Download Chapters • Lesson 18

Chapters • Lesson 18
How Humans Affect the Environment
Objectives: 2.1.1,2.2.1,2.2.2
Key term Words
• biodiversity • natural resource • deforestation • keystone species • pollution • global warming
• acid rain • eutrophication • pesticide • bioaccumulation • extinction
Getting the Idea
The organisms in a stable ecosystem are well suited to their environment. However, sudden
changes in environmental conditions, such as those caused by a fire or flood, can rapidly
change which populations can live in an ecosystem. Many human activities also affect the
populations that live in an ecosystem.
Human Population Growth
A major challenge affecting the environment is human population growth. Over the last 200
years, the total human population of the world has grown exponentially. Although the rate of
growth is slowing, the number of births each year continues to exceed the number of
deaths. As a result, the human population continues to increase. The graph below shows
how the total human population has grown over time and gives a projection of population
growth over the next few decades.
As with other populations, continued growth brings the human population closer to Earth's
carrying capacity. Recall that the carrying capacity of an ecosystem is the largest population
that it can support. The carrying capacity for humans is limited by available energy,
drinkable water, and nutrients. Researchers have attempted to address these limits.
However, developing new energy resources, improving the efficiency of existing energy
sources, recycling and conserving water, and increasing agricultural productivity cannot
increase Earth's carrying capacity as fast as the population is growing.
Environmental Changes Affect Biodiversity
Humans are part of Earth's biodiversity. Biodiversity refers to the number of different kinds
of organisms living on Earth or in an ecosystem. Many human activities can change
environmental conditions in ways that alter the biodiversity of an ecosystem.
Human actions can greatly affect Earth's biological, physical, and chemical processes. For
example, as the human population grows, people use more natural resources. A natural
resource is a part or product of the environment that is used by humans or other
organisms. Land, water, and air are examples of natural resources used by all organisms.
Organisms are also natural resources.
Earth's supply of many natural resources is limited, and people sometimes use more of a
resource than they actually need. When humans use too much of a resource, it may
become unavailable to other organisms. Human use of the resources in an ecosystem may
also physically alter the ecosystem. These changes can harm the species living there. For
example, people often clear land and move or fill waterways to make space for buildings
and roads. Undeveloped land and bodies of water provide habitats to many organisms.
When people alter an ecosystem, many organisms living there lose their habitats.
Organisms that are unable to move to another ecosystem may die.
Deforestation is one example of how human use of natural resources can affect other
species. Deforestation is the removal of all the trees in an area of forest. People may clear
land to make space available for farms, mines, or towns. People may also cut down all the
trees in an area in order to use the wood. Deforestation destroys habitats, forcing species to
move out of the area. This reduces the biodiversity of the area.
Not all changes to natural environments are as large as deforestation. However, the
removal of even one type of plant or animal can affect an entire ecosystem. Some
ecosystems contain organisms called keystone species. A keystone species is a species
that plays a critical role in the community structure of an ecosystem. The loss or decline in
population of a keystone species can affect many other populations in the ecosystem.
The eastern oyster is a keystone species in many estuaries along the coast of North
Carolina. The oysters serve three major roles in this ecosystem. One of these roles is as a
food source for crabs and other animals that live in the estuary. The oysters also help clean
the water in the estuary by removing excess algae, sediment, and pollutants as they feed.
In addition, the oysters build large reefs that provide a habitat to as many as 300 species of
fish, crustaceans, and other aquatic life. During the twentieth century, populations of
eastern oysters in North Carolina declined by as much as 90 percent. The causes of this
decline included the harvesting of too many oysters, water pollution, and habitat destruction.
Loss of the oysters has led to increased pollution in estuary waters and a decline in the
populations of several aquatic species.
Many human activities cause pollution. Pollution is the release of harmful substances or
energy into air, soil, or water. Burning fuels release various gases, which can pollute the air.
Recall from Lesson 15 that one of the gases released by burning fuels is carbon dioxide.
Since the Industrial Revolution, the amount of carbon dioxide in the atmosphere has
increased, largely because of the increased use of fossil fuels. Deforestation also affects
carbon dioxide levels and the carbon cycle because it reduces the number of plants that
take in carbon dioxide from the air.
