the lifespan of an island (Stuessy et al. 1998). In con-
trast, habitat islands exist typically within complex
landscape matrices, which are often rapidly and
dramatically changing over just a few years. Matrix
landscapes may be hostile to some but not all species
of the habitat islands, and isolation is thus of a dif-
ferent nature from islands in the sea. It cannot neces-
sarily be assumed that what goes for real islands also
works for habitat islands and vice versa.
As much of this book is concerned almost exclu-
sively with islands in the sea, their biogeographical
peculiarities, and their problems, it is important to
consider their modes of origin, environmental char-
acteristics, and histories. These topics form the bulk
of this chapter.
2.2 Modes of origin
Are islands geologically distinctive compared with
most of the land surface of the Earth? If we take this
question in two parts, continental shelf islands are
a pretty mixed bag, a reflection of their varied
modes of origin, whereas oceanic islands are fairly
distinctive geologically, being generally composed
of volcanic rocks, reef limestone, or both (Darwin
1842; Williamson 1981). The analysis that follows is
intentionally a simplified account of an extremely
complex reality.
The development of the theories of continental
drift and, more recently, plate tectonics, has revolu-
tionized our understanding of the Earth’s surface
and, along with it, our understanding of the distri-
bution and origins of islands. According to the latter
theory, the Earth’s surface is subdivided into some
seven major plates, each larger than a continent, and
a number of smaller fragments (Fig. 2.1). With the
exception of the Pacific plate and its subordinates
(Nazca, Cocos, Juan de Fuca, and Philippines), the
plates themselves are typically made up of two
parts, an oceanic part and a continental part. The sil-
icon/aluminium-rich granitic parts of the plates
are of relatively low density and these form the
continental parts, supporting the continents them-
selves (consisting of a highly varied surface geol-
ogy), extending to about 200 m below sea level
(Fig. 2.2). This zone, from 0 to 200 m, forms the
continental shelf and supports islands such as the
British Isles and the Frisian Islands (off Germany,
the Netherlands, and Denmark), typically involving
a mix of rock types and modes of formation, such
that any combination of sedimentary, metamorphic,
or igneous rocks may be found. As Williamson
(1981) noted, about the only generalization that can
be made is that the geological structures of conti-
nental shelf islands tend to be similar to parts of the
nearby continent.
In places, continental plate can be found at much
greater depths than 200 m below sea level, and can
then be termed sunken continental shelf. Islands
on these sections of shelf (ancient continental
islandssensuWallace: Box 2.1) are thus formed of
continental rocks; examples include Fiji and New
Zealand (Fig. 2.3; Williamson 1981). Typically, how-
ever, there is a steeply sloping transition zone from
shallow continental shelf down to c.2000 m or
more, where the basalticpart takes over and the
true oceanic islands occur. These are all volcanic in
origin, although in certain cases they may be com-
posed of sedimentary material, principally lime-
stones, formed as the volcanic core has sunk below
sea level. True oceanic plate islandshave never
been attached to a greater land mass. They may
grow through further volcanism, or subside, erode,
and disappear. In a geological sense, they tend to be
transitory: some may last only a few days, others
some millennia; relatively few last tens of millions
of years. Thus, in addition to the islands that sus-
tain terrestrial biota today, there are also a large
number of past and future islands, or seamounts,
found at varying depths below sea level. Flat-
topped seamounts, formed through the submer-
gence of limestone-topped volcanoes, are called
guyots, after the Swiss geologist Arnold Guyot
(Jenkyns and Wilson 1999).
Islands may originate by existing areas of land
becoming separated from other land masses to
which they were formerly connected, by erosion, or
by changes in relative sea level from a variety of
causes. Many islands have originated from volcan-
ism associated with plate movements. The form this
volcanism takes and the pattern of island genesis
involved, depend crucially on the nature of the
contact zone between plates, that is whether the
plates are moving apart, moving towards each12 ISLAND ENVIRONMENTS