HUMAN BIOLOGY

486 Chapter 25

endangered species
A species native to an
area that is vulnerable to
extinction.

sustainability An approach
for managing human popu‑
lation growth, resource use,
and the preservation of
wild habitats in ways that
will help ensure the long‑
term survival of the human
species.

Unfortunately, most of its native forest habitat on the island

of Madagascar has been cleared away to make room for

human enterprises. Climate change also has begun affect­

ing species such as the polar bear, which recently was

listed as endangered by the U.S. Environmental Protection

Agency (Figure 25.23). No giant asteroids have hit Earth for

65 million years. Yet a major extinction event is under way.

Throughout the world, human activities are swiftly driving

many species to extinction. An endangered species is

an endemic species extremely vulnerable to extinction.

Endemic means it originated in only one geographic region

and lives nowhere else.

In the United States we are destroying the habitats of

wild species at a dizzying pace. For instance, we have

logged more than 90 percent of old­growth forests and

drained half the wetlands, which filter human water

supplies and provide homes for waterfowl and juvenile

fishes. Hundreds of native species in these areas have gone

extinct and dozens more are endangered.

Marine resources are being overharvested

People have been looking to the seas as a major source of

food for the expanding human population, but there, too,

little attention has been given to sustainable use of marine

resources. One recent study estimated that for a third

of seafood species, the annual catch has plummeted by

90 percent. The threatened species include Atlantic cod,

black sea bass, and several shark species (Figure 25.24).

The principle of sustainability is the answer

We humans are the dominant species on our planet, but

many people have begun to wonder how long that will

be the case. As you have read in this chapter, by our sheer

numbers and many of our activities we have disturbed

many, if not virtually all, ecosystems on the planet. That

course cannot continue for much longer. Fortunately,

endangered species and the Loss of Biodiversity

Figure 25.22 Habitat loss and overexploitation of wildlife is a

major threat to the survival of many species. A Cities displace

wild species and require huge amounts of resources to sustain

the people who live in them. B Confiscated products made from

endangered species. It’s estimated that 90 percent of the illegal

wildlife trade, which threatens hundreds of species of animals

and plants, goes undetected.

A

B

Steve Hillebrand, U.S. Fish & Wildlife Service

James Marshall/Image Works/Time Life Pictures/Getty Images

n    Today human activities are a major cause of the rapid loss

of other species.

n    Link to Mass extinction 23.7

A major mass extinction is under way, and we humans are

largely responsible for it. By one estimate our actions are

leading to the premature extinction of six species an hour! At

present the major culprits are destruction of wildlife habitats

and the overexploitation of wild species for food or profit.

Habitat loss pushes species to the brink

The underlying causes of today’s rapid pace of species extinc­

tions are human population growth and economic policies

that promote unsustainable exploitation. As our population

grows, we clear, occupy, and dam­

age more land to supply food, fuel,

timber, and other resources (Figure

25.22A). In some regions, the com­

bination of rapid population growth

and poverty pushes the poor to cut

forests, grow crops on marginal land,

overgraze grasslands, and poach

endangered animals (Figure 25.22B).

Globally, tropical deforestation is

the greatest killer of species. The

loss of plant species is extremely

important because most animals depend directly or indi­

rectly on plants for food, and often for shelter as well. We

humans also have traditionally depended on plants as

sources of medicines. For example, we get two anticancer

drugs from a tropical plant called the rosy periwinkle.

25.9

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