Prioritizing species for conservation planning

The efforts to protect biological diversity must be prioritized because resources for nature conservation are limited. Conservation prioritization can be based on numerous criteria, from ecological integrity to species representation, but in this review I address only species-level prioritization. Criteria used for species prioritization range from aesthetical to evolutionary considerations, but I focus on the aspects that are biologically relevant. I distinguish between two main aspects of diversity that are used as objectives: Maintenance of biodiversity pattern, and maintenance of biodiversity process. I identify two additional criteria typically used in species prioritization that serve for achieving the objectives: The species' need of protection, and cost and effectiveness of conservation actions. I discuss how these criteria could be combined with either of the objectives in a complementarity-based benefit function framework for conservation prioritization. But preserving evolutionary process versus current diversity pattern may turn out to be conflicting objectives that have to be traded-off with each other, if pursued simultaneously. Although many reasonable criteria and methods exist, species prioritization is hampered by uncertainties, most of which stem from the poor quality of data on what species exist, where they occur, and what are the costs and benefits of protecting them. Surrogate measures would be extremely useful but their performance is still largely unknown. Future challenges in species prioritization lie in finding ways to compensate for missing information.