SHADES OF IRREPLACEABILITY - TOWARDS A MEASURE OF THE CONTRIBUTION OFSITES TO A RESERVATION GOAL

In most regions there are many possible ways of combining numbers of s ites into reserve systems that represent a range of natural features. The 'irreplaceability' of a site is operationally defined as the perce ntage of these alternative systems in which it occurs. This is a funda mental measure of the conservation value of the site in terms of its p otential contribution to the achievement of a reservation goal or, alt ernatively, the options for reservation that are lost if the site is l ost. The measure allows decisions to be made on the future of individu al sites in the context of their value, in combination with other site s, to the conservation of the full range of natural features in a regi on. It also provides a logical framework for the design of whole syste ms of reserves, with decisions proceeding from the most to the least i rreplaceable. Irreplaceability can be measured directly for small data sets but must be predicted for regional data sets. A promising approa ch to prediction is discussed that requires validation with more exten sive trials. The irreplaceability of a site depends on a specific rese rvation target and changes as some of the site's features become progr essively represented in reserves elsewhere. The concept of irreplaceab itity undermines notions of conservation value that are static or base d on a single static system of sites to achieve a reservation goal.