Shades of irreplaceability: towards a measure of the contribution of sites to a reservation goal

In most regions there are many possible ways of combining numbers of sites into reserve systems that represent a range of natural feasures. The irreplaceability of a site is operationally defined as the percentage of these alternative systems in which it occurs. This is a fundamental measure of the conservation value of the site in terms of its potential contribution to the achievement of a reservation goal or, alternatively, the options for reservation that are lost if the site is lost. The measure allows decisions to be made on the future of individual sites in the context of their value, in combination with other sites, to the conservation of the full range of natural features in a region. It also provides a logical framework for the design of whole systems of reserves, with decisions proceeding from the most to the least irreplaceable. Irreplaceability can be measured directly for small data sets but must be predicted for regional data sets. A promising approach to prediction is discussed that requires validation with more extensive trials. The irreplaceability of a site depends on a specific reservation target and changes as some of the site's features become progressively represented in reserves elsewhere. The concept of irreplaceability undermines notions of conservation value that are static or based on a single static system of sites to achieve a reservation goal.