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Island Biogeography and the Design of Natural Reserves
The flora and fauna of islands have played a central role in the development of ecological thought, from the early formulations of evolution and biogeography by Darwin and Wallace, through Mayr's demonstration of the role of geographic isolation in speciation, to the 5 analytical theory of island biogeography pioneered by MacArthur and Wilson. Some reasons why islands have been well suited to provoking or testing theoretical ideas are that they have definite boundaries, come in many different sizes and heights and remoteness, often have relatively simple communities of plants and animals, and serve as ready-made evolutionary laboratories offering replicate "natural experiments" in community assembly. Islands may be real islands in the ocean, or they may be virtual islands such as hilltops (where for many species the surrounding lowland presents a distributional barrier), lakes, or wooded tracts surrounded by open land. In particular, the natural reserves and wildlife refuges that are set aside from large areas bent to man's purposes may be thought of as islands in a sea of altered habitat. In view of the manifest destiny of much of the world's tropical rain forest, we may ask such questions as: How many species of Amazonian plants and animals will survive if only 1 per cent of the Amazonian rain forest can be preserved? At what rate will species be extinguished? Which species will most likely be lost? These are pressing questions, to which the theory of island biogeography holds at least some of the answers. Preston (1962) and MacArthur and Wilson (1963, 1967) have pointed out that the number of species on an island is set by a dynamic balance between immigration and extinction. For any particular island, the net extinction rate will increase as the total number of species present (S) increases, conversely, the net rate at which new species are added - the immigration rate - will decrease as S increases. This perception gives insight into the relation between an island's size and degree of isolation, and the equilibrium number of species on it. Species immigrate on to an island as a result of dispersal of colonists from continents or other islands: the more remote the island, the lower the immigration rate. Species established on an island run the risk of extinction due to fluctuation in population numbers: the smaller the island, the smaller is the population and the higher the extinction rate. Area also affects immigration and extinction in several other ways: through its relation to the magnitude of spatial and temporal variability in resources; by being correlated with the variety of available habitats; and by being correlated with the number of "hot spots", or sites of locally utilizable resource production for a particular species. All in all, the larger and less isolated the island, the higher is the species number at which it should equilibrate.
1. Skimming Read through the text very quickly and now choose the sentence which, in your opinion, best describes the subject of the passage.
(a) Islands are all threatened by destruction. (b) The Amazonian rainforest is being gradually destroyed. (c) Man must do something to protect wildlife. (d) Through the study of actual islands we can learn how to provide the best wildlife reserves. (e) Animals and plants on islands are in great danger.
2. Understanding the text: True or False? From the information contained in the text, decide whether the statements are true or false.
(a) The study of island biogeography is a recent phenomenon. (b) Islands provide good environments for evaluating theories. (c) Only real islands surrounded by water are suitable for study. (d) Nobody knows what is happening to the world's rainforests. (e) Studying islands may help to preserve some endangered species. (f) When the immigration rate rises, there is a decrease in the extinction rate in an island community. (g) The size of an island has no effect on the extinction rate of species. (h) There is a correlation between the immigration rate and the degree of isolation of an island (i) Community assembly can best be studied in a laboratory (j) MacArthur and Wilson were the first scientists to work on an analytical theory of island biogeography.
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