Accounting for non-independence of extinction probabilities in the derivation of conservation priorities based on Weitzman’s diversity concept

In the original formulation of Weitzman’s diversity concept, extinction probabilities of different operational taxonomic units (OTUs) are implicitly defined to be independent. In this study, it is shown, that dependencies like concurrence or synergistic relations between OTUs can be accounted for by assuming that joint extinction probabilities differ from the product of the extinction probabilities of the interacting OTUs. A fully analytical treatment is provided for the case of two interacting OTUs, and all cases are illustrated with an example data set. For the case of concurring OTUs, the following general results are derived: (a) concurrence between OTUs always reduces the amount of diversity expected to be conserved in the future; (b) concurrence has a more adverse effect on the more endangered OTUs involved; (c) concurrence can change conservation priorities between OTUs; and (d) with high levels of concurrence, investments in the conservation of some of the concurring OTUs can have a negative effect on the overall diversity conserved. In addition to conservation activities targeted towards some of the OTUs, reduction of the causes of concurrence may be a valid and cost-efficient alternative. For the case of synergistic dependencies between OTUs it was found, that such dependencies always increase the conservation potential of the involved OTUs but may lead to a change of priority ranking in extreme cases. Finally possible extensions and generalisations of the suggested approach are discussed and it is argued, that by adopting these extensions Weitzman’s diversity concept becomes an even more powerful and flexible tool to derive conservation priorities both in an ecological and in a farm animal context.