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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. 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.
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 the distribution and dynamics of galaxies; with dark diversity we can understand the composition and dynamics of ecological communities.
Habitat-specificity is making the distinction between dark diversity and beta diversity. If beta diversity is the association between Alpha diversity 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. 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 vegetation, bird count transect in a landscape, 50x50 km UTM grid cell).
To separate ecologically suitable species, different methods can be used. Environmental niche modelling can be applied to a large number of species. Expert opinion can be used. Data on species' habitat preferences is available in books, e.g. bird nesting habitats. This can also be quantitative, for example plant species , according to Ellenberg. A recently developed method estimates dark diversity from species co-occurrence matrices. An online tool is available for the co-occurrence method.
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.
Dark diversity can be useful in prioritizing nature conservation, 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, the dark diversity concept was used to explain mechanisms behind the plant diversity-productivity relationship.
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