, particularly in industrial working areas; planting vegetation to absorb and screen out noise pollution; and zoning urban areas to maintain a separation between residential areas and zones of excessive noise.

HISTORY �
As humans developed new technologies, the magnitude and severity of pollution increased.

Many historians speculate that the extensive use of lead plumbing for drinking water in Rome caused chronic lead poisoning in those who could afford such plumbing.

The mining and smelting of ores that accompanied the transition from the Stone Age to the Metal Age resulted in piles of mining wastes that spread potentially toxic elements such as mercury, copper, lead, and nickel throughout the environment.

Evidence of pollution during the early Industrial Revolution is widespread. Samples of hair from historical figures such as Newton and Napoleon show the presence of toxic elements such as antimony and mercury.

By the 1800s, certain trades were associated with characteristic occupational diseases: Chimney sweeps contracted cancer of the scrotum from hydrocarbons in chimney soot; hatters became disoriented, or "mad," from nerve-destroying mercury salts used to treat felt fabric; and bootblacks suffered liver damage from boot polish solvents.

During the 20th century, pollution evolved from a mainly localized problem to one of global consequences in which pollutants not only persisted in the environment, but changed atmospheric and climatic conditions.

The Minamata Bay disaster was the first major indication that humans would need to pay more attention to their waste products and waste disposal practices, in particular, hazardous waste disposal.

In the years that followed, many more instances of neglect or carelessness resulted in dangerous levels of contamination.

In 1978 it was discovered that the Love Canal housing development in New York state was built on a former chemical waste dump. The development was declared uninhabitable.

The world's worst industrial accident occurred in Bhopal, India in 1984. A deadly gas leaked from an American chemical plant, killing over 2000 people while injuring over 200,000.

The 1986 Chernobyl nuclear reactor accident demonstrated the dangerous contamination effects of large, uncontained disasters.

In an unprecedented action, pollution was used as a military tactic in 1991 during the conflict in the Persian Gulf. The Iraqi military intentionally released as much as 1 billion liters (336 million gallons) of crude oil into the Persian Gulf and set fire to over 700 oil wells, sending thick, black smoke into the atmosphere over the Middle East.

CONTROLLING POLLUTION �

Because of the many environmental tragedies of the mid-20th century, many nations instituted new, comprehensive regulations designed to repair the past damage of uncontrolled pollution and prevent future environmental contamination.

In the United States, the Clean Air Act (1970) and its amendments significantly reduced certain types of air pollution, such as sulfur dioxide emissions.

The Clean Water Act (1977) and Safe Drinking Water Act (1974) regulated pollution discharges and set water quality standards.

The Toxic Substances Control Act (1976) and the Resource Conservation and Recovery Act (1976) provided for the testing and control of toxic and hazardous wastes.

In 1980, Congress passed the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), also known as Superfund, to provide funds to clean up the most severely contaminated hazardous waste sites.

These and several other federal and state laws helped limit uncontrolled pollution, but progress has been slow and many severe contamination problems remain due to lack of funds for cleanup and enforcement.

International agreements have also played a role in reducing global pollution. The Montreal Protocol on Substances that Deplete the Ozone Layer (1987) set international target dates for reducing the manufacture and emissions of the chemicals, such as CFCs, known to deplete the ozone layer.

Regulations and legislation have led to considerable progress in cleaning up air and water pollution in developed countries.

Vehicles in the 1990s emit fewer nitrogen oxides than those in the 1970s did; power plants now burn low-sulfur fuels; industrial stacks have scrubbers to reduce emissions; and lead has been removed from gasoline.

Developing countries, however, continue to struggle with pollution control because they lack clean technologies and desperately need to improve economic strength, often at the cost of environmental quality.

The problem is compounded by developing countries attracting foreign investment and industry by offering cheaper labor, cheaper raw materials, and fewer environmental restrictions.

The assembly plants along the Mexican side of the Mexico-U.S. border, provide jobs and industry for Mexico, but are generally owned by non-Mexican corporations attracted to the cheap labor and lack of pollution regulation.

As a result, this border region, including the R�o Grande river, is one of the most heavily polluted zones in North America.

To avoid ecological disaster and increased poverty, developing countries will require aid and technology from outside nations and corporations, community participation in development initiatives, and strong environmental regulations.

Nongovernmental citizen groups have formed at the local, national, and international level to combat pollution problems worldwide.

Many of these organizations provide information and support for people or organizations traditionally not involved in the decision-making process.

The Pesticide Action Network provides technical information about the effects of pesticides on farmworkers. The Citizen's Clearinghouse for Hazardous Waste, established by veterans of the Love Canal controversy, provides support for communities targeted for hazardous waste installations.

Greenpeace International is an activist organization that focuses international attention on industries and governments known to contaminate land, sea, or atmosphere with toxic or solid wastes.

Friends of the Earth International is a federation of international organizations that fight environmental pollution around the world.

