Page 348 - Environment: The Science Behind the Stories

More species More species Proportionately fewer
colonize islands colonize larger species go extinct on
close to mainland islands larger islands

(a) Distance effect (b) Target size (c) Differential extinction
FIGURE 12.25 Islands that are larger or closer to a mainland support more species. According to island
biogeography theory, islands near a continent (a) receive more immigrants than distant islands. Larger islands
present fatter targets (b), so more species encounter large islands than small islands. Large islands also have
lower extinction rates (c), because larger area allows for larger populations.

the number of species on an island is expected to double as had vanished from Sequoia and Kings Canyon National
island size increases tenfold. This phenomenon is illustrated Parks, for example, and the white-tailed jackrabbit and spot-
by species-area curves (FIGURE 12.26). ted skunk no longer lived in Bryce Canyon National Park.
These patterns hold up for terrestrial habitat islands as In all, 42 species had disappeared. As island biogeography
well, such as forests fragmented by logging and road build- theory predicted, smaller parks lost more species than larger
ing. Small “islands” of forest lose diversity fastest, starting parks. Species were disappearing because the parks were too
with large species that were few in number to begin with. small to sustain their populations, Newmark concluded, and
One of the first researchers to show this experimentally because the parks had become too isolated to be recolonized
was University of Michigan graduate student William New- by new arrivals.
mark, who in 1983 examined historical records of mammal
sightings in North American national parks. The parks, Reserve design has consequences
increasingly surrounded by development, were islands of
natural habitat isolated by farms, ranches, roads, and cities. for biodiversity
Newmark found that many parks were missing a few
species they had held previously. The red fox and river otter Because habitat fragmentation is such a central issue in
biodiversity conservation, and because there are limits on
how much land can feasibly be set aside, conservation biolo-
100 Puerto Rico Hispaniola Cuba gists have argued heatedly about whether it is better to make CHAPTER 12 • FOREST S, FOREST MAN A GEMENT, AND PR O TECTED AREAS
Number of species on island 10 Saba Montserrat Jamaica large or several small,” this debate is complex, but it seems
reserves large in size and few in number, or many in number
but small in size. Nicknamed the SLOSS dilemma, for “single
clear that large species that roam great distances, such as
wildebeest and zebras (Chapter 11), benefit more from the
“single large” approach to reserve design. In contrast, crea-

do just fine in a number of small isolated reserves, if they
1 Redonda tures such as insects that live as larvae in small areas may
can disperse as adults by flying from one reserve to another.
1 10 100 1000 10,000 100,000 The SLOSS debate was one motivation for establishing the
Area of island (square miles) Amazonian forest fragmentation project described in our
Science behind the Story (pp. 348–349).
FIGURE 12.26 Larger islands hold more species.This species- A related issue is how effectively corridors of protected
area curve shows that the number of amphibian and reptile species
on Caribbean islands increases with island area. The increase is land allow animals to travel between islands of habitat. In
not linear, but logarithmic; note the scales of the axes. Data from theory, connections between fragments provide animals
MacArthur, R.H, and E.O. Wilson. The theory of island biogeography. © 1967 access to more habitat and encourage gene flow to maintain
Princeton University Press, 1995 renewed PUP. Reprinted by permission of populations in the long term. Many land managers now try to
Princeton University Press. join new reserves to existing reserves for these reasons. 347

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