HUMAN BIOLOGY

hUMaN iMpaCts oN the Biosphere 483

What are some major land-use issues?

• Energy- and water-intensive agricultural practices and the
  removal of vast tracts of forests are having major negative
  impacts on land ecosystems.


• Desertification of once-usable grassland or cropland is a
  growing problem.


• Rapid deforestation may be boosting the greenhouse effect.


• Deforestation promotes soil erosion and probably is contributing
  to greenhouse warming of the atmosphere.

taKe-Home messaGe

and gradually release water. Forests also help control soil
erosion, flooding, and sediment buildup in rivers, lakes,
and reservoirs.
Deforestation is the name for removal of all trees from
large tracts of land for logging, agricultural, or grazing
operations. The loss of vegetation exposes the soil, and this
promotes leaching of nutrients and erosion, especially on
steep slopes. Cleared plots soon become infertile and are
abandoned. The photograph in Figure 25.18A shows forest
destruction in the Amazon basin of South America.
Deforestation is linked with several ecological prob­
lems. One of the most troubling effects relates to the global
carbon cycle. Tropical forests absorb much of the sunlight
reaching equatorial regions of Earth’s surface. When the
forests are cleared, the land becomes “shinier” and reflects
more incoming energy back into space. The many millions
of photosynthesizing trees in these vast forests help sus­
tain the global cycling of carbon and oxygen. When trees
are harvested or burned, carbon stored in their biomass is
released to the atmosphere in the form of carbon dioxide—
and this may be boosting the greenhouse effect.
About half the world’s tropical forests have been cut
down for cropland, fuel wood, grazing land, and timber.
Deforestation is greatest in Brazil, Indonesia, Colombia,
Mexico, Nigeria, and CÔte d’Ivoire (Figure 25.18B). If clear­
ing continues at present rates, within a few years only Brazil

Figure 25.18 Huge tracts of tropical and temperate lands
are being deforested. A Forest destruction in South America.
B Orange shading on this map indicates countries where about
2,000 to 14,800 square kilometers of tropical forests are removed
annually. Gold indicates sites of “moderate” tropical deforestation
(100 to 1,900 square kilometers). C A huge clear-cut in a temperate
conifer forest in British Columbia. (A: R. Bieregaard/Science Source; B: © Cengage
Learning; C: Ted Kerasote/Science Source)

A

B

C

and the Democratic Republic of the Congo will have large

tropical forests. By 2035, most of their forests will be gone.

Conservation biologists are attempting to reverse the

trend. For example, in Brazil, a coalition of 500 groups

is working to preserve the country’s remaining tropical

forests. In Kenya, women have planted millions of trees.

Their success has inspired similar programs in more than

a dozen countries in Africa. In eastern North America, for­

ested land has increased in recent years due to regrowth

in logged areas and the creation of commercial tree plan­

tations. Elsewhere, such as the western United States,

British Columbia (western Canada), and Siberia, logging is

rapidly clearing vast tracts of old growth temperate forests

(Figure 25.18C).

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