There are three reasons why ecologists are interested in ecological diversity and its measurement. First, despite changing fashions and preoccupations, diversity has remained a central theme in ecology. The well documented patterns of spatial and temporal variation in diversity which intrigued the early investigators of the natural world (for example Clements, 1916; Thoreau, 1860) continue to stimulate the minds of ecologists today (Currie and Paquin, 1987; May, 1986). Second, measures of diversity are frequently seen as indicators of the wellbeing of ecological systems. Thirdly, considerable debate surrounds the measurement of diversity. Diversity may appear to be a straightforward concept which can be quickly and painlessly measured. This is because most people have a ready intuitive grasp of what is meant by diversity and have little difficulty in accepting, say, that tropical rain forests are more diverse than temperate woodlands or that there is a high diversity of organisms in coral reefs. Yet diversity is rather like an optical illusion. The more it is looked at, the less clearly defined it appears to be and viewing it from different angles can lead to different perceptions of what is involved. The problem has been exacerbated by the fact that ecologists have devised a huge range of indices and models for measuring diversity. Despite, or perhaps as a result of these, diversity has a knack of eluding definition and in one instance Hurlbert (1971) even went so far as to decry it as a ‘non-concept’.
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