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1,000 20
Near islands 16
Number of bird species (log scale) 100 Number of mammal species 8 4 0
Intermediate islands
Far islands
12

10
100
1,000
2

1 (a) 0 10 Area (km ) (log scale) 10,000 100,000
26 260 2,600 26,000 260,000 20
2
Area (km ) (log scale)
FIGURE 10.5 The combined effects of island area and island 16
distance from a mainland. Researchers examined the number 12
of bird species living on 25 islands in the South Pacific. Larger
islands contained more species, but island distance from a main- Number of mammal species 8
land source of birds (New Guinea) also mattered. For any given
island area, near islands contained more bird species than far 4
islands. (Data from MacArthur, R. H., and E. O. Wilson. 1963. An equilibrium theory of
insular zoogeography. Evolution 17: 373–387.) 0
0 60 120 180 240 300
(b) Isolation (km)
Observing the Natural Effect of Island FIGURE 10.6 Effects of mountaintop area and distance from
Distance a mainland on mammal species richness. Mountaintops in
the American Southwest are tundra and conifer habitats, which
We can see an example of how island area and distance to a are isolated from each other by the presence of other habitats
mainland affect species richness by examining the number between the mountaintops. (a) Mountaintops with larger areas
of bird species living on 25 islands in the South Pacific. In contain more mammal species. (b) Mountaintops that are closer
this region, island areas range from small to large; island dis- to large mountain ranges also contain more mammal species.
tances range from close to far from New Guinea, which is a (Data from Lomolino, M. V., et al. 1989. Island biogeography of montane forest mammals in
very large island and serves as a large potential source of col- the American Southwest. Ecology 70: 180–194.)
onizing bird species. You can see how island size and island
distance affect the number of species in FIGURE 10.5. Just as
we have seen in earlier studies, islands with larger areas con- they also found that mountaintops that were more distant
tain more species of birds. However, for islands of a particu- from the Southern Rocky Mountains and Mongollon Rim
2
lar area, such as 2,600 km , those near to New Guinea had contained fewer species of mammals (Figure 10.6b). Results
more bird species than those at intermediate distances or far such as these make a compelling case that island area and
from New Guinea. In short, the number of species on an island distance combine to determine the number of species
island is determined by both island size and island distance. present.
The combined effects of island area and island distance
can also be observed in the mammal species living on Experimentally Demonstrating the Effect
mountain tops in the American Southwest, which represent of Island Distance
another type of habitat “island.” These mountaintops contain
alpine tundra and coniferous forests, which are surrounded Thus far we have described patterns in nature and hypothe-
by very different biomes including woodlands, grasslands, sized that islands farther from a mainland have fewer species
and deserts. As a result, the mammals living on the moun- because they are less likely to be colonized from the main-
taintops are isolated on habitat islands, similar to birds living land. It would be even more convincing if we could conduct
on oceanic islands. The nearest large source of these moun- a controlled experiment to show this mechanism actually
taintop mammals is the southern Rocky Mountains and the operating. Several decades ago, researchers E. O. Wilson and
Mongollon Rim, which is a mountain range in northern Daniel Simberloff conducted an experiment that tested this
Arizona. When researchers counted the mammal species on hypothesis using small islands in Florida that typically con-
each mountain top, they found that larger mountain hab- tained a single mangrove tree. They began by visiting three
itats contained more mammals (FIGURE 10.6a). However, islands that were near, intermediate, or far from mainland

116 UNIT 2 ■ The Living World: Biodiversity
Uncorrected proofs have been used in this sample.
Copyright © Bedford, Freeman & Worth Publishers.
Distributed by Bedford, Freeman & Worth Publishers. Not for redistribution.

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