Another factor influencing diversity is the fact that it is usually lower on islands than on continents, even those of the same latitude. For diversity to be maintained in a particular location, there has to be the possibility of immigration from neighboring locations, because in any location, there is a finite rate of extinction. On island ecosystems, the probability of immigration of a species is low. Once a species does arrive, its probability of extinction is high, because of the difficulty for other individuals of the same species to reach the island (MacArthur and Wilson 1967).
Up to this point, we have been talking about diversity at the species level, that is, diversity of species within a community. Genetic diversity is another component of overall diversity, but determining genetic diversity is much more difficult than determining diversity at the species level. There is yet insufficient evidence to conclude that genetic diversity of trees in the tropics is higher than that of temperate-zone trees (Bawa and Krugman 1991).
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