Chapter 38 BSU Reading
38.1Pollution
Chemical Pollution
Learning Objective 38.1.1Compare how air and water are chemically polluted.
Our world is one ecological continent, one highly interactive biosphere, and damage done to any one
ecosystem can have ill effects on many others. Burning high-sulfur coal in Illinois kills trees in Vermont,
while dumping refrigerator coolants in New York destroys atmospheric ozone over Antarctica and leads to
increased skin cancer in Madrid. Biologists call such widespread effects on the worldwide
ecosystemglobal change. The pattern of global change that has become evident within recent years,
including chemical pollution, acid precipitation, the ozone hole, the greenhouse effect, and the loss of
biodiversity, is one of the most serious problems facing humanity's future.
The problem posed by chemical pollution has grown very serious in recent years, both because of the
growth of heavy industry and because of an overly casual attitude in industrialized countries. In one
example, a poorly piloted oil tanker named the Exxon Valdez ran aground in Alaska in 1989 and spilled oil
over many kilometers of North American coastline, killing many of the organisms that live there and
coating the land with a thick layer of sludge. If the tanker had been loaded no higher than the waterline,
little oil would have been lost, but it was loaded far higher than that, and the weight of the above-waterline
oil forced thousands of tons of oil out the hole in the ship's hull. Why do policies permit overloading like
this?
Air Pollution Air pollution is a major problem in the world's cities. In Mexico City, oxygen is sold routinely
on corners for patrons to inhale. Cities such as New York, Boston, and Philadelphia are known as gray-air
cities because the pollutants in the air are usually sulfur oxides emitted by industry. Cities such as Los
Angeles, however, are called brown-air cities because the pollutants in the air undergo chemical reactions
in the sunlight to form smog.
911 Air Pollution
Water Pollution Water pollution is a very serious consequence of our casual attitude about pollution.
“Flushing it down the sink” doesn't work in today's crowded world. There is simply not enough water
available to dilute the many substances that the enormous human population produces continuously.
Despite improved methods of sewage treatment, lakes and rivers throughout the world are becoming
increasingly polluted with sewage. In addition, fertilizers and insecticides also get washed from the land to
the water in great quantities.
Thames River
Agricultural Chemicals
Learning Objective 38.1.2Explain how biological magnification endangered bald eagles.
The spread of “modern” agriculture and particularly the Green Revolution, which brought high-intensity
farming to developing countries, have caused very large amounts of many kinds of new chemicals to be
introduced into the global ecosystem, particularly pesticides, herbicides, and fertilizers. Industrialized
countries like the United States now attempt to carefully monitor side effects of these chemicals.
Unfortunately, large quantities of many toxic chemicals, although no longer manufactured, still circulate in
the ecosystem.
For example, the chlorinated hydrocarbons, compounds that include DDT, chlordane, lindane, and
dieldrin, have been banned in the United States, where they were once widely used. They are still
manufactured in the United States and exported to other countries, where their use continues.
Chlorinated hydrocarbon molecules break down slowly and accumulate in animal fat tissue. Furthermore,
as they pass through a food chain, they become increasingly concentrated in a process called biological
magnification. Figure 38.1 shows how a minute concentration of DDT in plankton increases to
significant levels as it is passed up through this aquatic food chain. In the United States and elsewhere,
DDT caused serious ecological problems by leading to the production of thin, fragile eggshells in many
predatory bird species, such as peregrine falcons, bald eagles, osprey, and brown pelicans. In the late
1960s, DDT was banned in time to save the birds from extinction. Chlorinated compounds have other
undesirable side effects and exhibit hormonelike activities in the bodies of animals.
Figure 38.1 Biological magnification of DDT.Because DDT accumulates in animal fat, the compound
becomes increasingly concentrated in higher levels of the food chain.
Inherited Pollution
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Key Learning Outcome 38.1 All over the globe, increasing industrialization is leading to higher
levels of pollution.
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38.2Acid Precipitation
The Threat of Acid Rain
Learning Objective 38.2.1Discuss the sources and consequences of acid precipitation.
The smokestacks you see in figure 38.2 are those of a power plant that burns coal, sending the smoke
high into the atmosphere through these tall stacks. The smoke contains high concentrations of sulfur
dioxide and other sulfates, which produce acid when they combine with water vapor in the air. The first
tall stacks were introduced in Britain in the mid-1950s, and the design rapidly spread through Europe and
the United States. The intent of having tall smokestacks was to release the sulfur-rich smoke high in the
atmosphere, where winds would disperse and dilute it, carrying the acids far away.
Figure 38.2 Tall stacks export pollution.Tall stacks, as seen in this coal-burning power plant, send pollution far
up into the atmosphere.
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However, in the 1970s, scientists began noticing that the acids from the sulfur-rich smoke were having
devastating effects. Throughout northern Europe, lakes were reported to have suffered drastic drops in
biodiversity, some even becoming devoid of life. The trees of the great Black Forest of Germany seemed
to be dying, and the damage was not limited to Europe. In the eastern United States and Canada, many
of the forests and lakes have been seriously damaged.
It turns out that when the sulfur introduced into the upper atmosphere combined with water vapor to
produce sulfuric acid, the acid was taken far from its source, but it later fell along with water as acidic rain
and snow. This pollution-acidified precipitation is called acid rain (but the term acid precipitation is
actually more correct). Natural rainwater rarely has a pH lower than 5.6; however, rain and snow in many
areas of the United States have pH values less than 5.3, and in the Northeast, pHs of 4.2 or below have
been recorded, with occasional storms as low as 3.0.
Acid precipitation destroys life. Many of the forests of the northeastern United States and Canada have
been seriously damaged. In fact, it is now estimated that at least 1.4 million acres of forests in the
Northern Hemisphere have been adversely affected by acid precipitation (figure 38.3). In addition,
thousands of lakes in Sweden and Norway no longer support fish; these lakes are now eerily clear. In the
northeastern United States and Canada, tens of thousands of lakes are dying biologically as their pH
levels fall to below 5.0. At pH levels below 5.0, many fish species and other aquatic animals die, unable to
reproduce.
Figure 38.3 Acid precipitation.Acid precipitation is killing many of the trees in North American and European
forests. Much of the damage is done to the mycorrhizae, fungi growing within the cells of the tree roots. Trees need
mycorrhizae in order to extract nutrients from the soil.
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The solution seems like it would be easy—clean up the sulfur emissions. But there have been serious
problems with implementing this solution. First, it is expensive. Estimates of the cost of installing and
maintaining the necessary emission “scrubbers” in the United States are on the order of $5 billion a year.
An additional difficulty is that the polluter and the recipient of the pollution are far from one another, and
neither wants to pay so much for what they view as someone else's problem. Clean air legislation has
begun to address this problem by mandating some cleaning of emissions in the United States, although
