DISHARMONY, FILTERS, AND REGIONAL BIOGEOGRAPHY 57Box 3.2 Islands in the ocean: general features
In Box 2.1 we outlined the differing origins of
three classes of island. Here we summarize
aspects of their geology, geography,
and biology.
●Oceanic islandsare built over the oceanic
plate, are of volcanic or coralline formation, are
remote and have never been connected to
mainland areas, from which they are separated
by deep sea. They generally lack indigenous land
mammals and amphibians but typically have a
fair number of birds and insects and usually
some reptiles.
●Continental islandsare more varied geologi-
cally, containing both ancient and recent stratified
rocks. They are rarely remote from a continent
and always contain some land mammals and
amphibians as well as representatives of the
other classes and orders in considerable variety
(Wallace 1902), typically having been connectedto other land masses before the Holocene
transgression.
●Continental fragment islandsare in some
respects intermediate between the other two
classes, but being typically ancient and
long-isolated they are in other respects
biologically peculiar.
The biogeographical distinction between islands
of differing geological origins of course depends
greatly on the ability of potential inhabitants of an
island to disperse to it, whether over land or sea.
As a simplification, we may conceptualize an
oceanic island as one for which evolution is faster
than immigration, and a continental island as one
where immigration is faster than evolution
(Williamson 1981, p. 167). Thus, a particular island
may be thought of as essentially continental for
some highly dispersive groups of organisms (e.g.
ferns (Bramwell 1979) and some types of birds),General features of islands of different origin of relevance to their biogeography (modified from Fernández-Palacios 2004)
Features Continental islands Continental fragments Oceanic islands
Origin Interglacial sea-level rise New mid-ocean rift creation Between or within plate
submarine volcanic activity
End Glacial sea-level drop Collision with a continent Erosive break up and subsidence
Degree of isolation Small Variable; small for Cuba, Large (except Canaries)
large for New Zealand
Size From very small to very large Some small, mostly large Small (except Iceland)
Longevity Short (20–30 Ka) Long (50–150 Ma) Variable (hours–20 Ma)
as separate entities
Water gap depth Small (130 m) Large (1000 m) Large (1000 m)
Parent rocks Granites Granites Basalts
Erodibility Low Low High
Fragmentation Low Low (except Balearics and High (except lonely islands
Seychelles) such as Ascension or St. Helena)
Original biota Present Present Absent
Relictualism Lacking High Moderate
Speciation process Limited Vicariance Founder event, etc.
Endemism Variable according to latitude Very high High
(low in high latitudes,
often high in low latitudes)