Objectives
Explain how human activities can impact chemical cycles.
Explain how pollution can affect food chains.
Key Terms
deforestation
greenhouse effect
global warming
eutrophication
acid rain
pollution
biological magnification
ozone
Throughout this unit you have explored the complexity and
interconnectedness of ecosystems. Over the past few centuries, many
ecosystems have been affected by the rapidly growing human
population's need for resources. The effects of human activities are
sometimes felt in only a small area. Sometimes, though, the ecological
impact is more widespread or even global.
Impact on Chemical Cycles
Human activities can affect chemical cycling by literally moving
nutrients from one place to another. In contrast to a deer, which might eat
plants in a forest and return the raw materials to the same forest in its
waste, a human might eat a salad containing vegetables from many
different parts of the country. And the human's waste might be carried out
into the ocean in sewage, far from the sources of the raw nutrients. On an
even larger scale, some human activities can disrupt the processes within
global chemical cycles.
Carbon Cycle Impacts As you have read, the burning of wood and
fossil fuels is one source of carbon dioxide in the atmosphere. As nations
have become more industrialized, atmospheric carbon dioxide levels have
risen steadily (Figure 36-13). Deforestation, the clearing of forests for
agriculture, lumber, and other uses, also affects the carbon cycle by
eliminating plants that absorb carbon dioxide during photosynthesis.
Sometimes after being cut down, the trees are then burned, releasing more
carbon dioxide. Burning after deforestation in the tropics accounts for
about 20 percent of the carbon dioxide added to the atmosphere by
human activities. Worldwide burning of fossil fuels accounts for most of
the other 80 percent.
Figure 36-13
The zig-zag shape of this graph is due to seasonal
changes in atmospheric levels of carbon dioxide. Levels
decrease each year during the summer growing season
and increase during the winter. The overall level of
carbon dioxide has climbed during the last four
decades.
How are increased carbon dioxide levels in the atmosphere significant?
Carbon dioxide is one of a few gases in the atmosphere that allow
sunlight to pass through to Earth, but trap some heat as it is reflected and
radiated from Earth's surface (Figure 36-14). As a result, the temperature
of Earth's surface is higher than it would be if all heat escaped into space.
These gases act somewhat like the glass windows of a greenhouse that
allow light in, but prevent heat from escaping. Because of this analogy,
the process by which atmospheric gases trap heat is called the greenhouse
effect. As the levels of carbon dioxide and other "greenhouse gases" rise,
the greenhouse effect becomes stronger, trapping more heat in the
atmosphere and raising Earth's average temperature. Such an overall rise
in Earth's average temperature is called global warming.
Figure 36-14
The greenhouse effect is a natural process that stops all of the sun's heat
from escaping rapidly back to space. This process can be altered by human
activities that affect the levels of greenhouse gases in the atmosphere.
Using a variety of computer models, scientists make predictions about the
long-term effects of increasing carbon dioxide levels. Global warming
predictions range from just tenths of a degree to a few degrees Celsius.
Scientists also have different hypotheses about the possible effects of
global warming. For example, a few degrees of warming could cause
enough melting of glaciers and polar ice caps to cause a rise in sea levels,
flooding low-lying coastal areas. Small changes in temperatures could
also have large effects on weather, such as changing precipitation
patterns. The boundaries between biomes might shift, affecting the
species that live in them. Which species could move or adapt quickly
enough to survive these changes? The answer is unknown.
International cooperation and individual actions are all needed to lessen
the chance of these outcomes. Nations can identify ways that people in
the tropics can support themselves without burning forests. Nations can
also reduce use of fossil fuels by conserving energy and developing
alternative energy sources, such as wind, solar, and geothermal energy.
You can help by making your home more energy-efficient and by
walking, biking, or riding public transportation.
Nitrogen Cycle Impacts Human activities impact the nitrogen cycle
primarily by moving large amounts of nitrogen compounds into the water
or the air. For example, some sewage treatment plants release dissolved
nitrogen compounds into streams and rivers. Fertilizers applied to crops
are another source of nitrogen compounds—excess fertilizer may run off
into nearby streams and ponds. The high levels of nitrogen, often along
with phosphates, feed the rapid growth of algae in these bodies of water, a
condition called eutrophication. As the algae die, the bacteria
decomposing them can use up so much of the oxygen in the water that
there is no longer enough to support other organisms.
Smokestacks and automobile exhaust pipes release certain nitrogen and
sulfur compounds into the atmosphere. There these compounds combine
with water, forming nitric and sulfuric acids. Precipitation that carries this
acid back to Earth's surface is called acid rain. The Clean Air Act has
helped lessen the acid rain problem in the United States by reducing
levels of sulfuric acids. However, precipitation in some areas is still
acidic enough to cause damage.