much still remains to be done worldwide.
Key Learning Outcome 38.2 Pollution-acidified precipitation—loosely called acid rain—is
destroying forest and lake ecosystems in Europe and North America. The solution is to clean up
the emissions.
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38.3Global Warming
Learning Objective 38.3.1Explain the greenhouse effect.
For over 150 years, the growth of our industrial society has been fueled by cheap energy, much of it
obtained by burning fossil fuels—coal, oil, and gas. Coal, oil, and gas are the remains of ancient plants,
transformed by pressure and time into carbon-rich “fossil fuels.” When such fossil fuels are burned, this
carbon is combined with oxygen atoms, producing carbon dioxide (CO 2). Industrial society's burning of
fossil fuels has released huge amounts of carbon dioxide into the atmosphere. No one paid any attention
to this because the carbon dioxide was thought to be harmless and because the atmosphere was thought
to be a limitless reservoir, able to absorb and disperse any amount. It turns out neither assumption is true,
and in recent decades, the levels of carbon dioxide in the atmosphere have risen sharply and are
continuing to rise.
What is alarming is that the carbon dioxide doesn't just sit in the air doing nothing. The chemical bonds in
carbon dioxide molecules transmit radiant energy from the sun but trap the longer wavelengths of infrared
light, or heat, that are reflected off the earth's surface and prevent them from radiating back into space.
This creates what is known as the greenhouse effect. Planets that lack this type of “trapping”
atmosphere are much colder than those that possess one. If the earth did not have a “trapping”
atmosphere, the average earth temperature would be about –20°C, instead of the actual +15°C.
Global Warming
Global Warming Due to Greenhouse Gases
Learning Objective 38.3.2Assess the argument that global warming is the consequence of increased
carbon dioxide in the atmosphere.
The rise in average global temperatures during recent decades, a profound change in the earth's
atmosphere referred to as global warming (shown as the red line in figure 38.4), is correlated with
increased carbon dioxide concentrations in the atmosphere (the blue line). The suggestion that global
warming might in fact be caused by the accumulation of greenhouse gases (carbon dioxide, CFCs,
nitrogen oxides, and methane) in the atmosphere has been controversial and is examined in detail in the
Inquiry & Analysis feature at the end of this chapter. After serious examination of the evidence, the
overwhelming consensus among scientists is that greenhouse gases are indeed causing global warming.
Figure 38.4 The greenhouse effect.The concentration of carbon dioxide in the atmosphere has shown a steady
increase for many years (blue line). The red line shows the average global temperature for the same period of time.
Note the general increase in temperature since the 1950s and, specifically, the sharp rise beginning in the 1980s.
Data from the National Center for Atmospheric Research and other sources.
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Increases in the amounts of greenhouse gases increase average global temperatures from 1° to 4°C,
which could have serious impact on rain patterns, prime agricultural lands, and sea levels.
Effects on Rain Patterns Global warming is predicted to have a major effect on rainfall patterns. Areas
that have already been experiencing droughts may see even less rain, contributing to even greater water
shortages. Recent increases in the frequency of El Niño events (see chapter 36) and catastrophic
hurricanes may indicate that climatic changes due to global warming are already beginning to occur.
Karoo Global Warming
Effects on Agriculture Both positive and negative effects of global warming on agriculture are predicted.
Warmer temperatures and increased levels of carbon dioxide in the atmosphere would be expected to
increase the yields of some crops but have a negative impact on others. Droughts that may result from
global warming will also negatively affect crops. Plants in the tropics are growing at near their maximal
temperature limits; any further increases in temperature will probably begin to have a negative impact on
agricultural yields of tropical farms.
Rising Sea Levels Much of the water on Earth is locked into ice in glaciers and polar ice caps. As global
temperatures increase, these large stores of ice have begun to melt. Most of the water from the melted
glaciers ends up in the oceans, causing water levels to rise (but because the Arctic ice cap floats, its
melting will not raise sea levels, any more than melting ice raises the level of water in a glass). Higher
water levels can be expected to cause increased flooding of low-lying lands.
There is considerable disagreement among governments about what ought to be done about global
warming. The federal Clean Air Act of 1990 and the Kyoto Treaty have established goals for reducing the
emission of greenhouse gases. Countries across the globe are making progress toward reducing
emissions, but much more needs to be done.
Key Learning Outcome 38.3 Humanity's burning of fossil fuels has greatly increased atmospheric
levels of CO2, leading to global warming.
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38.4The Ozone Hole
Destroying the Earth's Radiation Shield
Learning Objective 38.4.1Explain how chemical coolants caused the ozone hole over Antarctica.
For 2 billion years, life was trapped in the oceans because radiation from the sun seared the earth's
surface unchecked. Nothing could survive that bath of destructive energy. Living things were able to leave
the oceans and colonize the surface of the earth only after a protective shield of ozone had been added
to the atmosphere by photosynthesis. Imagine if that shield were taken away. Alarmingly, it appears that
we are destroying it ourselves. Starting in 1975, the earth's ozone shield began to disintegrate. Over the
South Pole in September of that year, satellite photos revealed that the ozone concentration was
unexpectedly less than elsewhere in the earth's atmosphere. It was as if some “ozone eater” were
chewing it up in the Antarctic sky, leaving a mysterious zone of lower-than-normal ozone concentration,
an ozone hole. For many years after that, more of the ozone was depleted, and the hole grew bigger and
deeper. The satellite image in figure 38.5 shows lower levels of ozone as light purple (Antarctica is also
colored purple, indicating that the ozone hole completely covers it). The graph indicates the size of the
ozone hole over a 10-year period, with the largest hole appearing in September 2000 (the blue line).
Figure 38.5 The ozone hole over Antarctica.For decades, NASA satellites have tracked the extent of ozone
depletion over Antarctica. Every year since 1975, an ozone “hole” has appeared in August when sunlight triggers
chemical reactions in cold air trapped over the South Pole during Antarctic winter. The hole intensifies during
September before tailing off as temperatures rise in November–December. In 2000, the 28.4-million-square-kilometer
hole (purple in the satellite image) covered an area larger than the United States, Canada, and Mexico combined, the
largest hole ever recorded. In September 2000, the hole extended over Punta Arenas, a city of about 120,000 people
in southern Chile, exposing residents to very high levels of UV radiation.
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What is eating the ozone? Scientists soon discovered that the culprit was a class of chemicals that
everyone had thought to be harmless: chlorofluorocarbons (CFCs). CFCs were invented in the 1920s,
a miracle chemical that was stable, harmless, and a near-ideal heat exchanger. Throughout the world,
CFCs are used in large amounts as coolants in refrigerators and air conditioners, as the gas in aerosol
dispensers, and as the foaming agent in Styrofoam containers. All of these CFCs eventually escape into
the atmosphere, but no one worried about this until recently, both because CFCs were thought to be
chemically inert and because everyone tends to think of the atmosphere as limitless. But CFCs are very
stable chemicals and have continually accumulated in the atmosphere.
