movement with a volume of c.400 km^3 , generating
a mega-tsunami that transported boulders weigh-
ing more than 1000 t on to elevations of 11 m on the
Bahamas Archipelago, located more than 3000 km
westwards (Hearty 1997, in Whelan and Kelletat
2003). Ward and Day (2001) have highlighted the
great instability of the Cumbre Vieja massif, the
active southern summit of La Palma, where seven
volcanic eruptions have been concentrated in the
last five centuries. It has been predicted that
Cumbre Vieja will collapse to the west in the coming
millennia, generating a mega-tsunami likely to
devastate the east coast of North America on the far
side of the Atlantic.
2.6 Summary
Two broad classes of island are identified, true
islands surrounded by water, and habitat islands
surrounded by a contrasting matrix. True islands can
be subdivided into: island continent, oceanic islands,
continental fragments, continental shelf islands, and
islands within lakes. This chapter is concerned
almost exclusively with islands located in the oceans
of the world. These islands are rarely ancient in geo-
logical terms, and in many cases are significantly less
ancient biologically than geologically. Oceanic
islands have volcanic foundations, are concentrated
in a number of distinctive inter- and intraplate
settings and have commonly experienced a dynamic
history involving varying degrees of lateral and
vertical displacement, the latter confounded by, but
often in excess of, eustatic sea-level changes. In the
tropics and subtropics, upward growth of reef-form-
ing corals at times of relative or actual subsidence
has led to the formation of numerous islands of only
a few metres elevation, contrasting with the gener-
ally steep topography of the high volcanic islands, to
a large degree according with Darwin’s 1842 theories
on coral reef formation.
Some of the more obviously important environ-
mental features of islands are directly related to
these geological factors and include characteristic
topographic, climatic, and hydrological phenom-
ena. Subsurface water storage is a particularly
important feature of both volcanic and low,
sedimentary islands: the possession of a subsurface
freshwater lens is a characteristic of all but the
smallest islets. Island environments might be
thought to have been shielded from the full ampli-
tude of continental climatic fluctuations, but the
palaeoecological record indicates significant climatic
changes within the late Pleistocene and Holocene
on a range of islands, including examples from the
tropics and subtropics. With limited scope for range
adjustments within the island setting, the biogeo-
graphical significance of such environmental
changes should not be ignored.
Shorter term environmental variation, perturba-
tion, or disturbances (broadly overlapping cate-
gories) characterize many island environments, not
least for those islands which lie in the tropical
cyclone belt and for those which remain volcani-
cally active. Recent bathymetric, geomorphological,
and volcanological advances have revealed just
how dynamic oceanic island environments are.
Island surfaces are constantly in flux, not only built
by volcanism, and reduced by subaerial and coastal
erosion, but also subject to transforming cata-
strophic slope failures. Some appreciation of the
distinctive nature of the origins, environmental
characteristics, and history of islands is almost
self-evidently important to an understanding of the
biogeography of islands, although, in this scene-
setting chapter, the biogeographical content has
intentionally been kept within limits.SUMMARY 45