Biology Reference
In-Depth Information
the island has been isolated, and size and diversity of habitats on the island. Islands that were
once connected to the mainland are called continental islands. Some of them separated from a
mainland when the ancient continent of Gondwanaland split up, beginning about 200 million
years ago. These include Madagascar, New Zealand, the Seychelles, New Guinea, and New
Caledonia, off the northeastern coast of Australia. They are loaded with ancient endemic spe-
cies, many rare, and most quite different from the closest mainland flora and fauna. Other
continental islands separated from adjacent mainland only a few thousand years ago when
rising sea levels caused by melting glaciers severed their lowland connections. For example,
Sri Lanka was connected to India only a few thousand years ago, and Trinidad was connec-
ted to Venezuela until about 11,000-15,000 years ago. On each of these islands, the flora and
fauna are quite similar to those of the continent nearby because insufficient time has elapsed
for much evolutionary change to happen.
Other island groups, such as the Hawaiian and Juan Fernández Islands, are oceanic islands
formed by volcanic action. They were never connected to a continental landmass. Thus, all
organisms living on them must have dispersed across oceanic barriers. This is why the Juan
Fernández Islands, although formed 1 to 6 million years ago, have few endemic vertebrates
and only a modest number of endemic plants. Also, the few islands in the archipelago are
too close to one another to isolate populations. The fauna and flora of nearshore oceanic is-
lands, such as those in the Gulf of California, often tend to look quite similar to those on the
mainland. They are within easy reach of mainland species that can fly or raft over on floating
vegetation or whose seeds arrive on the winds. Habitat diversity also plays a key role in the
amount of speciation that takes place. Like New Guinea, some of the most ecologically di-
verse islands—such as New Caledonia and Madagascar—are covered in mountain chains or
bisected by plateaus. The resulting rain shadow creates wet forests on the island's windward
side and dry forests on its leeward side, the different habitats favoring different species.
New Guinea not only embodies areas of exceptional isolation conducive to speciation.
Much of the island has another quality that makes it an ideal natural laboratory for the
study of rarity and abundance—remote mountain ranges marked by the virtual absence of
human interference. Today, natural patterns of rarity and abundance on virtually all equat-
orial islands have become increasingly obscured by the destructive spread of invasive spe-
cies—goats, cattle, pigs, cats, dogs, rats, rabbits—and by the logging of native forests, con-
version of cutover land to agriculture, and other forms of development and exploitation that
have followed the arrival of humans.
A scientist in New Guinea, in contrast, can still observe the interplay of geology, evolution,
and rarity in an all but undisturbed venue. The mountain walls, deep gorges, and numerous
rivers create barriers that prevent recently arrived species from spreading and swamping the
local biota. Some of the high ranges even check the spread of humans. New Guinea is about
twice the size of California but remains sparsely populated. With so few people—about 7
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