It turned out that the CFCs were causing mischief the chemists had not imagined. High over the South
and North Poles, nearly 50 kilometers up, where it is very, very cold, the CFCs stick to frozen water vapor
and act as catalysts of a chemical reaction. Just as an enzyme carries out a reaction in your cells without
being changed itself, so the CFCs catalyze the conversion of ozone (O 3) into oxygen (O2) without being
used up themselves. Very stable, the CFCs in the atmosphere just keep at it—little machines that never
stop. They are still there, still doing it, today. The drop in ozone worldwide is now over 3%.
Ultraviolet radiation is a serious human health concern. Every 1% drop in the atmospheric ozone content
is estimated to lead to a 6% increase in the incidence of skin cancers. At middle latitudes, the drop of
approximately 3% that has occurred worldwide is estimated to have led to an increase of perhaps as
much as 20% in lethal melanoma skin cancers.
Experts generally agree that the amount of ozone-killing chemicals in the upper atmosphere is leveling off
since more than 180 countries in the 1980s signed an international agreement that phases out the
manufacture of most CFCs. The 2005 ozone hole peaked at about 25 million square kilometers (the size
of North America), below the 2000 record size of about 28.4 million square kilometers. Current computer
models suggest the Antarctic ozone hole should recover by 2065, and the lesser-damaged ozone layer
over the Arctic by about 2023.
Key Learning Outcome 38.4 CFCs are catalytically destroying the ozone in the upper atmosphere,
exposing the earth's surface to dangerous radiation. International attempts to solve the problem
appear to be succeeding.
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38.5Loss of Biodiversity
Factors Responsible for Extinction
Learning Objective 38.5.1Discuss the impact of three factors thought to play key roles in many
extinctions.
Just as death is as necessary to a normal life cycle as reproduction, so extinction is as normal and
necessary to a stable world ecosystem as species formation. More than 99% of species known to science
are now extinct. However, current rates of extinctions are alarmingly high. The extinction rate for birds
and mammals was about one species every decade from 1600 to 1700, but it rose to one species every
year during the period from 1850 to 1950 and four species per year between 1986 and 1990. It is this
marked increase in the rate of extinction that is the heart of the biodiversity crisis.
What factors are responsible for extinction? Biologists have identified three factors that seem to play a
key role in many extinctions: habitat loss, species overexploitation, and introduced species (figure 38.6).
Figure 38.6 Factors responsible for animal extinction.These data represent known extinctions of mammals
in Australia, Asia, and the Americas. Some extinctions have more than one cause.
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Habitat Loss Habitat loss is the single most important cause of extinction. Given the tremendous
amounts of ongoing destruction of all types of habitat, from rain forest to ocean floor, this should come as
no surprise. Natural habitats may be adversely affected by human influences in four ways: (1) destruction,
(2) pollution, (3) human disruption, and (4) habitat fragmentation (dividing up the habitat into small
isolated areas). As you can see in figure 38.7, destruction of rain forest habitat is occurring rapidly in
Madagascar, which is endangering species.
Hungry Reindeer
Figure 38.7 Extinction and habitat destruction.The rain forest covering the eastern coast of Madagascar, an
island off the coast of East Africa, has been progressively destroyed as the island's human population has grown.
Ninety percent of the original forest cover is now gone. Many species have become extinct, and many others are
threatened, including 16 of Madagascar's 31 primate species.
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Species Overexploitation Species that are hunted or harvested by humans have historically been at
grave risk of extinction, even when the species populations are initially very abundant. There are many
examples in our recent history of overexploitation: passenger pigeons, bison, many species of whales,
commercial fish such as Atlantic bluefin tuna, and mahogany trees in the West Indies are but a few.
Ocean Fishing Ban
Introduced Species Occasionally, a new species will enter a habitat and colonize it, usually at the
expense of native species. Colonization occurs rarely in nature, but humans have made this process
more common, with devastating ecological consequences. The introduction of exotic species has wiped
out or threatened many native populations. Species introductions occur in many ways, usually
unintentionally. Plants and animals can be transported in nursery plants, as stowaways in boats, cars,
and planes, and as beetle larvae within wood products. These species enter new environments where
they have no native predators to keep their population sizes in check, and they crowd out native species.
Key Learning Outcome 38.5 Loss of biodiversity can be attributed to one of a few main causes,
including habitat loss, overexploitation, and introduced species.
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Today's Biology
The Global Decline in Amphibians
Sometimes important things happen right before our eyes, without anyone noticing. That thought
occurred to David Bradford as he stood looking at a quiet lake high in the Sierra Nevada Mountains of
California in the summer of 1988. Bradford, a biologist, had hiked all day to get to the lake, and when he
got there, his worst fears were confirmed. The lake was on a list of mountain lakes that Bradford had
been visiting that summer in Sequoia-Kings Canyon National Parks while looking for a little frog with
yellow legs. The frog's scientific name was Rana muscosa, and it had lived in the lakes of the parks for as
long as anyone had kept records. But this silent summer evening, the little frog was gone. The last major
census of the frog's populations within the parks had been taken in the mid-1970s, and R. muscosa had
been everywhere, a common inhabitant of the many freshwater ponds and lakes within the parks. Now,
for some reason Bradford did not understand, the frogs had disappeared from 98% of the ponds that had
been their homes.
After Bradford reported this puzzling disappearance to other biologists, an alarming pattern soon became
evident. Throughout the world, local populations of amphibians (frogs, toads, and salamanders) were
becoming extinct. Waves of extinction have swept through high-elevation amphibian populations in the
western United States and have cut through the frog populations of Central America and coastal
Australia.
Amphibians have been around for 350 million years, since long before the dinosaurs. Their sudden
disappearance from so many of their natural homes sounded an alarm among biologists. What are we
doing to our world? If amphibians cannot survive the world we are making, can we?
In 1998, the U.S. National Research Council brought scientists together from many disciplines in a
serious attempt to address the problem. After years of intensive investigation, they have begun to sort out
the reasons for the global decline in amphibians. Like many important questions in science, this one does
not have a simple answer.
Five factors seem to be contributing in a major way to the worldwide amphibian decline: (1) habitat
deterioration and destruction, particularly clear-cutting of forests, which drastically lowers the humidity
(water in the air) that amphibians require; (2) the introduction of exotic species that outcompete local
amphibian populations; (3) chemical pollutants that are toxic to amphibians; (4) fatal infections by
pathogens; and (5) global warming, which is making some habitats unsuitable.
Infection by parasites appears to have played a particularly important role in the western United States
and coastal Australia. Amphibian ecology expert James Collins of Arizona State University has reported
one clear instance of infection leading to amphibian decline. When Collins examined populations of
