Measuring biodiversity: The importance of species similarity

Background/Question/Methods

Biodiversity is associated to properties of ecosystem function and productivity or at the molecular level it can describe genetic diversity with connections to quantities such  as host-pathogen fitness. Diversity is clearly an important concept and it begs the question how should one measure diversity. There are literally dozens of measures of diversity in the literature, perhaps one of the most common examples being species richness or simply the number of species in the community concerned. Even such a simple definition raises questions, what defines a species? Sometimes this question is simple to answer, but for microbial communities, for example,  there is no simple definition of a species and yet we still wish to quantify diversity.

To address this issue we present a natural family of diversity measures which not only takes into account relative abundance of species, but also accounts for differences or similarities between species. This latter point allows us to deal with communities where the notion of species is unclear.

Results/Conclusions

We demonstrate that our new measure of diversity is not simply an addition to the already long list of indices: instead, a single formula subsumes many of the most popular indices, including Shannon’s, Simpson’s, species richness, and Rao’s quadratic entropy. These popular indices can then be used and understood in a unified way, and the relationships between them are made plain. The new measures are, moreover, effective numbers, so that percentage changes and ratio comparisons of diversity value are meaningful.

We illustrate these measures with the use of  diversity profiles, a simple graphical tool which provides information about the shape of a community and how it's diversity changes according to the importance that is placed on rare or common species. The comparison is simple, but reveals important insights into a communities diversity while also being transparent about how diversity is defined, so rather than simply settling for one measure of diversity many can be considered and thus more information about the comity can be revealed.