Introduction

Why the number of species varies from hot spots of species 1 r

. . place to place, and from time to time, richness r r are questions that present themselves not only to ecologists but to anybody who observes and ponders the natural world. They are interesting questions in their own right - but they are also questions of practical importance. A remarkable 44% of the world's plant species and 35% of vertebrate species (other than fish) are endemic to just 25 separate 'hot spots' occupying a small proportion of the earth's surface (Myers et ah, 2000). Knowledge of the spatial distribution of species richness is a prerequisite for prioritizing conservation efforts both at a large scale (setting global priorities) and at a regional and local scale (setting national priorities). This aspect of conservation planning will be discussed in Section 22.4.

It is important to distinguish be-biodiversity and tween species richness (the number of species rich ness species present in a defined geographical unit - see Section 16.2) and biodiversity. The term biodiversity makes frequent appearances in both the popular media and the scientific literature - but it often does so without an unambiguous definition. At its simplest, biodiversity is synonymous with species richness. Biodiversity, though, can also be viewed at scales smaller and larger than the species. For example, we may include genetic diversity within species, recognizing the value of conserving genetically distinct sub-populations and subspecies. Above the species level, we may wish to ensure that species without close relatives are afforded special protection, so that the overall evolutionary variety of the world's biota is maintained as large as possible. At a larger scale still, we may include in biodiversity the variety of community types present in a region - swamps, deserts, early and late stages in a woodland succession, and so on. Thus, "biodiversity' may itself,

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