salamanders living on the Kaibab Plateau along the Grand Canyon rim, he found many sick salamanders.
Their skin was covered with white pustules, and most infected ones died, their hearts and spleens
collapsed. The infectious agent proved to be a virus common in fish called a ranavirus. Ranavirus isolated
by Collins from one sick salamander would cause the disease in a healthy salamander, so there was no
doubt that ranavirus was the culprit responsible for the salamander decline on the Kaibab Plateau.
Ranavirus outbreaks eliminate small populations, but in larger ones, a few individuals survive infection,
sloughing off their pustule-laden skin. These populations slowly recover.
A second kind of infection, very common in Australia but also seen in the United States, is having more
widespread effects. Populations infected with this microbe, a kind of fungus called a chytrid (pronounced
“kit-rid,” see chapter 18), do not recover. Usually a harmless soil fungus that decomposes plant material,
this particular chytrid (with the Latin name of Batrachochytrium dendrobatidis) is far from harmless to
amphibians. It dissolves and absorbs the keratinous mouthparts of amphibian larvae, killing them.
This killer chytrid appeared in Australia near Melbourne in the early 1980s. Now almost all Australia is
affected. How did the disease spread so rapidly? Apparently it traveled by truck. Infected frogs moved all
across Australia in wooden boxes with bunches of bananas. In one year, 5,000 frogs were collected from
banana crates in one Melbourne market alone.
In other parts of the world, infection does not seem to play as important a role as acid precipitation,
habitat loss, and introduction of exotic species. This complex pattern of cause and effect only serves to
emphasize the take-home lesson: Worldwide amphibian decline has no one culprit. Instead, all five
factors play important roles. It is their total impact that has shifted the worldwide balance toward
extinction.
To reverse the trend toward extinction, we must work to lessen the impact of all of these factors. It is
important that we not get discouraged at the size of the job, however. Any progress we make on any one
factor will help shift the balance back toward survival. Extinction is only inevitable if we let it be.
38.6Reducing Pollution
Legislating Environmental Protection
Learning Objective 38.6.1Describe how economists estimate the “optimum” amount of pollution.
Human activities are placing a severe stress on the biosphere, and we must quickly find ways to reduce
the harmful impact. There are four key areas in which it will be particularly important to meet the
challenge successfully: reducing pollution, preserving nonreplaceable resources, finding new energy
sources, and curbing population growth.
To solve the problem of industrial pollution, it is first necessary to understand the cause of the problem. In
essence, it is a failure of our economy to set a proper price on environmental health. To understand how
this happens, we must think for a moment about money. The economy of the United States (and much of
the rest of the industrial world) is based on a simple feedback system of supply and demand. As a
commodity gets scarce, its price goes up, and this added profit acts as an incentive for more of the item
to be produced; if too much is produced, the price falls and less of it is made because it is no longer so
profitable to produce it.
This system works very well and is responsible for the economic strength of our nation, but it has one
great weakness. If demand is set by price, then it is very important that all the costs be included in the
price. Imagine that the person selling the item were able to pass off part of the production cost to a third
person. The seller would then be able to set a lower price and sell more of the item! Driven by the lower
price, the buyer would purchase more than if all the costs had been added into the price.
Unfortunately, that sort of pricing error is what has driven the pollution of the environment by industry. The
true costs of energy and of the many products of industry are composed of direct production costs, such
as materials and wages, and of indirect costs, such as pollution of the ecosystem. Economists have
identified an “optimum” amount of pollution based on how much it costs to reduce pollution versus the
social and environmental cost of allowing pollution. The economically optimum amount of pollution is
indicated by the blue dot in figure 38.8. If more pollution than the optimum is allowed, the social cost is
too high, but if less than the optimum is allowed, the economic cost is too high.
Figure 38.8 Is there an optimum amount of pollution?Economists identify the “optimum” amount of pollution
as the point at which eliminating the next unit of pollution (the marginal cost of pollution abatement) equals the cost in
damages caused by that unit of pollution (the marginal cost of pollution).
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The indirect costs of pollution are usually not taken into account. However, the indirect costs do not
disappear because we ignore them. They are simply passed on to future generations, which must pay the
bill in terms of damage to the ecosystems on which we all depend. Increasingly, the future is now. Our
world, unable to support more damage, is demanding that something be done—that we finally pay up.
Antipollution Laws
Two effective approaches have been devised to curb pollution in this country. The first is to pass laws
forbidding it. In the last 20 years, laws have begun to significantly curb the spread of pollution by setting
stiff standards for what can be released into the environment. For example, all cars are required to have
effective catalytic converters to eliminate automobile smog. Similarly, the Clean Air Act of 1990 requires
that power plants eliminate sulfur emissions. They can accomplish this by either installing scrubbers on
their smokestacks or by burning low-sulfur coal (clean-coal technology), which is more expensive. The
effect is that the consumer pays to avoid polluting the environment. The cost of the converters makes
cars more expensive, and the cost of the scrubbers increases the price of the energy. The new, higher
costs are closer to the true costs, lowering consumption to more appropriate levels.
Pollution Taxes
A second approach to curbing pollution has been to increase the consumer costs directly by placing a tax
on the pollution, in effect an artificial price hike imposed by the government as a tax added to the price of
production. This added cost lowers consumption too, but by adjusting the tax, the government can
attempt to balance the conflicting demands of environmental safety and economic growth. Such taxes,
often imposed as “cap-and-trade pollution permits,” are becoming an increasingly important part of
antipollution laws.
Key Learning Outcome 38.6 Free market economies often foster pollution when prices do not
include environmental costs. Laws and taxes are being designed in an attempt to compensate.
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38.7Preserving Nonreplaceable Resources
Not All Damage Can Be Repaired
Learning Objective 38.7.1Evaluate the importance of three nonrenewable resources.
Among the many ways ecosystems are being damaged, one problem stands out as more serious than all
the rest: consuming or destroying resources that we all share in common but cannot replace in the future
(figure 38.9). Although a polluted stream can be cleaned, no one can restore an extinct species. In the
United States, three sorts of nonreplaceable resources are being reduced at alarming rates: topsoil,
groundwater, and biodiversity.
Figure 38.9 The tragedy of the commons.In a now-famous essay, ecologist Garrett Hardin argues that
destruction of the environment is driven by freedom without responsibility.
Reprinted with permission from “The Tragedy of the Commons,” by G. Hardin, Science, 162, p. 1244.
Copyright 1968 AAAS.
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Topsoil
The United States is one of the most productive agricultural countries on Earth, largely because much of
it is covered with particularly fertile soils. Our Midwestern farm belt sits astride what was once a great
