What is Biodiversity?

There’s been a lot of talk of ‘biodiversity’ in San Francisco recently. The city’s ‘Recreation and Open Space Element’ (ROSE) mentions it without clearly defining it. The Natural Areas Program claims to preserve it. There’s a new position, the Director of Biodiversity Coordinator (currently Peter Brastow, formerly of Nature in the City) within San Francisco’s Department of the Environment.

One of our readers, puzzled by all the discussion, asked a simple question of UC Davis Professor Arthur Shapiro, who gave a talk at the Commonwealth Club a few days ago. Instead of the two-line answer they expected,  he sent this detailed response — which he kindly permitted us to publish.

Mt Davidson woodland path


A buzzword.
Biodiversity means whatever you want it to mean. I hate the word. Here’s why.

The following is from the introductory biology textbook we use at U.C. Davis, Life: The Science of Biology, (10th edition, Sadava et al., p.1229 — yes, I said p. 1229!):

“…the term BIODIVERSITY, a contraction of ‘biological diversity,’ has multiple definitions. We may speak of biodiversity as the degree of genetic variation within a species….Biodiversity can also be defined in terms of species richness in a particular community.  At a larger scale, biodiversity also embraces ecosystem diversity—particularly the complex interactions within and between ecosystems….One conspicuous manifestation of biodiversity loss is species extinction…”

Got that?

The glossary at the back of the book defines “biodiversity hot spot” (itself ambiguous, conflating numbers of species and degree of endemism), but NOT biodiversity itself. One can see why.

Where did this verbal monstrosity come from?

Anise swallowtail butterfly breeds on fennelThe raw number of species in a defined area or system – what many of us call “species richness”—is a useful number. There are more species of butterflies in Brazil than in California, and more in California than in Alaska. That is true even if we pro-rate species number by area, and it is not trivial to ask why.

But there is more to biodiversity than mere numbers of species. Ecologists are also interested in how individuals are divided among species, that is, the distribution of commonness and rarity among species. You can have a “community” consisting of exactly two species.  It could have, say, 50 individuals of each species, or it could have 99 of one and 1 of the other—or any ratio in between. Does this matter? Why? What can those numbers tell us?


A century ago a Danish plant ecologist named Christen Raunkiaer observed that there was a statistical regularity to this; he called it the “law of frequency.” In subsequent years it was found to hold for bird censuses and moths collected at lights, as well as for old-field plants.  A whole series of mathematical models developed over the years attempted to account for this regularity by means of assumptions about how species interacted—competing for resources, for example. These exercises were at the core of community ecology for several decades, and were seen as immensely important.

During World War II an applied mathematician named Claude Shannon, working on war-related communications problems at Bell Labs, developed a formula that concisely expressed the information content of a message. Ecologists discovered the Shannon formula in the 1960s and realized it could easily be adapted to give a single number that combined the number of species in a community and their relative abundances.

Thus whole communities could be compared efficiently, a potentially informative and useful tactic in trying to understand how multispecies systems worked. The number generated by the Shannon formula came to be called diversity, and the formula became the first and most widely-used of several diversity indices. I learned it in high school and I still use it in teaching. Diversity had two components, then:

  • Species richness and
  • “Equitability,” (the difference between a 50:50 and a 99:1 community).

And we were off and running. Now everything could be quantified with a diversity index: “foliage height diversity” in a forest canopy, or “aspect diversity” in moth faunas (how many wing shape-pattern themes could be recognized?). The number of uses and abuses of the term multiplied like rabbits. By 1971 things had gotten so bad that a paper was published caustically titled “The nonconcept of species diversity.” It was widely applauded for its candor. Unfortunately, the author ended up inventing his own new measure of diversity—one he thought was better than the old ones.


But things could get worse. And they did. With the passage of the Endangered Species Act, which opened the door to protection of endangered subspecies (keep in mind that there is no concept of the subspecies; a subspecies is whatever some taxonomist says it is) and even “distinct population segments” (no one knows what that means), genetics got in on the diversity game. Now we would not be content with diversity at the species level; we needed to get inside species.

In the scramble to define what might be protectable, a search was launched for “evolutionarily significant units.” With modern molecular-genetic tools, we quickly learned that taxonomic subspecies may be genomically nearly identical, while organisms indistinguishable by the naked eye may be wildly different. Defining diversity at the genetic level is still, well, challenging.

One very useful dimension of biodiversity is known as alpha, beta and gamma diversity:

  • Alpha diversity is species richness at the local level.
  • Beta diversity is a measure of how much the biota of different localities within a region differ among themselves—that is, how quickly species composition “turns over” in space [i.e. when you have many different little ecosystems next to each other].
  • Gamma diversity is at a large spatial scale.

The Bay Area has phenomenally high beta diversity in almost everything.


So what is biodiversity? 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.

Somehow or other concepts of “quality” have gotten mixed in, too. When you clear-cut a redwood forest (which has very low species richness), the early-successional communities that develop on the site, which may be dominated by “invasive weeds,” will have both much higher species numbers and a richer distribution of species abundances than the forest they replaced. But early-successional communities don’t get any respect despite being more diverse and despite the supposition that biodiversity is good. Because they’re made up of the ‘wrong’ species—whatever that means.

Because biodiversity, after all, is only a buzzword.

tony holiday glen canyon 7881491_orig



5 Responses to What is Biodiversity?

  1. milliontrees says:

    Thank you for publishing Professor Shapiro’s lucid explanation of the slippery concept of “biodiversity.” Clearly this word is used differently by native plant advocates who are under the mistaken impression that it refers exclusively to native plants. They also seem to confer “bonus points” if the native species is rare.

    As Professor Shapiro explains, science considers ALL species, regardless of their origins, when quantifying biodiversity. Therefore, San Francisco and its suburban neighbors are considerably more biodiverse now than they were prior to the arrival of Europeans. And if native plant advocates succeed in their crusade to eradicate non-native species, San Francisco will once again be less biodiverse.

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  3. Dave says:

    We never hear what will be gained in biodiversity by removing certain plants and species because they are nonnative. For example, the Feathered Friends of Glen Canyon say that there are more than 100 species recorded in the canyon. