How often have you seen those wonderful advertisements inviting you to have a holiday on a tropical island (Fig. )What is it about islands, whether in the tropics or polar regions, that suggests romance, excitement and adventure? Is it because of a sense of escape from the pressures and stress of a bustling way of life, or the opportunity to savour sun-soaked beaches, or the adventure of rocky unexplored shores, or perhaps the chance of seeing unique island wildlife? It is for all these reasons that there is a growing tourist industry for many islands around the world. The wildlife of islands, especially oceanic islands, has long been of special significance in biology, ecology, conservation and biogeography. Studies of island species have also been of historical significance for evolutionary biology. Many of the world's islands have high levels of endemic flora and fauna; that is, taxa found only on a particular island and no other place.
Island biota has often been devastated by the effects of introduc-ed species, whether that be by accident or by deliberate means. In the last few hundred years human beings have been the main carriers of introduced species to islands, sometimes with drastic consequences for the indigenous biota.
For example, information assembled by the World Conservation Monitoring Centre shows that most vertebrate extinctions have been on islands and a high proportion of endangered vertebrates occur on islands. A growing tourism industry could, without careful planning, contribute further to the continuing and growing loss of wildlife on islands.
We commence this chapter by describing what an island is and then Fe: brief introduction to the historically important island biogeographical stu-dies. Then we mention some intriguing aspects of island biology and final introduce the topic of island conservation and restoration.
Islands and island biogeography It seems curious to ask `What is an island?' The answer is straightforward DUE there are different kinds of island. An island is an area of land permanently surrounded by water. Islands occur in rivers, within lakes, in estuaries no some are oceanic. On the basis of geological history, at least three types of oceanic island have been recognised (resulting from movements of the Earth's tectonic plates).
There are oceanic ridge islands, so called 'hot spot' islands and those of island arcs. The first two result from volcanic activity but form as the volcano emerges from the ocean floor. Islands of island arcs are also volcanic but are formed where geological plates collide; the descending plane gives rise to series of volcanic activity and the visible results are islands distributed along an arc. But the question 'What is an island?' is important because island biogeog-raphy can include studies of more than just pieces of land surrounded by water. At the very least, in biogeography, it is important to state how the term Island' is used.
For some people, island biogeography includes islands as defined above and also 'mainland islands' or 'island habitats', for example the upper levels on mountains or remnants of woodland surrounded by agricul-tural land. The theories of island biogeography have often been dis-cussed along with theories about the effects of fragmentation of habitats and biological communities, hence sometimes mixing the ecology of islands and the ecology of isolated habitats (sometimes called habitat fragments or 'island habitats'). Islands are small areas of land surrounded by water whereas isolated habitats include remnants of what were once larger expanses of biological communities.
Other examples include lakes and ponds where populations of a species are isolated from each other. There are also examples of isolated alpine plant and animal communities. Mixing the biogeography of islands with the spatial ecology of mainland habitats has commonly and regrettably occurred with respect to ideas about the design of nature reserves and other kinds of designated conservation area. For reasons which we hope will become clear, our appraisal of the application of island biogeography to nature reserve design has been given over to Habitat fragmentation. In we use the term island to refer only to a piece of land surrounded by water.