prairie. The soil of that ecosystem accumulated bit by bit from countless generations of animals and
plants until, by the time humans came to plow, the humus-rich soil extended down several feet.
We cannot replace this rich topsoil, the capital upon which our country's greatness is built, yet we are
allowing it to be lost at a rate of centimeters every decade. Our country has lost one-quarter of its topsoil
since 1950! By repeatedly tilling (turning the soil over) to eliminate weeds, we permit rain to wash more
and more of the topsoil away, into rivers and eventually out to sea. New approaches are desperately
needed to lessen the reliance on intensive cultivation. Some possible solutions include using genetic
engineering to make crops resistant to weed-killing herbicides and terracing to recapture lost topsoil.
Groundwater
A second resource that we cannot replace is groundwater, water trapped beneath the soil within porous
rock reservoirs called aquifers. This water seeped into its underground reservoir very slowly during the
last ice age over 12,000 years ago. We should not waste this treasure, for we cannot replace it.
In most areas of the United States, local governments exert relatively little control over the use of
groundwater. As a result, a very large portion is wasted watering lawns, washing cars, and running
fountains. A great deal more is inadvertently being polluted by poor disposal of chemical wastes—and
once pollution enters the groundwater, there is no effective means of removing it. Some cities, like
Phoenix and Las Vegas, may completely deplete their groundwater within several decades.
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Biodiversity
The number of species in danger of extinction during your lifetime is far greater than the number that
became extinct with the dinosaurs. This disastrous loss of biodiversity is important to every one of us,
because as these species disappear, so does our chance to learn about them and their possible benefits
for ourselves. The fact that our entire supply of food is based on 20 kinds of plants, out of the 250,000
available, should give us pause. Like burning a library without reading the books, we don't know what it is
we waste. All we can be sure of is that we cannot retrieve it. Extinct is forever.
Over the last 20 years, about half of the world's tropical rain forests have been either burned to make
pasture land or cut for timber (figure 38.10). Over 6 million square kilometers have been destroyed.
Every year, the rate of loss increases as the human population of the tropics grows. About 160,000
square kilometers were cut each year in the 1990s, a rate greater than 0.6 hectares (1.5 acres) per
second! At this rate, all the rain forests of the world will be gone in your lifetime. In the process, it is
estimated that one-fifth or more of the world's species of animals and plants will become extinct—more
than a million species. This would be an extinction event unparalleled for at least 65 million years, since
the Age of Dinosaurs.
Figure 38.10 Tropical rain forest destruction.(a) These fires are destroying the rain forest in Brazil, which is
being cleared for cattle pasture. (b) The flames are so widespread and so high that their smoke can be viewed from
space. (c) The long-term consequences of deforestation can be seen in the bare slopes of Atlantic rain forest hills in
Rio de Janeiro State, Brazil.
D
You should not be lulled into thinking that loss of biodiversity is a problem limited to the tropics. The
ancient forests of the Pacific Northwest are being cut at a ferocious rate today, largely to supply jobs (the
lumber is exported), with much of the cost of cutting it down subsidized by our government (the Forest
Service builds the necessary access roads, for example). At the current rate, very little will remain in a
decade. Nor is the problem restricted to one area. Throughout our country, natural forests are being
“clear-cut,” replaced by pure stands of lumber trees planted in rows like so many lines of corn. It is difficult
to scold those living in the tropics when we ourselves do such a poor job of preserving our own country's
biodiversity.
But what is so bad about losing species? What is the value of biodiversity? Loss of a species entails three
costs: (1) the direct economic value of the products we might have obtained from species; (2) the indirect
economic value of benefits produced by species without our consuming them, such as nutrient recycling
in ecosystems; and (3) their ethical and aesthetic value. It is not difficult to see the value in protecting
species that we use to obtain food, medicine, clothing, energy, and shelter, but other species are vitally
important to maintaining healthy ecosystems; by destroying biodiversity, we are creating conditions of
instability and lessened productivity. Other species add beauty to the living world, no less crucial because
it is hard to set a price upon.
Key Learning Outcome 38.7 Nonreplaceable resources are being consumed at an alarming rate all
over the world; key among them are topsoil, groundwater, and biodiversity.
Page 813
38.8Curbing Population Growth
The Human Explosion
Learning Objective 38.8.1Describe the growth of the human population over the last 10,000 years.
If we were to solve all the problems mentioned in this chapter, we would merely buy time to address the
fundamental problem: There are getting to be too many of us.
Humans first reached North America at least 12,000 to 13,000 years ago, crossing the narrow straits
between Siberia and Alaska, and moving swiftly to the southern tip of South America. By 10,000 years
ago, when the continental ice sheets withdrew and agriculture first developed, about 5 million people lived
on Earth, distributed over all the continents except Antarctica. With the new and much more dependable
sources of food that became available through agriculture, the human population began to grow more
rapidly. By the time of Christ, 2,000 years ago, an estimated 130 million people lived on Earth. By the
year 1650, the world's population had doubled, and doubled again, reaching 500 million. Starting in the
early 1700s, changes in technology have given humans more control over their food supply, have led to
the development of cures for many diseases, and have produced improvements in shelter and storage
capabilities that make humans less vulnerable to climatic uncertainties. These changes allowed humans
to expand the carrying capacity of the habitats in which they lived and thus to escape the confines of
logistic growth and reenter the exponential phase of the sigmoidal growth curve, shown by the explosive
growth infigure 38.11.
Figure 38.11 Growth of the human population.Over the past 300 years, the world population has been
growing steadily. Currently, there are over 7 billion people on the earth. Mexico City (inset photo), one of the world's
largest cities, has about 20 million inhabitants.
D
Although the human population has grown explosively for the last 300 years, the average human birthrate
has stabilized at about 20 births per year per 1,000 people worldwide. However, with the spread of better
sanitation and improved medical techniques, the death rate has fallen steadily, to its present level of 2 per
1,000 per year. The difference between birth- and death rates amounts to a population growth rate of
1.2% per year, which seems like a small number, but it is not, given the large population size.
The world population reached 7 billion people in 2011, and the annual increase now amounts to about 78
million people, which leads to a doubling of the world population in about 60 years. Put another way,
about 214,000 people are added to the world population each day, or almost 152 every minute. At this
rate, the world's population will continue to grow and perhaps stabilize at a figure around 10 billion. Such
growth cannot continue, because our world cannot support it. Just as a cancer cannot grow unabated in
your body without eventually killing you, so humanity cannot continue to grow unchecked in the biosphere
without killing it.
Most countries are devoting considerable attention to slowing the growth rate of their populations, and
there are genuine signs of progress, but the world population may still gain another 1 to 4 billion people