Scientists think that increased levels of carbon dioxide are a major cause of a form of
climate change known as global warming. Global warming is an increase in average
atmospheric temperatures around the world. Rising temperatures appear to be harming
many natural environments. For example, polar ice caps, sea ice, and glaciers are melting
and not being replaced by fresh ice. Sea ice is an important part of polar bears' habitat, so
the loss of ice is threatening polar bear populations. Some scientists are concerned that the
melting of ice caps will lead to a rapid rise in sea levels and flood many coastal cities.
Global warming is also expected to cause more droughts, floods, and other extreme
weather that can damage or destroy an ecosystem. Slower, long-term climate changes can
also alter ecosystems as an area becomes too hot, cold, wet, or dry for the species that live
there.
Burning fossil fuels also produces pollutants such as compounds of sulfur and nitrogen.
These compounds can combine with water in the atmosphere to produce acid rain. Acid
rain is rain that has a pH lower than 5.6. Acid rain can harm ecosystems by damaging
plants. This damage, in turn, is harmful to animals that feed on the plants. Acid rain can also
change the pH levels of aquatic habitats, which can kill off or reduce the populations of
some species that live there.
The improper disposal of solid wastes and chemicals, or the reckless use of chemicals, can
pollute water and soil. Oil spills pollute lakes, rivers, and oceans. Pollutants on land or in
water can make an ecosystem unfit for the organisms that normally live there. Pollutants
can also make freshwater unusable for drinking, cooking, and irrigation, and even for
manufacturing.
The release of pollutants into the environment can have harmful and sometimes irreversible
consequences. Eutrophication occurs when excess nutrients are released into a body of
water. This happens when large amounts of sewage, fertilizer, or sediment enter the water.
In North Carolina, waste lagoons on hog farms are one source of these nutrients. The
nutrients, usually phosphorus or nitrogen compounds such as nitrates, can seep into the
soil and be carried into aquatic ecosystems by groundwater or runoff. The excess nutrients
cause an algal bloom, a population explosion of algae. As they use up the nutrients, the
algae exceed the carrying capacity of the environment and begin to die in large numbers.
When the algae die, decomposers become active and begin to use more oxygen.
Decomposition of the dead algae decreases the oxygen available to other organisms, killing
many fish and other organisms that live in an affected body of water.
Pesticides are chemicals that are designed or used to kill pest animals, such as certain
insects and rodents. The use of pesticides helps reduce disease and increase food
production worldwide. However, pesticides can make an ecosystem unstable by sickening
or killing animals other than the target pests. For example, pesticides used on lawns and
fields can harm beneficial insects, such as bees, which pollinate crops and many other
plants. Runoff can carry pesticides into bodies of water, where they may harm aquatic
plants and animals.
Some pesticides are taken in by organisms as they feed, without harming the organisms. A
pesticide that an organism eats can be passed on to other organisms through the food
chain. Although the pesticide may not immediately kill or sicken the organism that
consumes it, it may be stored and build up in the organism's tissues over time. An increase
in the amount of a substance, such as a pesticide, in the tissues of an organism is called
bioaccumulation. Pesticides that accumulate in an animal's body are passed on to
predators that feed on that animal. Over time, tiny quantities of pesticides become
increasingly concentrated in the cells and tissues of organisms that feed at higher trophic
levels. This process, in which chemicals become more concentrated at higher feeding
levels, is called biomagnification.
In some cases, biomagnification does not directly harm the organisms that ingest a
pesticide but affects their offspring. Populations of predatory birds such as bald eagles and
peregrine falcons decreased in the 1960s because biomagnification of a pesticide called
DDT made the shells of their eggs brittle. As a result, fewer chicks hatched. Fortunately,
scientists discovered the cause of the population declines. The work of these scientists led
to a ban on the use of DDT.
Activities with Direct Effects on Species
A reduction in biodiversity has far-reaching effects because every species on Earth
depends to one degree or another on many other organisms. If one species vanishes, other
organisms may be endangered. Biodiversity tends to sustain all forms of life on Earth,
including humans.
