Application of a taxon priority system for conservation planning by selecting areas which are most distinct from environments already reserved

Measures of phylogenetic diversity have been used to establish conservation priorities amongst groups of taxa. We adapt one such measure ('Phylogenetic Diversity' or 'PD') to a hierarchical environmental classification and use its algorithm to design a conservation reserve system (using the Northern Territory of Australia as a case study). This approach accords priority selection to sites which are most dissimilar to the environments already secured in the existing conservation network, and hence rapidly increased equitability in reservation percentage at higher levels of the hierarchy (broad environmental groups). Variation within these broad classes (i.e. environmental classes defined at a lower level of the hierarchy) is included in selections, but this inclusion tends to occur later in the selection process. This approach results in an eventual complete solution (to the goal of 5% reservation of all recognised environmental classes), which is 18% more expensive than a minimum-set solution which treats all environmental classes as equally distinct, but its advantage of rapid securement of main-environments would be compelling where only a limited number (less than the minimum set) of additions to the existing conservation network is achievable in the forseeable future. The approach is further contrasted with a prioritisation mechanism ('irreplaceability') associated with minimum-set approaches. Irreplaceability is largely a measure of limited distribution, and we argue that this may not be an important ranking criterion if the limited environment is defined at a low classification level and closely related environments are already well-represented in the existing conservation network. Copyright (C) 1996 Elsevier Science Limited.