0198566123.pdf

14 ISLAND ENVIRONMENTS

12 ° 10 ° 8 ° 6 ° 4 ° 2 °

Longitude West

0 °

Atlantic Irish shelf Ireland Britain North Sea

Metres

1000

1000

2000

3000

0

North

Channel

Figure 2.2A section across the British Isles at 55N. The section passes through Londonderry in Northern Ireland, Cairnsmore of Fleet
in Scotland, and Newcastle upon Tyne in England. (Redrawn from Williamson 1981, Fig. 1.2.)

Box 2.1 Islands in the ocean: an appraisal of Alfred Russel Wallace’s classification

In his seminal book entitled Island life(3rdedition

1902, first published 1880) Alfred Russel Wallace

offered a first classification of ‘true’ islands,

according to their geological origin and biological

properties. Although derived long before the

development of plate tectonic theory and at a

time where scientists were only just beginning to

understand the importance of the glaciation

events, his classification, illustrated in the table

below, remains fundamental.

●Continental islands (or recent continental

islandssensuWallace) constitute emergent frag-

ments of the continental shelf, separated from the

continents by narrow, shallow waters. This separa-

tion is often recent, as a consequence of postglacial

sea-level rise (c.130 m), which has isolated the

species that were already on these islands from their

mainland conspecifics. Pronounced sea-level change

connected with glaciation has occurred repeatedly

during the Pleistocene, resulting in reduced isolation

of all continental shelf islands and, for many, joining

them to the adjacent continental land mass. In such

cases, they are termed land-bridge islands, and

their effective age as an island is some 10 000 years

or less. They will remain islands only until the next

glaciation event, perhaps in another 10 000 years or

so. Because of their origin, continental islands are

very similar in geological and biological features to

the continents.

●Continental fragmentsor micro-continents

(ancient continental islands sensuWallace): once

part of the continents, tens of millions of years ago,

tectonic drift started to separate these fragments

from the mainland, with the species they carried.

Now the waters between them and the continents

are wide and deep and the long isolation has

allowed both the persistence of some ancient line-

ages and the development of new species in situ.

These continental fragments lose their status only

when, after tens of millions of years of drift, they

finally collide with a different continent, forming a

new peninsula. This was the case for the Indian sub-

continent, which split from the Gondwanaland

supercontinent during the Early Cretaceous

(c.130 Ma) and began a lonely trip northwards,

finally colliding with Eurasia c.50 Ma, in the process

forming the Himalayan range (Stanley 1999;

Tarbuck and Lutgens 2000).

●Oceanic islands originate from submarine

volcanic activity, mostly with basaltic foundations.

Oceanic islands have never been connected to the

continents and so are populated initially by species

that have dispersed to the islands from elsewhere—

subsequently enriched by speciation. They form in