0198566123.pdf

elevated. Christmas Island in the Indian Ocean
exemplifies this. It has been elevated to form a
plateau of 150–250 m above sea level, but in parts
reaches 360 m. The general aspect of the island is of
coastal limestone cliffs 5–50 m high, which carries
with it the implication that the area currently avail-
able for seaborne propagules to come ashore is con-
siderably less than the island perimeter, a factor in
common with many islands of volcanic origin.
It is becoming apparent that the relative elevation
of particular islands may be subject to influence by
the behaviour of others in the vicinity. The loading
of the ocean floor by a volcano appears to cause
flexure of the lithosphere, producing both near-
volcano moating and compensatory uparching
some distance from the load; thus one repeated
pattern in the South Pacific is the association within
island groups of young volcanoes ( 2 million
years) with raised reef, or makatea islands, as exem-
plified respectively by Pitcairn and Henderson
Islands (Benton and Spencer 1995). There are many
other examples of islands that contain substantial
amounts of limestone as a product of uplift: one that
will be considered below is Jamaica.

Over time, sea levels have fluctuated markedly,

particularly during the Quaternary glacial–inter-

glacial cycles. These fluctuations, in combination with

island subsidence, wave action, and subaerial erosion,

have resulted in many islands declining below the

current sea level. In nautical parlance, they are known

asbanksif they are less than 200 m from the surface

(Menard 1986). Once they have sunk below about

200 m, they are effectively below the range of eustatic

sea level and in this situation will rarely re-emerge

as islands. Flat-topped sunken islands are termed

guyots, and although they were once thought to be

predominantly erosional features, most are in fact flat-

topped through accretion of carbonate sediments

(Jenkyns and Wilson 1999). The distribution of guy-

ots, and particularly of banks, may be crucial to an

understanding of the historical biogeography of

contemporary islands (Diamond and Gilpin 1983; and

compare McCall 1997 with Rogers et al. 2000).

Eustatic changes in sea level

From a biogeographical viewpoint, it does not matter

greatly whether the changing configuration of an

24 ISLAND ENVIRONMENTS

Region of water temperature

over 20 ̊C (68 ̊F) in the

coldest month

Figure 2.6Region of water temperatures exceeding 20C in the coldest month, which corresponds roughly to the major reef regions of the
world’s oceans. (Redrawn from Mielke 1989, Fig. 7.9.)