3 Species Rarity : Definition , Causes , and Classification

In virtually all ecological communities around the world, most species are represented by few individuals, and most individuals come from only a few of the most common species. Why this distribution of species abundances is so regularly observed among different taxonomic sets in geographically diverse systems is a question that has received considerable theoretical and empirical investigation (Preston 1948, 1962; Harte et aL 1999; Hubbell.2001). Understanding the mechanisms leading to the pattern of few common and many rare species extends beyond basic interest in how natural communities are assembled. It is also of great practical importance to conservation science since human uses of ecosystems can greatly affect the pattern of commonness and rarity in the biota inhabiting those same ecosystems (Lubchenco et aL 1991). Because budgets for biodiversity conservation are limited, a common strategy for allocating scarce conservation resources has been to focus on species that are thought to have the highest extinction risk (Sisk et aL 1994; Flather et al. 1998). What is the ecological justification for a conservation paradigm focused on rare species? How does one go about distinguishing rare from nonrare species? And what factors contribute to species rarity? We address these questions so as to provide a foundation for why resource managers need to be concerned about rare species and how they can identify them. We also discuss the implications that various causes of rarity have on the choice, and likely success, of management actions to protect and enhance populations of rare species.