General Comments on Ecology

The term ecology was introduced by the German biologist Ernst Heinrich Haeckel in 1866; it is derived from the Greek oikos ("household"), sharing the same root word as economics. Thus, the term implies the study of the economy of nature.

Modern ecology, in part, began with Charles Darwin. In developing his theory of evolution, Darwin stressed the adaptation of organisms to their environment through natural selection. Also making important contributions were plant geographers, such as Alexander von Humboldt, who were deeply interested in the "how" and "why" of vegetational distribution around the world.

THE EARTH'S BIOSPHERE �

The thin mantle of life that covers the earth is called the biosphere.

Ecosystems �
A useful way of looking at the terrestrial and aquatic landscapes is to view them as ecosystems, a word coined in 1935 by the British plant ecologist Sir Arthur George Tansley to stress the concept of each locale or habitat as an integrated whole.

A system is a collection of interdependent parts that function as a unit and involve inputs and outputs. The major parts of an ecosystem are the producers (green plants), the consumers (herbivores and carnivores), the decomposers (fungi and bacteria), and the nonliving, or abiotic, component, consisting of dead organic matter and nutrients in the soil and water.

Inputs into the ecosystem are solar energy, water, oxygen, carbon dioxide, nitrogen, and other elements and compounds.

Outputs from the ecosystem include water, oxygen, carbon dioxide, nutrient losses, and the heat released in cellular respiration, or heat of respiration. The major driving force is solar energy.

Energy and Nutrients �
Ecosystems function with energy flowing in one direction from the sun, and through nutrients, which are continuously recycled.

Light energy is used by plants, which, by the process of photosynthesis, convert it to chemical energy in the form of carbohydrates and other carbon compounds.

This energy is then transferred through the ecosystem by a series of steps that involve eating and being eaten, or what is called a food web.

Each step in the transfer of energy involves several trophic, or feeding, levels: plants, herbivores (plant eaters), two or three levels of carnivores (meat eaters), and decomposers.

Only a fraction of the energy fixed by plants follows this pathway, known as the grazing food web.

Plant and animal matter not used in the grazing food chain, such as fallen leaves, twigs, roots, tree trunks, and the dead bodies of animals, support the decomposer food web.

Bacteria, fungi, and animals that feed on dead material become the energy source for higher trophic levels that tie into the grazing food web. In this way nature makes maximum use of energy originally fixed by plants.

The number of trophic levels is limited in both types of food webs, because at each transfer a great deal of energy is lost (such as heat of respiration) and is no longer usable or transferable to the next trophic level.

Thus, each trophic level contains less energy than the trophic level supporting it. For this reason, as an example, deer or caribou (herbivores) are more abundant than wolves (carnivores).

Because most plants and animals go uneaten, nutrients contained in their tissues, after passing through the decomposer food web, are ultimately released by bacterial and fungal decomposition, a process that reduces complex organic compounds into simple inorganic compounds available for reuse by plants.

Imbalances �
Within an ecosystem nutrients are cycled internally. But there are leakages or outputs, and these must be balanced by inputs, or the ecosystem will fail to function.

Nutrient inputs to the system come from weathering of rocks, from windblown dust, and from precipitation, which can carry material great distances.

Varying quantities of nutrients are carried from terrestrial ecosystems by the movement of water and deposited in aquatic ecosystems and associated lowlands.

Erosion and the harvesting of timber and crops remove considerable quantities of nutrients that must be replaced. The failure to do so results in an impoverishment of the ecosystem. This is why agricultural lands must be fertilized.

If inputs of any nutrient greatly exceed outputs, the nutrient cycle in the ecosystem becomes stressed or overloaded, resulting in pollution.

Pollution can be considered an input of nutrients exceeding the capability of the ecosystem to process them.

Nutrients eroded and leached from agricultural lands, along with sewage and industrial wastes accumulated from urban areas, all drain into streams, rivers, lakes, and estuaries. These pollutants destroy plants and animals that cannot tolerate their presence or the changed environmental conditions caused by them; at the same time they favor a few organisms more tolerant to changed conditions.

Thus, precipitation filled with sulfur dioxide and oxides of nitrogen from industrial areas converts to weak sulfuric and nitric acids, known as acid rain, and falls on large areas of terrestrial and aquatic ecosystems. This upsets acid-base relations in some ecosystems, killing fish and aquatic invertebrates, and increasing soil acidity, which reduces forest growth in northern and other ecosystems that lack limestone to neutralize the acid.

POPULATIONS AND COMMUNITIES �
The functional units of an ecosystem are the populations of organisms through which energy and nutrients move. A population is a group of interbreeding organisms of the same kind living in the same place at the same time .

Groups of populations within an ecosystem interact in various ways. These interdependent populations of plants and animals make up the community, which encompasses the biotic portion of the ecosystem.

Habitat and Niche �
The community provides the habitat--the place where particular plants or animals live. Within the habitat, organisms occupy different niches.