before it stabilizes. No one knows whether the world can support so many people.
Population Growth Rate Declining
Learning Objective 38.8.2State by what percentage the world's human population is changing each
year.
The world population growth rate has been declining, from a high of 2.0% in the period 1965–70 to 1.2%
in 2011. Nonetheless, because of the larger population, this amounts to an increase of 78 million people
per year to the world population, compared to 53 million per year in the 1960s.
The United Nations attributes the decline to increased family planning efforts and the increased economic
power and social status of women. As family size decreases in developing countries, education programs
improve, leading to increased education levels for women, which in turn tends to result in further
decreases in family size.
No one knows whether the world can sustain today's population of 7 billion people, much less the far
greater numbers expected in the future. We cannot reasonably expect to expand the world's carrying
capacity indefinitely. The population will begin scaling back in size, as predicted by logistic growth
models; indeed, it is already happening. In the sub-Saharan area of Africa, population projections for the
year 2025 have been scaled back from 1.33 billion to 1.05 billion because of the impact of AIDS. If we are
to avoid catastrophic increases in death rates, the birthrates must continue to fall.
Page 814
Population Pyramids
Learning Objective 38.8.3Explain why population pyramids with broader bases indicate more rapid
future population growth.
While the human population as a whole continues to grow rapidly, this growth is not occurring uniformly
over the planet. Some countries, like Mexico, are currently growing rapidly. Figure 38.12 shows how
Mexico's birthrate, while declining (the blue line), still greatly exceeds its death rate (the red line), which is
stabilizing. There is often a correlation in how developed a country is and how rapidly its population
grows.Table 38.1 compares three countries that differ in how well developed they are. Ethiopia, a
developing country, has a higher fertility rate, which results in a higher birthrate than either Brazil or the
United States. But Ethiopia also has a much higher infant mortality rate and a lower life expectancy.
Overall, the population in Ethiopia will double much more quickly than the population of Brazil or the
United States. The rate at which a population can be expected to grow in the future can be assessed
graphically by means of a population pyramid—a bar graph displaying the numbers of people in each age
category (some examples are shown in figure 38.13). Males are conventionally shown to the left of the
vertical age axis (colored blue here) and females to the right (colored red). In most human population
pyramids, the number of older females is disproportionately large compared with the number of older
males, because females in most regions have a longer life expectancy than males. This is apparent in the
upper portion of the 2005 U.S. pyramid.
Figure 38.12 Why Mexico's population is growing.The death rate (red line) in Mexico has been falling, while
the birthrate (blue line) remained fairly steady until 1970. The difference between birth- and death rates has fueled a
high growth rate. Efforts begun in 1970 to reduce the birthrate have been quite successful. Although the growth rate
remains rapid, it is expected to begin leveling off in the near future as the birthrate continues to drop.
D
Figure 38.13 Population pyramids.Population pyramids are graphed according to a population's age
distribution. Kenya's pyramid has a broad base because of the great number of individuals below child-bearing age.
When all of the young people begin to bear children, the population will experience rapid growth. The 2005 U.S.
pyramid demonstrates a larger number of individuals in the “baby boom” cohort—the pyramid bulges because of an
increase in births between 1945 and 1964, as shown at the base of the 1964 pyramid. The 25–to–34 cohort in the
1964 pyramid represents people born during the Depression and is smaller in size than the cohorts in the preceding
and following years.
D
TABLE 38.1
A COMPARISON OF 2006 POPULATION DATA IN DEVELOPED AND DEVELOPING COUNTRIES
United States
(highly developed)
Brazil (moderately
developed)
Ethiopia
(developing)
Fertility rate
2.1
2.3
5.4
Doubling time at current
rate (yr)
72.2
55.5
27.9
Infant mortality rate
(infant deaths/1,000
births)
6.5
27
77
Life expectancy (yr)
78
72
49
$44,260
$8,800
$1,190
Per capita income (U.S.
dollar equivalent)
Viewing such a pyramid, one can predict demographic trends in births and deaths. In general, rectangular
pyramids are characteristic of countries whose populations are stable; their numbers are neither growing
nor shrinking. A triangular pyramid, like the 2005 Kenya pyramid, is characteristic of a country that will
exhibit rapid future growth, as most of its population has not yet entered the child-bearing years. Inverted
triangles are characteristic of populations that are shrinking.
Compare the differences in the population pyramids for the United States and Kenya in figure 38.13. In
the somewhat more rectangular population pyramid for the United States in 2005, the cohort (group of
individuals) 40 to 59 years old represents the “baby boom,” the large number of babies born following
World War II. When the media refers to the “graying of America,” they are referring to the aging of this
disproportionately large cohort that will impact the health-care system and other age-related systems in
the future. The very triangular pyramid of Kenya, by contrast, predicts explosive future growth. The
population of Kenya is predicted to double in less than 20 years. However, it is important to note that
these estimates do not take into account the huge impact that natural disasters and epidemics such as
AIDS will have on population sizes. In sub-Saharan Africa, the AIDS epidemic has reduced the life
expectancy at birth by 20 years. Figure 38.14 shows two population pyramid projections for Botswana,
Africa, where over 36% of the population is living with HIV or AIDS. The uncolored portions of the bars
indicate the population projections in 2025 without the effect of the AIDS epidemic, and the colored bars
reflect actual projections with AIDS.
Figure 38.14 Projected AIDS effect on Botswana population (year 2025).
D
Page 815
The Level of Consumption in the Developed World Is Also a Problem
We in the developed countries of the world need to pay more attention to lessening the impact of our
resource consumption. Indeed, the wealthiest 20% of the world's population accounts for 86% of the
world's consumption of resources and produces 53% of the world's carbon dioxide emissions, whereas
the poorest 20% of the world is responsible for only 1.3% of consumption and 3% of CO 2 emissions.
One way of quantifying this disparity is by calculating what has been termed the ecological
footprint,which is the amount of productive land required to support an individual at the standard of living
of a particular population through the course of his or her life. As figure 38.15 illustrates, the ecological
footprint of an individual in the United States is more than 10 times greater than that of someone in India.
Based on these measurements, researchers have calculated that resource use by humans is now onethird greater than the amount that nature can sustainably replace; if all humans lived at the standard of
living in the developed world, two additional planet Earths would be needed.
Figure 38.15 Ecological footprint of individuals in different countries in 2003.The ecological footprint
calculates how much land is required to support a person through his or her life, including the acreage used for
production of food, forest products, and housing, in addition to the forest required to absorb the carbon dioxide
produced by the combustion of fossil fuels.
D
Key Learning Outcome 38.8 The problem at the core of all other environmental concerns is the
rapid growth of the world's human population. Serious efforts are being made to slow its growth.
Reviewing What You Have Learned
Global Change
Pollution


