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60%
Mortality
Percentage of sapling trees 40
Recruitment
50

30

20
10

0
(a) (b) Small Medium Large
Island size
FIGURE 10.4 Effects of islands on ecological relationships. (a) When the Caroni River was
dammed to form Lago Guri in Venezuela, a set of islands was created. On the smallest islands,
the populations of primary consumers were too small to support predators, so the predators went
extinct. (b) As a result of not having predators, the primary consumers on the smallest islands
increased in number and this caused increased herbivory on the seedling and sapling trees,
resulting in high sapling mortality and a lower percentage of seedlings surviving into the sapling
stage (i.e., recruitment). Error bars are standard errors. (Data from Terborgh, J., et al. 2006. Vegetation dynamics of
predator-free land-bridge islands. Journal of Ecology 94: 253–263.) (a: Peter Langer/DanitaDelimont.com)

exist. As we noted in our discussion of trophic levels and the sapling stage. As you can see from this example, smaller
rule of 10 percent ecological efficiency in Module 7, small islands not only have fewer species, but the species can also
populations of producers and primary consumers make it experience quite different ecological relationships, including
difficult to support populations of secondary and tertiary in some cases a complete lack of large predators.
consumers. As a result, we would expect smaller islands to
not support large predators.
A nice test of this hypothesis was possible when the
Caroni River in Venezuela was dammed to create a lake 10-2 What determines the number of species on
known as Lago Guri. As the water rose behind the dam, the islands?
surrounding landscape was flooded and the highest points
in the forested landscape became islands in the lake, as you The number of species on islands
can see in FIGURE 10.4a. While the landscape originally depends on rates of colonization and
contained large predators, the smallest islands did not con-
tain enough primary consumers to support predator pop- extinction
ulations, so the predators on these islands went extinct. In
contrast, the larger islands continued to support larger pop- We can now understand that the size of an island is one factor
ulations of primary consumers and this allowed predators that greatly impacts the number of species that it can sup-
to persist. This mixture of small islands without predators port. As scientists investigated the number of species living
and large islands with predators meant that the two types on islands, they noticed that distance between a habitat and
of islands experienced very different community structures. a source of colonizing species is a second factor that affects
The lack of predators on the small islands allowed the pri- species richness. For example, oceanic islands that are more
mary consumers such as howler monkeys (Alouatta seniculus), distant from continents generally have fewer species than
iguana lizards, and leaf-cutter ants to become much more islands that are closer to continents. Distance matters because,
abundant and consume more of the plants on the island. In while many species can disperse short distances, only a few
Figure 10.4b, you can see the impact of this increased her- can disperse long distances. In other words, if two islands
bivory in terms of the higher mortality of sapling trees and are the same size and contain the same resources, the nearer
the lower recruitment of sapling trees, which was defined island should accumulate more species than the farther island
by the researchers as the survival of seedling trees into the because it has a higher rate of immigration by new species.

ModUle 10 ■ Island Biogeography 115
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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