HUMAN BIOLOGY

482 Chapter 25

deforestation The removal
of trees from large tracts
of land.

desertification The conver‑
sion of agricultural land or
grassland to a desertlike
state, often due to overgraz‑
ing of livestock.

Overgrazing of livestock on marginal lands is a prime

cause of desertification—the conversion of grasslands or

cropland to a desertlike state that does not readily support

useful plants (Figure 25.17). Worldwide, this has happened

to about 10 million square kilometers over the past 50 years,

and the trend is continuing.

Development of land for housing, towns, and cities also

has an ecological price. One of the most important is the

loss of habitat used by other species. In areas where water

is scarce, such as much of the western United States, hous­

ing developments also increase the pressure on limited

water supplies and political wrangling over access to them.

As climate change alters conditions in these areas, these

problems will be increasingly difficult to solve. We will

return to the topic of habitat losses in Section 25.9.

Deforestation has global repercussions

The world’s great forests influence the bio sphere in many

ways. Like giant sponges, forested watersheds absorb, hold,

problems with Land use and Deforestation

Figure 25.17 Desertification has become a major problem in

parts of Africa. A A satellite photo taken in 2005. Red lines mark

the huge area where desertification has occurred. B People

who live in this area can barely eke out a living on the land.

(A: © Cengage Learning/NASA; B: Romano Cagoni/Peter Arnold, Inc.)

Sahara Desert

Sahel

A

B

Figure 25.16 Agriculture requires large amounts of land

and water. Rice must grow in shallow water of continuously

flooded fields. It is a major staple food crop for tens of millions

of people in Asia.

Charles O’Rear/Documentary Value/Corbis

n    The demand for food, housing, and forest products has

led to widespread deforestation, agricultural practices that

harm the environment, and the loss of habitat for other

species.

n    Link to Genetic engineering of plants 21.11

Like other ecological problems, those related to land use

are linked to the rapid growth of the human population.

People must have places to live and grow food, as well as

materials to build homes and use as fuel.

Feeding and housing billions of humans

requires land and other scarce resources

Only 25 percent of Earth is dry land, and only a fraction of

that is available for human use. Today, almost a quarter of

Earth’s land is being used for agri­

culture. Scientists have made valiant

efforts to improve crop production on

existing land. As part of the “green

revolution,” research has been geared

to improving the varieties of crop

plants for higher yields and exporting

modern agricultural practices and

equipment to developing countries.

Unfortunately, the green revolution is based on huge

inputs of fertilizers and pesticides and ample irrigation to

sustain high­yield crops (Figure 25.16). It is based also on

fossil fuel energy to drive farm machines. Crop yields are

four times as high as from traditional methods. But modern

practices use up to 100 times more energy and minerals

such as nitrogen and phosphorus. Also, there are signs that

limiting factors are coming into play to slow down further

increases in crop yields.

25.7

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