What is “dark diversity” and how can we use it to guide conservation and restoration?

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References: Lewis, R.J., Bello, F., Bennett, J.A., Fibich, P., Finerty, G.E., Götzenberger, L., Hiiesalu, I., Kasari, L., Lepš, J., Májeková, M., Mudrák, O., Riibak, K., Ronk, A., Rychtecká, T., Vitová, A. and Pärtel, M. (2017), Applying the dark diversity concept to nature conservation. Conservation Biology, 31: 40-47. DOI:10.1111/cobi.12723

Pärtel, M., Szava-Kovats, R., & Zobel, M. (2011). Dark diversity: Shedding light on absent species. Trends in Ecology & Evolution, 26(3), 124-128. DOI: https://doi.org/10.1016/j.tree.2010.12.004

The core of ecology is devoted to studying the interactions among species and their environment. But why are some species present and others absent in an environment? Think of a region of forest that has been converted to an agricultural field. The species that were thriving in the forest now have become absent because they are not tolerant of the new environmental conditions imposed upon them in the agricultural field.

Only a subset of all species in a region can tolerate the ecological conditions of a given site (the site-specific species pool). Of those, not all are realized in the local species pool. These absent species form what is called the dark diversity of a community. Authors Lewis et al. (2017) believe that the dark diversity concept can be used to complement and further develop conservation prioritization and management decisions.

What is dark diversity?

The term “dark diversity” was specifically chosen because it is inspired by dark matter. Dark diversity, like dark matter, cannot be observed or measured directly. The concept was developed by three researchers from the University of Tartu and states the definition of dark diversity as species in the region that can potentially inhabit particular ecological conditions of a local community but are absent from that community (Pärtel Szava-Kovats, & Zobel, 2011).

The concept of local and dark diversity is like other concepts such as alpha, beta, and gamma diversity (see Fig. 1). To refresh your memory or introduce you to these concepts let me define them. Alpha diversity is the local observed diversity in a habitat, gamma diversity is the observed diversity in a region, and beta diversity, often called species turnover, is the diversity among regions. Essentially, beta diversity tells us about the number of communities in a region.

Figure 1: Dark diversity is part of the species pool, which is different but related to other ecological concepts such as alpha-beta-gamma diversity. Figure from Lewis et al. 2017.

How can it be applied?

The concept of dark diversity is promising for prioritizing management efforts because finding large dark diversity is a sign of reduced local biodiversity as well as the potential for restoring a damaged ecosystem. Let me highlight three examples Lewis et al. (2017) give of how dark diversity can be used as a conservation management tool, and at the same time, convince you that absent species can offer us valuable information as observable species.

Dark diversity and the “completeness index”

One example the authors offer is using the concept to identify conservation priority to a habitat by formulating a “completeness index” of a habitat or region relative to its species pool. More “complete” habitats or regions would have more observable species than absent species. Think of a ratio of observable to dark (e.g., 200:5 would have 200 observed diversity and 5 dark diversity).

High completeness (i.e., high observed diversity and lower dark diversity) indicates than an area would have a high conservation priority. Areas with higher completeness indices are particularly vulnerable regions which, if lost or degraded, would significantly contribute to biodiversity loss. An area that could have high completeness is a remnant forest that has been unmanaged, species-rich tropical habitats, or isolated island habitats where there are many endemic species (only found in this area).

Not only can dark diversity be used to derive a “completeness index” but it can be applied at any scale (e.g., within one tree, within a park, within a mountain range), and to compare communities of different trophic levels such as plants, insects, and birds.

Dark diversity and restoration

The authors propose another example focusing on restoration ecology, suggesting that knowledge of a region’s dark diversity can tell us how potentially successful the restoration will be. For example, you have a habitat where many species are hidden in the dark diversity species pool (i.e., the species exists in the wider region and has the probability to disperse and establish in the local species pool), then the chances of successful restoration in this site should be good. In contrast, if you have a region where there is little dark diversity then restoration success will likely not be needed, or unsuccessful in the short term. If dark diversity in an area remains high, there is a possibility for the missing species to return. The concept states that the dark diversity species may have been present in an ecosystem and may return if favorable conditions are met.

Dark diversity and invasion ecology

The last example the authors offer us is using information gathered about dark diversity to combat invasions of unwanted species such as invasive species. The same way dark diversity provides information that helps prevent absences of species we want to conserve, it can also highlight the prominence of an absent species.

Understanding the process of invasion is critical to conservation efforts. Species richness has been used in the past to predict invasion resistance, but hypotheses today are dismissed (Moore et al., 2001) because many times it is poorly supported by field experiments in natural communities. Rather, a better predictor is habitat-specific species pools that reflect species ecological preferences (Vellend, 2010). Using this as a measure by “completeness ratios” may offer a suitable measure of community saturation, broadly defined as the possibility of more species existing in habitat than are present at a particular point in time because resources are not used by the existing species, and this can be a proxy for invisibility.

Just as the above-mentioned completeness index can be used as a tool for establishing restoration priorities, the completeness ratio can be used as a tool to compare biotic resistance among different habitat types or regions, which helps us better understand the relationship between species diversity and invasion.

Summary

There are many reasons to expect an understanding of dark diversity to contribute towards understanding patterns of species diversity. In the same way patterns of observable species provide important importation for conservation biology so too can patterns of absent species (dark diversity). Current examples provided by Lewis et al. (2017) are already promising tools for conservation management, but I am excited to see where future research and development takes this concept into the future.

References:

Moore JL, Mouquet N, Lawton J H, Loreau M. (2001). Coexistence, saturation and invasion resistance in simulated plant assemblages. Oikos 94:303–314.

Vellend M. (2010). Conceptual synthesis in community ecology. The Quarterly Review of Biology 85:183–206

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