Seven Kinds of Rarity
Biodiversity is not just the number of species in an area. An area that contained twenty species that were all relatively common would be more diverse than one that contained nineteen rare species and one common species. What do we mean when we say a species is rare? Should it just be based on population size? Deborah Rabinowitz proposed that we should consider rarity along three different axes. The first axis is whether the species has a high or a low population size. The second is whether the species has a large or small geographic range. The third axis is whether the species can occur in a broad range of habitats or whether it is restricted to a more narrow range. According to Rabinowitz, a species could be considered common if, and only if, it had a high population size, large geographic range, and occurred in broad range of habitats; all other species were rare. But they could be rare in different senses. Given that there are three binary criteria, there would be two to the third power, or eight, categories with only one being common; thus, there would be seven different kinds of rarity. Rabinowitz used these criteria to classify wild flower species in Great Britain. While thirty-six percent of the species fell into the "common" category, the most prevalent category comprised species that were widely distributed and had high population sizes, but were restricted in their use of habitat. One lesson from this study is that many species that are abundant and widespread may be subject to extinction if their habitat were degraded.
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