Why should we reconsider using species richness in spatial conservation prioritization?

Species richness has been largely used for determining site conservation values, but this has not been assessed in the context of marine mammal conservation. In our study, we assessed the effectiveness of species richness as a surrogate of marine mammal species representation at the global scale and compared it to the use of complementarity as an alternative approach. We obtained 134 marine mammal distribution maps from the IUCN Red List database and used two complementarity algorithms to calculate conservation priorities. To determine the effectiveness of species richness as a surrogate, we calculated the Species Accumulation Index (SAI) scores for marine mammal groups. Our findings indicate that both complementarity approaches are consistently more effective surrogates of marine mammal species representation across all groups and grains tested. Our study strongly supports the use of complementarity as supposed to richness as the preferred method to select sites conservation values for marine mammals. To the best of our knowledge, we are the first to assess the effectiveness of richness and compare it to results obtained by the use of complementarity in global marine mammal species spatial representation.