382 CHAPTER 15 Biodiversity and Conservation
(Figure 15.6c). Smaller patches tend to support fewer
species than larger areas, because they provide fewer eco-
logical niches and cannot host species with large ranges.
Habitat destruction, fragmentation, and degradation are
happening worldwide, causing the extinction of many
species and reducing genetic diversity within many sur-
viving species.
Africa provides a vivid example of the conflict over land
use between growing human populations and other spe-
cies. African elephants are nomads that require a large area
of natural landscape in which to forage for the hundreds
of kilograms of food that each consumes daily. People in
Africa are increasingly pushing into the elephants’ terri-
tory to grow crops and graze farm animals. The elephants
often trample or devour crops; they have even killed peo-
ple. Farmers cannot shoot at or kill elephants because they
are a protected species (although the animals are killed by
poachers seeking their ivory). Researchers have found that
elephants move out of areas that become too crowded with
people. Unfortunately, the wild areas to which elephants
can move are steadily shrinking, and elephant populations
are declining (Figure 15.6 d). One of the great challenges
is finding a way to allow people and elephants to coexist in
an increasingly crowded world. A promising solution intro-
duced in 2011 established a “bee fence” that exploits ele-
phants’ fear of bees to keep them within protected areas.Pollution Human-produced acid precipitation, strato-
spheric ozone depletion, and climate change degrade
even wilderness habitats that are considered natural and
undisturbed. Acid precipitation is thought to have con-
tributed to the decline of large stands of forest trees and
the biological death of many freshwater lakes. Because
ozone in the upper atmosphere shields the ground from
a large proportion of the sun’s harmful ultraviolet (UV)
radiation, ozone depletion in the upper atmosphere
represents a threat to all terrestrial life. Overwhelming
evidence links climate warming, caused in part by an
increase in atmospheric carbon dioxide released when
fossil fuels are burned, to biodiversity declines. Further
climate change is expected to increase the rate of extinc-
tion, particularly in such affected regions as polar areas.
Such habitat modifications particularly reduce the bio-
logical diversity of species with narrow and rigid environ-
mental requirements. Other types of pollutants that affect
organisms include industrial and agricultural chemicals,
organic pollutants from sewage, acid mine drainage, and
thermal pollution from heated industrial wastewater.Human Causes of Species Endangerment
Scientists generally agree that the single greatest threat
to biological diversity is loss of habitat. Pollution, the
spread of invasive species, and overexploitation are also
important. Underlying these direct effects are increases
in human population, economic activity, and uses of
technology, as well as social, political, and cultural fac-
tors (Figure 15.6a).
Habitat Destruction, Fragmentation, and
Degradation Most species facing extinction today
are endangered because of the destruction, fragmen-
tation, or degradation of habitats by human activities
( Figure 15.6b and c; also see Figure 13.6b, c, and d).
We demolish or alter habitats when we build roads,
parking lots, bridges, and buildings; clear forests to
grow crops or graze domestic animals; and log forests
for timber. We drain marshes to build on aquatic habi-
tats, thus converting them to t errestrial ones, and we
flood terrestrial habitats when we build dams. Explora-
tion for and mining of minerals, including fossil fuels,
also destroy habitats. Habitats are altered by outdoor
recreation, including using off-road vehicles, hiking
off-trail, golfing, skiing, and camping. Because most
organisms depend almost exclusively on a particular
type of environment, habitat destruction reduces their
biological range and ability to survive.
As the human population has grown, the need for
increased amounts of food has resulted in a huge con-
version of natural lands into croplands and permanent
pastures. According to the U.N. Food and Agriculture
Organization, total agricultural lands (croplands and
pasturelands) currently occupy 38 percent of Earth’s
land area. Agriculture also has a major impact on aquatic
ecosystems because it diverts water for irrigation. Little
habitat remains for many endangered species. For exam-
ple, human population growth and resource extraction
have destroyed most of the grizzly bear’s U.S. habitat—it
now occupies about 2 percent of its original habitat in
the lower 48 states.
Habitat fragmentation, the breakup of large areas of
habitat into small, isolated patches (that is, islands), is a
major threat to the long-term survival of many species.
In ecological terms, island refers not only to any land-
mass surrounded by water (Figure 15.6b) but also to any
isolated habitat surrounded by an expanse of unsuitable
territory. Therefore, a small patch of forest surrounded
by agricultural and suburban lands is considered an island