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We are members of the Andrew Gonzalez lab , in the Biology Department at McGill University.
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Saturday, February 5, 2011

The Dark side of diversity

Recently Pärtel et al. (2010) introduced a simple but intriguing idea that the fraction of 'missing' species from a site provides insights that can be used to understand biodiversity loss.

In cosmology dark matter is thought to make up the dominant fraction of the matter in the known universe. Ordinary matter, the stuff we see, makes up a small fraction of total matter. Can the same be said of species diversity at a given site? The authors of this paper define dark diversity as the fraction of species we do not see at a site but are potentially capable of inhabiting it within the regional pool. Ecologists have long known that local diversity is typically a small fraction of the total diversity in the regional pool, but few have focused on quantifying the variation in dark diversity from site to site. A global analysis presented for plant diversity reveals interesting spatial patterns in dark diversity. In particular, dark diversity is relatively lower in temperate regions where 'ordinary' diversity represents a greater ratio of the total potential pool than in many parts of the tropics. 

The value of the concept can be seen when we ask whether dark diversity in a locality is increasing rapidly over time, in particular, because of local extinction due to human disturbance. As long as dark diversity is high the potential for ecosystem restoration remains. However, extensive and persistent extinction would also erode dark diversity, and eventually reduce the options for restoration; a worrying possibility for many areas of the world. 

The authors offer some advice about how to decide whether a species can potentially inhabit a site but this is a nontrivial decision. For example, many species may persist as sink populations. By definition these species could not exist in a community in the absence of immigration. The only way to measure this component of local diversity is to isolate a community. Should sink species ('shadow' diversity?) be excluded from consideration? These and other questions arise when thinking about dark diversity, but this does not detract from the value of the concept. This notion is ripe for theoretical analysis.


Pärtel, M., Szava-Kovats R., Zobel M. (2010) Dark diversity: shedding light on absent species. TREE online early.

1 comment:

  1. I've always been a little skeptical of analyses of "community saturation", or other measures of the proportion of species in a regional pool present, or in this case absent, at a local site. I see 2 problems:

    1. If a species is not observed at a site ("dark diversity"), it doesn't necessarily mean it is not present, only that it was not detected. I have more confidence in observed species than unobserved ones, unless you have a good idea of how representative your sampling is.

    2. From a conservation perspective, I don't see this as a useful metric on its own, because there are two ways to increase the proportion of regional species present at a local site: you can increase local diversity, or *decrease* regional diversity. A region composed of many homogenous sites of a few species may appear "saturated", yet if this situation arises following many extinctions at the regional scale, there may in fact be many opportunities for invasion (or speciation).

    It's certainly intriguing to look at absence, rather than presence, but I'll have to see how the authors address these issues.