A niche is the functional role of a species in a community--that is, its occupation, or how it earns its living.

For example, the scarlet tanager lives in a deciduous forest habitat. Its niche, in part, is gleaning insects from the canopy foliage.

Population Growth Rates �

Populations have a birth rate (the number of young produced per unit of population per unit of time), a death rate (the number of deaths per unit of time), and a growth rate.

The major agent of population growth is births, and the major agent of population loss is deaths. When births exceed deaths, a population increases; and when deaths exceed additions to a population, it decreases. When births equal deaths in a given population, its size remains the same, and it is said to have zero population growth.

When introduced into a favorable environment with an abundance of resources, a small population may undergo geometric, or exponential growth, in the manner of compound interest.

Many populations experience exponential growth in the early stages of colonizing a habitat because they take over an underexploited niche or drive other populations out of a profitable one.

Those populations that continue to grow exponentially, however, eventually reach the upper limits of the resources; they then decline sharply because of some catastrophic event such as starvation, disease, or competition from other species.

In a general way, populations of plants and animals that characteristically experience cycles of exponential growth are species that produce numerous young, provide little in the way of parental care, or produce an abundance of seeds having little food reserves.

These species, usually short-lived, disperse rapidly and are able to colonize harsh or disturbed environments. Such organisms are often called opportunistic species.

Other populations tend to grow exponentially at first, and then logistically--that is, their growth slows as the population increases, then levels off as the limits of their environment or carrying capacity are reached.

Through various regulatory mechanisms, such populations maintain something of an equilibrium between their numbers and available resources.

Animals exhibiting such population growth tend to produce fewer young but do provide them with parental care; the plants produce large seeds with considerable food reserves.

These organisms are long-lived, have low dispersal rates, and are poor colonizers of disturbed habitats. They tend to respond to changes in population density (the number of organisms per unit area) through changes in birth and death rates rather than through dispersal.

As the population approaches the limit of resources, birth rates decline, and mortality of young and adults increases.

Community Interactions �
Major influences on population growth involve various population interactions that tie the community together. These include competition, both within a species and among species and predation, including parasitism.

Competition �

When a shared resource is in short supply, organisms compete, and those that are more successful survive.

Within some plant and animal populations, all individuals may share the resources in such a way that none obtains sufficient quantities to survive as adults or to reproduce.

Among other plant and animal populations, dominant individuals claim access to the scarce resources and others are excluded. Individual plants tend to claim and hold onto a site until they lose vigor or die. These prevent other individuals from surviving by controlling light, moisture, and nutrients in their immediate areas.

Many animals have a highly developed social organization through which resources such as space, food, and mates are apportioned among dominant members of the population.

Such competitive interactions may involve social dominance, in which the dominant individuals exclude subdominant individuals from the resource; or they may involve territoriality, in which the dominant individuals divide space into exclusive areas, which they defend.

Subdominant or excluded individuals are forced to live in poorer habitats, do without the resource, or leave the area. Many of these animals succumb to starvation, exposure, and predation.

Competition among members of different species results in the division of resources in a community.

Certain plants, for example, have roots that grow to different depths in the soil.

Some have shallow roots that permit them to use moisture and nutrients near the surface.

Others growing in the same place have deep roots that are able to exploit moisture and nutrients not available to surface-rooted plants.

Predation �
One of the fundamental interactions is predation, or the consumption of one living organism, plant or animal, by another.

While it serves to move energy and nutrients through the ecosystem, predation may also regulate population and promote natural selection by weeding the unfit from a population.

Thus, a rabbit is a predator on grass, just as the fox is a predator on the rabbit.

Predation on plants involves defoliation by grazers and the consumption of seeds and fruits. The abundance of plant predators, or herbivores, directly influences the growth and survival of the carnivores.

Thus, predator-prey interactions at one feeding level influence the predator-prey relations at the next feeding level.

Succession and Climax Communities �
Ecosystems are dynamic, in that the populations constituting them do not remain the same.

This is reflected in the gradual changes of the vegetational community over time, known as succession.

It begins with the colonization of a disturbed area, such as an abandoned crop field or a newly exposed lava flow, by species able to reach and to tolerate the environmental conditions present.

Mostly these are opportunistic species that hold on to the site for a variable length of time. Being short-lived and poor competitors, they are eventually replaced by more competitive, longer-lived species such as shrubs, and then trees.

In aquatic habitats, successional changes of this kind result largely from changes in the physical environment, such as the buildup of silt at the bottom of a pond.

As the pond becomes more shallow, it encourages the invasion of floating plants such as pond lilies and emergent plants such as cattails.

The pace at which succession proceeds depends on the competitive abilities of the species involved; tolerance to the environmental conditions brought about by changes in vegetation; the interaction with animals, particularly the grazing herbivores; and fire.

Eventually the ecosystem arrives at a point called the climax, where further changes take place very slowly, and the site is dominated by long-lived, highly competitive species.