How to Define ‘Biodiversity’

trail under treesSomeone asked UC Davis Professor Arthur Shapiro for a definition of Biodiversity. After all, it’s apparently a key concept in some of the policy-making in San Francisco open spaces.

“A buzzword,” he responded. “Biodiversity means whatever you want it to mean.”

He amplified this definition in an article that’s been published on, HERE.


In brief: Biodiversity starts with a simple count of all the species in a defined area. That’s also called “species richness.”

But just the number of species isn’t enough. Ecologists want to tell the difference between a community that has, say 95 blackbirds and 5 sparrows, and one that has 50 blackbirds and 50 sparrows. This is called “equitability.”

Eventually, a “diversity index” was developed that could measure both the number of species, and the frequency with which they occurred. This tool was then generalized for use in many ways, so diversity indexes could be calculated for almost everything.


The Endangered Species Act was passed to protect animals and plants that were at risk of extinction. But in addition to protecting endangered species, it allowed for the protection of subspecies as well as distinct population segments. Neither of these things was actually defined in biological terms, leaving them open to interpretation.

Genetic analysis seemed to offer a solution. Theoretically, it should be possible to make a genetic analysis of any living thing, and decide whether it’s a species or subspecies or distinct population segment based on its DNA. But instead, it muddied the waters still more, because some plants or animals that looked very alike turned out to be unrelated; while others that looked very different were genetically very similar.

So it’s still a rather confused area (as was evident from the great Manzanita kerfuffle). No one knows how to define biodiversity at the genetic level.


Professor Shapiro gave some further measures of biodiversity:

  • Alpha diversity, which measure species richness at a local level. In San Francisco, if you counted up all the species of birds and animals and insects and plants, that would give you alpha diversity.
  • Beta diversity measures the difference in diversity between various areas in a region – how many different little ecosystems you have within the region. In San Francisco, for instance, you have many different habitats – dense forests beside open scrub land beside well-watered backyard gardens, and potentially different plants and animal life in each. In the San Francisco Bay Area, beta diversity is high for most species.
  • Gamma diversity is diversity over large areas.

Finally, then, how do you define biodiversity? Here’s what Professor Shapiro says:

  • It’s species richness plus
  • the distribution of abundance and rarity, plus
  • the geography of all that, plus
  • the amount of genetic variation in selected species of interest, plus
  • whatever you please.

nectaring on ivyHe adds, “Somehow, concepts concepts of “quality” have gotten mixed in, too.” He gives the example of a redwood forest – which actually has low biodiversity – being clear-cut. Left alone, the clear-cut area would fill up with a lot of other plants, many of which would be considered “invasive weeds.” That replacement vegetation would be more diverse – but it would be made up of the “wrong species, whatever that means.”

Because, he points out “Biodiversity is only a buzzword.”



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4 Responses to How to Define ‘Biodiversity’

  1. takebackthegreen says:

    Thank you, SMSF! It is refreshing to hear someone inject common sense into an often laughably ill-informed discussion. It will be interesting to hear the ways the “Natural Areas” church tries to discredit a scientist who studies the issue for a living. He must be employed by Big Something, right?

  2. takebackthegreen says:

    Big Eucalyptus? Is that a thing? Thanks again for the information.

  3. Filip says:

    @takebackthegreen Just like this site and its commenters keep suggesting that UCSF and Sutro Stewards are acting under direction of Big Herbicide?

  4. Pingback: Sutro Forest: Risk and Uncertainty | Save Mount Sutro Forest

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