It is possible to specify species that belong to a particular species pool but that are not locally present. We call this unaccounted set of species ‘dark diversity’.
So it is a set of species that we, for whatever reason, expect to be at a locality, but are not observed there.
I raised a concern that, for the sake of completeness, we should also consider light diversity , which would be the set of species that are not in the species pool, but we observe them anyway.
- Observations of yet undescribed species.
- Observations of vagrant and migrant species (species that do not reproduce at the locality).
- Aliens (thanks Marten).
- Species from taxonomic groups that are not part of our research, such as a cow in the middle of a botanical plot.
Depending on the context, light diversity could be also called inconvenient diversity, nuisance diversity, Jedi diversity (thanks Dylan), unexpected diversity, surprising diversity, undesirable diversity, avoided diversity, or denied diversity.
In order to make sense, dark diversity requires species pool to be philosophically realistic, i. e. we need to believe in the existence of a real species pool out there in the real world. In contrast, light diversity requires species pool to be nominalistic or conceptual, i.e. it is the species pool that we have in mind, or that we agree on in a broader community.
The idea of light diversity was received with a lukewarm amusement, and I am not sure how useful it could be, but then I am also unsure why we should have dark diversity. Perhaps I should just get back to work, to the usual and dull grey diversity.
Pärtel M., Szava-Kovats R. & Zobel M. (2011) Dark diversity: shedding light on absent species. Trends in Ecology and Evolution , 26 : 124-128.