38.1.1Pollution leads to global change because its effects can spread far from the source. Air and
water become polluted when chemicals that are harmful to organisms are released into the
ecosystem.
38.1.2The use of agricultural chemicals, such as pesticides, herbicides, and fertilizers, has been
widespread with devastating effects on animals. Biological magnification occurs when harmful
chemicals become more concentrated as they pass up through the food chain, as shown here
fromfigure 38.1.
Acid Precipitation

38.2.1The burning of coal releases sulfur into the atmosphere, where it combines with water vapor,
forming sulfuric acid. This acid falls back to Earth in rain and snow, commonly called acid rain, far
from the source of the pollution, killing animals and vegetation.
Global Warming


38.3.1The burning of fossil fuels releases carbon dioxide into the atmosphere, more carbon dioxide
than can be cycled back through the ecosystem. It remains in the atmosphere, where it traps
infrared light (heat) from the sun, a phenomenon known as the greenhouse effect.
38.3.2The average global temperatures have been steadily increasing, a process known as global
warming. Global warming is predicted to have major impacts on global rain patterns, agriculture, and
rising sea levels.
The Ozone Hole


38.4.1Ozone (O3) forms a protective shield in the earth's upper atmosphere that blocks out harmful
UV rays from the sun. In the mid-1970s, scientists determined that ozone was being depleted.
•Chlorofluorocarbons (CFCs), used in refrigeration systems, react with ozone, converting it to
oxygen gas (O2), which doesn't block UV rays. This reduction in ozone over the South Pole, as
shown here from figure 38.5, is resulting in dangerously high levels of radiation reaching the earth.
Loss of Biodiversity