Extinction is the permanent dying out of a species or population. Some scientists estimate
that one plant or animal species becomes extinct every hour. Although some extinctions
result from natural events, many are caused by human activities. Habitat destruction by
pollution or the clearing of land is a major cause of extinction. Other human activities that
can lead to extinction include overhunting, overfishing, and the introduction of invasive
species into an ecosystem.
Land provides habitats to a great many organisms, including humans. A growing human
population needs more land for homes, businesses, farms, and roads. In North Carolina,
population growth in the Piedmont area, or Piedmont urbanization, is increasing the
demand for land in this area. This demand often leads to habitat destruction as land is
cleared for human use.
Coastal regions of the state are also being developed to support a growing human
population. Construction in these areas has led to changes that may increase beach
erosion, in which sand is carried away from beaches. Some practices intended to make
beaches cleaner and more appealing to humans, such as raking and grooming, have also
increased erosion. Beach erosion can make homes in coastal areas more vulnerable to
damage from severe storms, such as hurricanes. Changes in the structure of beaches may
also change natural habitats and alter beach ecosystems.
Both freshwater and saltwater habitats provide abundant food for humans and other
organisms. These ecosystems are fragile and can be damaged or destroyed through
overuse. Overfishing is the harvesting of fish or shellfish faster than the population can
renew its numbers. As you read earlier in the lesson, overfishing is one cause of the decline
in eastern oyster populations in North Carolina estuaries. Such overfishing is a direct threat
to biodiversity because if too many fish are harvested, species may die out.
Overhunting means killing animals at a rate faster than the population can renew its
numbers. Over time, hunting for food, fur, feathers, and other body parts has wiped out
some species and threatened others with extinction. Species endangered by hunting
include blue whales, tigers, snow leopards, elephants, and rhinoceroses. In the United
States, sport hunting nearly wiped out the American bison.
Human activities can introduce new species into environments. Sometimes this is intentional.
For example, people may plant new flowers and vegetables in their gardens. The plants may
then spread to neighboring environments. Other species are introduced when people release
unwanted pets into the wild. Humans also accidentally carry species to new environments. For
example, insects may be transported around the world in luggage or in shipments of food.
Rodents that make their way onto ships can be transported from their native habitat to a new
location.
An invasive species is an introduced species that thrives in a new environment and has adverse
effects on that environment. Recall that all species in an ecosystem compete for resources.
Sometimes a species introduced to a new ecosystem has no natural predators there. Without
predators, the species may reproduce unchecked. As the population increases, it uses more
resources, competing with native species. If the species is very successful in its new
environment, it can outcompete native species and even bring about their extinction. A well
known invasive species in North Carolina ecosystems is kudzu. In the late 1800s, this plant was
imported from Asia to help control soil erosion. Although the plant is well suited to this job,
kudzu has also outcompeted native plants for resources. In forested areas, kudzu quickly
overtakes and smothers small ground plants. As the kudzu continues growing, it climbs tall
trees. In time, climbing kudzu vines become large and dense enough to kill the trees. Because
the plant grows rapidly and few animals feed on kudzu, it has become an invasive weed in
many forested areas of the state.
Focus on Inquiry
Sharing information is an essential part of science. Scientists often share information with others
in their field by publishing accounts of their research in scientific journals—magazines used for
communicating scientific findings. Scientists also communicate with one another and with the
public by speaking at lectures and press conferences, or in interviews. When scientists share
information about their research, they explain the goal of the work, how the research was done,
and their conclusions. They also report honestly and objectively about the information or data
they gather.
Sharing information allows scientists to find out what is already known about a topic. You can
find scientific information in a variety of places. Sources of scientific information include news
articles, scientific journals, government publications, and Web sites. However, you need to
remember that some of these sources are more reliable than others.
In this lesson, you read about kudzu. Kudzu is not the only invasive species affecting
ecosystems in your state. Do research to find out about three other invasive species found in
North Carolina. You may want to contact a local agricultural extension or the state fish and
wildlife service for information. Try to find out when and how each species was introduced into
the state and why it causes a problem for native organisms. If possible, obtain a picture of the
organism that other people can use to recognize it. Organize the information you find in a data
table like the one shown below. Then write a summary of your results or prepare a presentation
that can be used to share the information with your classmates.
List the sources of information you used