descriptionTổng quan
When a naturalist says that a bird or a plant is rare, he or she may mean one of several different things (Harper 1981, Rabinowitz 1981). The concept of rarity can refer to one of three characteristics: geographic range, habitat specificity, or local population size, and a classification based on the interplay between these variables yields seven different types of rarity. For example, certain species may be locally abundant over a wide geographic range, but found in only a very specific habitat, whilst others may be found in several habitats, but only in small numbers and in geographically restricted area. We must therefore recognise that the kinds of management which will be appropriate for protecting species threatened with extinction will vary.
Classic rare species are often those which have a small geographic range and narrow habitat specificity. Many plants of this type are restricted endemics, and are often endangered or threatened (Rabinowitz 1981). Other rare species have very large geographic ranges and occur widely in different habitats but are always at low density. These species are ecologically interesting but almost never appear on lists of endangered species. So the important point is that not all rare species are problems for conservationists.
The reasons why a particular species is rare vary. In some cases we can observe a species declining over time: for example, the African elephant population since 1950. This is a direct result of ivory poaching (Caughley et al. 1990). But not all species that have declined to rarity are so well understood. Some plant and animal species undergo bursts of colonisation and decay so that they persist as a mosaic of increasing and declining populations (Harper 1981).
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