Dark diversity

Dark diversity is the set of species that are absent from a study site but present in the surrounding region and potentially able to inhabit particular ecological conditions. It can be determined based on species distribution, dispersal potential and ecological needs.^[1] The term was introduced in 2011 by three researchers from the University of Tartu and was inspired by the idea of dark matter in physics since dark diversity too cannot be directly observed.^[2][3][4]

Overview

Common cowslip (Primula veris)

is a frequent plant species in European grasslands. If the environmental conditions of a studied grassland patch are suitable, but the cowslip is missing from it, the plant belongs to the dark diversity for that given grassland.

Dark diversity name is borrowed from dark matter: matter which cannot be seen and directly measured, but its existence and properties are inferred from its gravitational effects on visible matter. Similarly, dark diversity cannot be seen directly when only the sample is observed, but it is present if broader scale is considered, and its existence and properties can be estimated when proper data is available. With dark matter we can better understand distribution and dynamics of galaxies; with dark diversity we can understand composition and dynamics of ecological communities.

Habitat specificity and scale

Dark diversity is the counterpart of observed diversity (

Habitat-specificity is making the distinction between dark diversity and beta diversity. If beta diversity is the association between alpha and gamma diversity, dark diversity connects alpha diversity and habitat-specific (filtered) species pool. Habitat-specific species pool only these which can potentially inhabit focal study site.^[2] Observed diversity can be studied at any scale, and sites with varying heterogeneity. This is also true for dark diversity. Consequently, as local observed diversity can be linked to very different sample sizes, dark diversity can be applied at any study scale (1x1 m sample in a vegetation, bird count transect in a landscape, 50x50 km UTM grid cell).

Methods to estimate dark diversity

Region size determines likelihood of dispersal to study site and selecting appropriate scale depends on research question. For a more general study, a scale comparable to

To separate ecologically suitable species, different methods can be used.

Dark diversity allows meaningful comparisons of biodiversity. The community completeness index can be used:

${\displaystyle \log \left({\frac {\text{observed diversity}}{\text{dark diversity}}}\right)}$.^[10]

This express the local diversity at the relative scale, filtering out the effect of regional species pool. For example, if completeness of plant diversity was studied at the European scale, it did not exhibit the latitudinal pattern seen with observed richness and species pool values. Instead, high completeness was characteristic to regions with lower human impact, indicating that anthropogenic factors are among the most important local scale biodiversity determinants in Europe.^[11]

Dark diversity studies can be combined with functional ecology to understand why species pool is poorly realized in a locality. For example, if functional traits were compared between grassland species in observed diversity and dark diversity, it becomes evident, that dark diversity species have in general poorer dispersal abilities.^[12]

Dark diversity can be useful in prioritizing nature conservation,^[13] to identify in different regions most complete sites. Dark diversity of alien species, weeds and pathogens can be useful to prepare for future invasions in time.

Recently, dark diversity concept was used in to explain mechanisms behind plant diversity-productivity relationship.^[14]

See also

• Measurement of biodiversity
• Species diversity
• Species pool

References