Page 303 - Environment: The Science Behind the Stories

1 Original habitat

2 Gaps form as habitat
Figure 11.10 Development of land for housing is one way in becomes fragmented
which habitat is altered or destroyed.

those habitats are destroyed outright, but habitats are also lost
when they are altered through more subtle processes, includ-
ing fragmentation and other forms of degradation. Because
organisms have adapted over thousands or millions of years
to the habitats in which they live, any major change in their 3 Gaps become larger;
habitat is likely to render it less suitable for them. fragments become smaller
Many human activities alter, degrade, or destroy habitat. and more isolated
Housing development supplants diverse natural ecosystems
with simplified human-made ones, driving many species from
their homes (Figure 11.10). Farming replaces diverse natural
communities with simplified ones of only a few plant spe-
cies. Grazing modifies the structure and species composition
of grasslands, and it can lead to desertification (p. 241). Clear-
ing forests removes the food, shelter, and other resources that
forest-dwelling organisms need to survive. Dams turn riv- 4 Species disappear due to
ers into reservoirs upstream and affect water conditions and habitat fragmentation
floodplain communities downstream.
Habitat loss occurs most commonly through gradual,
piecemeal degradation, such as habitat fragmentation (Figure Figure 11.11 Habitat fragmentation occurs as human
11.11). When farming, logging, road building, or development impact creates gaps that expand and eventually come to
dominate the landscape, stranding islands of habitat. As
intrude into an unbroken expanse of forest or grassland, they habitat becomes fragmented, fewer populations can persist, and
break up a continuous area of habitat into an array of frag- numbers of species in the fragments decline.
ments, or patches. As habitat fragmentation proceeds across
a landscape, animals and plants requiring the habitat disap-
pear from one fragment after another. Fragmentation can also
prevent animals from moving from place to place, and this is according to UNEP data. For example, the prairies native to
the concern of conservationists fighting the proposal to build North America’s Great Plains are today almost entirely con-
a highway through the Serengeti. In response to habitat frag- verted to agriculture. Less than 1% of original prairie habitat
mentation, conservationists have designed landscape-level remains. As a result, grassland bird populations have declined
strategies to optimize the arrangement of areas to be preserved by an estimated 82–99%.
(pp. 345–350). Of course, our habitat alteration benefits some species.
Habitat loss has affected nearly every biome (Figure 11.12). Animals such as house sparrows, pigeons, gray squirrels,
Over half of the world’s temperate forests, grasslands, rats, and cockroaches thrive in cities and towns. However, the
and shrublands had been converted by 1950 (mostly for species that benefit from our modification of natural habi-
agriculture). Today habitat is being lost most rapidly in tropi- tats are relatively few; for every species that wins, more lose.
cal rainforests, tropical dry forests, and savannas. Furthermore, the species that do well in our midst tend to be
Habitat loss is the primary source of population declines weedy generalists that are in little danger of disappearing any
302 in 83% of threatened mammals and 85% of threatened birds, time soon.

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