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.

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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.

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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.

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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.

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List the sources of information you used

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