Senior Associate, World Resources Institute, Washington, D.C.
Identifying the elements of biological diversity and monitoring their changes through time is a daunting task. Biologists have long recognized that the full array of biological diversity will never be known completely—that is, not all species and ecosystems will be identified, named, cataloged, and studied in any detail before many of them are lost. For example, it is likely that there are many more than 10 million species living today. Only 1.4 million of these have been described and named, and a tiny fraction of them have been studied thoroughly for potential use by humans. Ecosystems also vary greatly in size, composition, complexity, and distribution, and it is not uncommon for ecologists to differ in describing and defining them. For example, despite many studies by vegetation ecologists and biogeographers in the United States, today there is no single, agreed-upon vegetation classification that can be used by the federal land-management agencies or by the many state and private organizations that could productively use a national classification scheme.
All this makes the work of systematically conserving species and ecosystems more difficult. It presents a real problem when we try to determine how well various ecosystems are protected or represented in the global, national, and state systems of protected areas (Harrison et al., 1984).
To tackle this problem, conservation biologists for years have intentionally or unwittingly used the process called gap analysis to establish short-term and longer-term conservation priorities. The concept is deceptively simple, if not simplistic: