Evaluating the performance of rarity as a surrogate in site prioritization for biodiversity conservation

To solve the minimum-set coverage problem in conservation planning (representing the greatest number of species in the least number of sites), several metrics can be used to prioritize sites based on their conservation importance. Traditionally, species richness has been the most widely used approach, but previous studies suggest that it is one of the least effective. Alternative metrics such as complementarity algorithms have produced better results, for which they have been used in recent conservation studies. In this study, we assessed the performance of a new potential surrogate: rarity indices. Specifically, we tested rarity-weighted richness (RWR), index of summed rarity (ISR)and index of relative rarity (IRR), to determine if they are effective surrogates of biodiversity when solving the minimum-set coverage problem. We tested the rarity indices in 14 datasets spanning varying extents and grains, and found that rarity consistently outperformed species richness, even when accounting for the differences in rarity index performance. In some cases, rarity outperformed the complementarity solution, suggesting that it is a promising alternative surrogate that can be easily tested in situations where conservation action resources are limited. Rarity indices have been seldom assessed for their surrogacy effectiveness, and to the best of our knowledge, we are the first to test IRR and ISR in this context. Our results justify the need for future studies to elucidate the importance and applicability of rarity indices in conservation planning.