Why is this important? The identification and protection of rare species remains central to the task of conservation. However, conservationists now know that there are several quite distinctive ways in which species can be rare. In addition not all forms of rarity are equally significant for regional conservation planning. Translating the different meanings of rarity into appropriate policy directives at a local scale requires research into both the patterns of rarity and the mechanisms that distinguish rare species from common ones.
Rarity is an important correlate of extinction vulnerability (one of a number of such correlates, it should be noted) (Davies et al. 2000); that is, rare species are more likely to go extinct than common ones. Rare species are therefore afforded special attention in conservation circles (Lawler et al. 2003). The familiar policy terms endangered, threatened, vulnerable species, rely heavily on rarity for designating extinction risk. It is, however, an ecological truism that rarity is common. This important concept alludes to the fact that a relatively few species may be abundant in an assemblage, but the majority of species are rare, often being represented by as few as a single specimen in ecological surveys (Preston 1948, Magurran and Henderson 2003). For example oak and hickory species typically dominate in an Oak Woodland, whereas most other species in the will occur in relatively low numbers. Since most species are not commonly encountered, it is clearly useful to distinguish species that are inherently rare (those species whose relative rarity is related to, for example, its life history strategy and may therefore have a stable, non threatened population) from those that are both rare and vulnerable to immanent extinction (Maina and Howe 2000). For instance, a species that is has a small population but which has a low variability in their populations numbers, rarely fluctuating from year to year, may have a reduced risk of extinction (Pimm et al. 1988). The determination that a rare species has a conservation priority status should be based on objective criteria (Ledant 1991, Mace and Lande 1991). Delineating the type of rarity being addressed in a given conservation strategy can be a crucial criterion. Both research and action that promotes rare species conservation must carefully also consider the issue of the geographical scale on which rarity is assessed (Fagan et al. 2005). A species with scarce populations in one area may be geographically widespread and may therefore not be regarded as having as high a priority as a species with small population in a very restricted geographical range. With this is mind, it is important that conservation reserve selection and design procedures that employ rarity as a decision criterion are implemented with a sensitivity not only to local needs but also takes the regional context into consideration (with a view to maximizing regional diversity) (Rodrigues and Gaston 2002). Such sensitivity to the multiple scales contributing to rarity is embedded in recent attempts to classifying different types of rarity.