Water Cycle Impacts One human activity that can impact the water
cycle is deforestation. A primary way that fresh water returns to the
atmosphere is transpiration from dense tropical forests. As a result,
tropical deforestation greatly reduces the amount of water vapor added to
the atmosphere. This changes precipitation patterns and affects
ecosystems. Drawing water from rivers or underground aquifers for
household use or crop irrigation also affects the water cycle. If the rate of
water use is faster than the rate at which the water cycle can replace it, the
river or aquifer may eventually run dry.
Case Study: Hubbard Brook Experimental Forest About 40
years ago, scientists started a long-term study on the effects of
deforestation on chemical cycling in a New Hampshire forest. The study
measured chemical cycling in an area of the forest where all the trees
were cut down (the altered area) compared to an untouched area (the
reference area). Scientists recorded nutrient levels in streams flowing out
of each area. For several years following the cut, the water volume
flowing from the altered area increased by 30 to 40 percent. Without trees
to absorb excess water, the water ran off from the soil. Most remarkable
was the loss of nitrate along with the water, which increased 60-fold (or
6000 percent) in concentration in the stream flowing from the altered area
(Figure 36-16). However, as new plants grew in the treatment area,
transpiration increased and runoff decreased. The Hubbard Brook
research shows that plants play a large role in chemical cycling in a forest
ecosystem. In Online Activity 36.4, you can use a virtual laboratory to
collect and analyze water and soil samples in the Hubbard Brook Forest.
Figure 36-16
Deforestation in the altered area of
Hubbard Brook Forest increased the
concentration of nitrates in runoff for
several years.
Other Effects of Pollution
The addition of substances to the environment that result in a negative
effect is called pollution. You have just read how pollution can affect
chemical cycles. Two more examples show how pollution can affect food
chains and Earth's atmosphere.
Biological Magnification As organisms take in nutrients and water
from the environment, they may also take in pollutants. Though some
pollutants may be excreted, others accumulate in an organism's tissues.
For example, a group of chemicals called PCBs are soluble in lipids and
collect in the fatty tissues of animals. PCBs that are disposed in industrial
wastes can remain in the environment for a long time. In the Great Lakes,
studies detected PCBs in the tissues of organisms throughout the food
web. As higher trophic levels fed on each other, the PCBs accumulated in
each predator's fatty tissues at even higher concentrations. As you can see
in Figure 36-17, the concentration of PCBs increased from 0.025 parts per
million (ppm) in phytoplankton to 124 ppm in herring gull eggs. The
process by which pollutants become more concentrated in successive
trophic levels of a food web is called biological magnification.
One of the first studied examples of biological magnification involved the
pesticide DDT. Before 1971, DDT was used in the United States to control
mosquitoes and agricultural pests. As with PCBs, DDT concentrates in
fats and becomes magnified at each step of the food chain. When
ecologists studying a decline in the populations of top-level consumers
such as pelicans, ospreys, and eagles found high levels of DDT in the
birds' eggs, they realized that DDT was a serious environmental problem.
DDT caused the birds' egg shells to be easily breakable, thus fewer young
survived to hatch. When DDT was banned in the United States,
populations of these birds made a dramatic recovery.
Figure 36-17
In this Great Lakes food chain, the
concentration of PCBs measured in herring
gull eggs was almost 5000 times higher than
that measured in phytoplankton. The
concentration increased at each successive
trophic level.
Damage to the Ozone Shield
Some pollution in the atmosphere affects a gas called ozone (O3) that has
particular importance to living things. The ozone layer, a region of the
atmosphere between 17 and 25 kilometers above Earth's surface, contains
concentrations of ozone that absorb ultraviolet radiation, shielding
organisms from its damaging effects.
Scientists have recorded the thinning of the ozone layer since the 1970s.
A major contributor to the destruction of the layer are
chlorofluorocarbons (CFCs) released from aerosol cans, refrigeration
units, and certain manufacturing processes. These compounds destroy
ozone (Figure 36-18).
Figure 36-18
Free chlorine atoms in the atmosphere react with and destroy
ozone molecules. Over time, the loss of ozone has resulted in an
ozone "hole"—an area of very low ozone concentrations located
over Antarctica.
The consequences of ozone depletion for humans may include an
increase in health problems such as skin cancer and cataracts, caused by
more intense ultraviolet radiation reaching Earth's surface. The radiation
may also harm crops and other producers.
International efforts may help to minimize ozone loss and to aid in its
recovery. Nearly 200 nations, including the United States, are working to
eliminate the use of ozone-destroying chemicals. For example, CFCs have
been banned in many countries.
Concept Check 36.4
1. Describe how increased quantities of carbon dioxide in the atmosphere
may contribute to global warming.
2. Give an example showing how pollution relates to biological
magnification.
3. How can deforestation impact the carbon and water cycles?
4. What is the relationship between chlorofluorocarbons and ozone?
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