Dark diversity in the dark: a new approach to subterranean conservation

When trying to predict biodiversity patterns, species absences in a community can be as informative as species presences. The concept of dark diversity considers geographical and ecological filters to set an expected species pool and to compare it with the observed species pool, through an index known as community completeness. Completeness shows no relationship with latitude, allowing the comparison of different communities and regions concerning community saturation. Here we propose the use of these methods to a better understanding of subterranean biodiversity patterns. We applied patterns of co-occurrence among phylogenetically related species to set the theoretical species pool and then compared it with the observed richness, using isopods as model taxon. Except for one cave, dark diversity was equal or higher than observed richness. Even though completeness was low in most cases, those caves with higher completeness represent a valuable sample of regional subterranean species pool and may act as a repository of diversity. Our study showed that the dark diversity approach is adaptable to studies of subterranean communities and may be coupled with other conservation tools towards more effective management decisions.