of continental rock stranded out in the oceans by
plate tectonic processes.
●Continental shelf islands are those islands
located on the continental shelf. Many of these
islands have been connected to mainland during
the Quaternary ice ages (formally, the last
1.8 million years), although cooling actually began
earlier), as these were periods of significantly lower
sea levels. The most recent period of connection for
these so called ‘land-bridge’islands ended follow-
ing the transition from the Pleistocene into the
Holocene. The Holocene began about 11 500 years
ago, but seas took several thousand years to rise to
their present levels.
●Finally, islands occurring within freshwater
bodies, both lakes and large rivers, are likely to be
more comparable to islands in the sea than to habi-
tat islands, and hence can also be considered ‘true’
islands.
Habitat islands are essentially all forms of insular
system that do not qualify as being ‘real islands’.
This means, for terrestrial systems, discrete patches
of a particular habitat type surrounded by a matrix
of strongly contrasting terrestrial habitats. For
aquatic systems we can consider similarly discrete
habitat types separated by strongly contrasting
aquatic environments (e.g. shallow benthic envi-
ronments isolated by deep water) also to constitute
habitat islands. Both habitat and lake islands can, of
course, occur within the true islands. Moreover, a
more refined classification of marine islands has to
recognize that there are, for example, many forms
and ages of continental shelf islands, and that there
are anomalous islands midway in character
between oceanic and continental.
The literature and theory of island biogeography
has been built up from a consideration of all forms of
island, from thistle-head habitat islands (Brown and
Kodric-Brown 1977) to Hawaii (e.g. Wagner and
Funk 1995). True islands have the virtue of having
clearly defined limits and properties, thus providing
discrete objects for study, in which such variables as
area, perimeter, altitude, isolation, age, and species
number can be quantified with some degree of objec-
tivity, even if these properties can vary hugely overTYPES OF ISLANDS 11Table 2.1A simple classification of island types distinguishing: (1) classic types of ‘real’ island, being land surrounded by open water, from
(2) habitat islands, for which the contrast between the ‘island’ and the surrounding matrix is less stark but still sufficient to represent a barrier or
filter to population movements. Australia, given its huge size, is essentially continental in character and in practice is not treated as an island in
the present work
Type of island Examples
Land surrounded by water
Island continent Australia
Oceanic islands Hawaii, Canaries
Continental fragments Madagascar, New Caledonia
Continental shelf islands British Isles, Newfoundland
Islands in lakes or rivers Isle Royale (Lake Superior), Barro Colorado island (Lake Gatún),
Gurupá island (River Amazon)
Habitat islands
Patches of a distinct terrestrial habitat
isolated by a hostile matrix Great-Basin (USA) mountain tops surrounded by desert
Woodland fragments surrounded by agricultural land
Thistle heads in a field
Continental lake (Baikal, Titicaca)
Marine habitat islands The fringing reef around an isolated oceanic island
Coral reefs separated from other reefs by stretches of seawater
Seamounts (submerged or not yet emerged mountains below sea level)
Guyots (submerged flat-topped former islands, i.e. a type of seamount)