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a larger tract of land will contain more species than multiple
smaller tracts that together comprise a similar total area.
The research also informs us that protecting tracts of habitat
islands closer to a large, protected habitat will protect more
species than protecting tracts of habitat islands that are
farther away. Thus, understanding island biogeography has
real-world applications to conserving biodiversity around
the world.

10-3 Why are some species on islands particularly
vulnerable to invasive species?

Some species on islands have evolved

to be specialists and are vulnerable FIGURE 10.11 Brown tree snake. The brown tree snake was
introduced to the island of Guam where it decimated the popula-
to invasive species tions of several island species, including birds, bats, and lizards.

(John Mitchell/Science Source)
When it comes to the conservation of biodiversity, species
living on islands are of high concern because they have
often evolved to be specialists. Why is this the case? Species eventually causing the decline or extinction of nine species
on islands are commonly much more isolated than species of birds, three species of bats, and several species of lizards.
on the mainland and they interact with a smaller number of The native fauna did not evolve with tree snakes, so they
other species. For example, many species living on islands had no defenses against this new predator. Similarly, the
lack predators and do not come into contact with the many birds of the Hawaiian Islands evolved without the patho-
pathogens that exist on the mainland. As a result, these spe- gen that causes avian malaria and is carried by mosquitoes.
cies have not evolved predator defenses. Many island species By the early 1900s, both mosquitoes and the avian malaria
of birds, for instance, have evolved to build their nests on pathogen were accidentally introduced into Hawaii and the
the ground since there are no nest predators on the island to populations of several native bird species experienced dra-
avoid. Some island birds, such as the species that live on the matic declines, with several going extinct.
islands of New Zealand, have historically lacked predators
and have also lost the ability to fly, which is an energeti- AP Exam Tip
®
cally costly activity. Finally, the environments of islands may
contain a relatively small diversity of food items, so island All species are native to some ecosystem. If the AP ®
species can evolve to specialize on the types of food that are Environmental Science Exam asks you to list an inva-
present. Under the pristine conditions of an isolated island, sive species you should list the species and where it
these specialized evolved responses can be beneficial to the is a problem. You will not be asked to know all of the
survival and reproduction of the island species. invasive species found in the world.
While being specialized can be beneficial under isolated
island conditions, it can make these species highly vulnera-
ble to invasive species. For example, numerous islands have
been invaded by accidentally introduced mice and rats when The vulnerability of island species becomes clear when
humans visit and inhabit the islands. These rodents are gen- we consider the percentage of extinct species that are island
eralists, allowing them to live under a wide range of condi- species. According to the Convention on Biological Diver-
tions and eat a wide range of food items. Moreover, they can sity, there have been 724 animal extinctions in the past
either consume the food required by the native specialists 400 years and nearly half of those have been island species.
or even serve as predators on the island fauna. One of the Moreover, it is estimated that at least 90 percent of extinct
best examples of an invasive predator wreaking havoc on birds were birds living on islands. As you can see, the unique
native island species happened when the brown tree snake conditions that favor the evolution of specialists on islands
( Boiga irregularis ) was accidentally introduced to the island makes them particularly vulnerable to the typically gener-
of Guam as ships were using the island as a port for trans- alist invasive species. This knowledge, combined with our
porting goods after World War II ( FIGURE 10.11 ). The snakes understanding of island biogeography, provides the insights

began to consume several species of native island animals. we need to help conserve biodiversity on islands and conti-
The population quickly grew to more than 2 million snakes, nents throughout the world.

ModUle 10 ■ Island Biogeography 119
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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