Page 347 - Environment: The Science Behind the Stories

FIGURE 12.24 Forests are being fragmented,
with ecological consequences. Fragmenta-
tion results from clear-cutting (a), agriculture,
and residential development. Shown in (b) are
historical changes in forested area in a region
of Wisconsin. Fragmentation affects forest-
dwelling species such as the wood thrush
(c), whose nests are parasitized by cowbirds
that thrive in surrounding open country. Source
(b): Curtis, J.T., 1956. The modification of mid-latitude
grasslands and forests by man. In Thomas, W.L. Jr., Ed.,
Man’s role in changing the face of the earth. ©1956. Used
by permission of the publisher, University of Chicago Press.

(a) Fragmentation from clear-cuts in Mount Hood National Forest, Oregon

1831 1882 1950
(b) Fragmentation of wooded area (green) in Cadiz Township, Wisconsin (c) Wood thrush

and other factors, including agriculture and residential devel- 1963, this theory explains how species come to be distributed
opment (FIGURE 12.24b). Even in areas where forest cover is among oceanic islands. Since then, researchers have applied
increasing (such as the northeastern United States), regrowing it to “habitat islands”—patches of one habitat type isolated
forests are becoming fragmented into ever-smaller parcels. within “seas” of others.
When forests are fragmented, many species suffer. For Island biogeography theory explains how the number of
some, a fragment may simply not contain enough area; bears, species on an island results from a balance between the num-
mountain lions, and other animals that need large ranges in ber added by immigration and the number lost through extir-
which to roam may disappear. For other species, the prob- pation. It predicts an island’s species richness based on the
lem may lie with edge effects, impacts that result because the island’s size and its distance from the mainland:
conditions along a fragment’s edge differ from conditions in
the interior. Bird species that thrive in the interior of forests • The farther an island lies from a continent, the fewer
may fail to reproduce when forced near the edge of a fragment species tend to find and colonize it. Thus, remote
(FIGURE 12.24c). Their nests often are attacked by predators and islands host few species because of low immigration
parasites that favor open habitats or travel along habitat edges. rates (FIGURE 12.25a). This is the distance effect.
Because of edge effects, avian ecologists judge forest frag- • Larger islands have higher immigration rates because
mentation to be a main reason why populations of many North they present fatter targets for dispersing organisms to
American songbirds are declining. encounter (FIGURE 12.25b).

• Larger islands have lower extinction rates because more
Insights from islands warn us of habitat space allows for larger populations, which are less likely
fragmentation to drop to zero by chance (FIGURE 12.25c).

In assessing the impacts of habitat fragmentation on popu- Together, the latter two trends give large islands more spe-
lations, ecologists and conservation biologists have leaned cies than small islands—a phenomenon called the area effect.
on concepts from island biogeography theory. Introduced Large islands also contain more species because they tend
346 by E.O. Wilson (p. 311) and ecologist Robert MacArthur in to possess more habitats than smaller islands. Very roughly,

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