38.5.1The current rate of species loss is alarmingly high. Three factors mostly responsible include
loss of habitat, overexploitation, and the introduction of exotic species. Loss of habitat is the most
devastating.
Saving Our Environment
Reducing Pollution

38.6.1Human activities are placing severe stress on the biosphere. Reducing pollution requires
examining the costs associated with pollution. Antipollution laws and pollution taxes are ways to
begin factoring in the costs of pollution.
Preserving Nonreplaceable Resources

38.7.1The consumption or destruction of nonreplaceable resources is perhaps the most serious
problem humans face. Topsoil, necessary for agriculture, is being depleted rapidly. Groundwater,
which percolates through the soil to underground reservoirs, is our primary source of drinking water,
but it is being wasted and polluted. Biodiversity is being reduced through extinctions, due primarily to
loss of habitat, such as rain forests (figure 38.10).
Curbing Population Growth



38.8.1The root of all environmental problems is the rapid growth of the human population. More
people means more resources are depleted, more land is developed, and more pollution is created.
38.8.2Technology has allowed the human population to grow exponentially for the last 300 years to
a current population of over 7 billion.
38.8.3Human populations grow at different rates, with developing countries' populations growing
more rapidly than developed countries' populations. However, it takes more resources to support
populations in developed countries.
Solving Environmental Problems
Preserving Endangered Species


38.9.1In an attempt to slow the loss of biodiversity, recovery programs are under way, designed to
save endangered species. These programs include habitat restoration, breeding in captivity, and
sustaining genetic diversity (figures 38.17).
38.9.2Keystone species exert particularly strong influences on the structure and functioning of their
ecosystems.
Finding Cleaner Sources of Energy


38.10.1The burning of fossil fuels leads to pollution, depletion of valuable resources, and global
warming. Alternative sources of energy are needed, and some countries have looked to nuclear
power, but nuclear power has its own drawbacks. Renewable energy sources such as solar and
wind power are promising alternatives.
38.10.2Ethanol distilled from sugars obtained from plants like corn and sugar cane, as well as
sugars obtained by fermentation of cellulose biomass, offer significant potential as renewable energy
sources.
Individuals Can Make the Difference

38.11.1There are environmental success stories, where one or a few individuals made a difference
and reversed an ecological disaster.
Page 824
Test Your Understanding





38.1.1“Gray-air cities” are the result of
a. biological magnification of air pollutants.
b. chlorinated hydrocarbons as a major air pollutant.
c. pesticides as a major air pollutant.
d. sulfur oxides as a major air pollutant.
Answer
38.1.2Explain why exposure to even very tiny amounts of chemical pollutants like BPA that mimic
estrogen can, over time, be hazardous to your health. Should one sex be more concerned than the
other? Explain.
Answer
38.2.1The main cause of acid rain is
a. car and truck exhaust.
b. coal-powered industry.
c. chlorofluorocarbons.
d. chlorinated hydrocarbons.
Answer
38.3.1The sun's radiant heating and so-called “greenhouse effect” are the only reasons Earth's
surface is not as cold as the dark side of the moon. Greenhouse warming increases the average
temperature of the earth by about
a. 70 degrees C.
b. 35 degrees C.
c. 10 degrees C.
d. 45 degrees C.
•If there are many greenhouse gases, why is only carbon dioxide often mentioned as the cause of
global warming?
a. The other gases do not cause global warming.
b. Carbon dioxide is the only gas that traps radiant energy from the sun.
c. Carbon dioxide is not only a greenhouse gas, but it also catalyzes the production of
CFCs.
d. Carbon dioxide is being released by industrial society in huge amounts.
Answer










38.3.2Global warming affects all of the following except
a. rain patterns.
b. sea levels.
c. ozone levels.
d. agriculture.
Answer
38.4.1Destruction of the ozone layer is due to
a. car and truck exhaust.
b. coal-powered industry.
c. chlorofluorocarbons.
d. chlorinated hydrocarbons.
Answer
38.5.1The factor most responsible for present-day extinctions is
a. habitat loss.
b. overexploitation of species.
c. introduction of new species.
d. All of these are equally responsible.
Answer
38.6.1Free market economies often promote pollution because
a. environmental costs are hardly ever recognized as part of the economy.
b. supply never keeps up with demand, so industry must increase output to address the
demand.
c. the costs of energy and raw materials are so variable.
d. laws about pollution are unenforceable.
•Economists tend to look at the world in terms of costs. They say, “If you increase pollution, then you
increase the social costs to people's health, but if you try to decrease pollution, then you increase
the economic costs of cleaning it up.” Discuss whether you think these two costs are paid equally by
the same set of people.
Answer
38.7.1Preserving biodiversity is
a. needed to preserve possible direct value from species, such as new medicines.
b. not needed as extinction is a “natural” cycle and should not be disturbed.
c. needed to make sure all niches are filled.
d. not needed as it interferes with industrial development.
•Which of the following factors is not thought to be contributing to the global decline of amphibians?
a. pollution
b. infections by pathogens
c. loss of genetic diversity
d. global warming
•If 99% of the species that ever existed are now extinct, why is there such concern about current
extinction rates?
Answer
38.8.1Which factor is not responsible for the large increase in the human population over the last
300 or so years?
a. larger and more reliable food reserves from the modernization of farming techniques
b. decreasing mortality rate due to improvements in medicine
c. increasing amounts of open space as countries develop
d. increased sanitation practices
Answer
38.8.2The world human population growth rate
a. is increasing exponentially.
b. is declining.
c. has reached a plateau.
d. is increasing at a slow rate.

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

Answer
38.8.3Human population pyramids with a broad base
a. are characteristic of countries whose population sizes are stable.
b. are correlated with countries being well-developed economically.
c. contain a disproportionate number of older females.
d. will experience rapid future growth.
Answer
38.9.1Removal of the endangered black-footed ferret and California condor populations from the
wild for breeding programs in zoos and field laboratories are examples of preservation through
a. pristine restoration.
b. habitat restoration.
c. habitat rehabilitation.
d. captive propagation.
Answer
38.9.2If the removal of a species causes an ecosystem to collapse, that species is known as a(n)
a. keystone species.
b. endangered species.
c. threatened species.
d. None of the above.
Answer
38.10.1Renewable energy approaches include all of the following except
a. scrubbers.
b. solar panels.
c. wind farms.
d. cellulose fermentation.
Answer
38.10.2Hemicellulose is
a. a cross-linked glucose polymer.
b. partially digested cellulose.
c. a five-carbon sugar polymer.
d. a complex of cellulose with lignin.
Answer
38.11.1Consider the stories of Marion Stoddart and W. T. Edmondson, and the following statement
by anthropologist Margaret Mead: “Never doubt that a small group of thoughtful, committed citizens
can change the world. Indeed it's the only thing that ever has.” What could you do to make your
neighborhood/area/community a better, healthier place? What will you do?
Answer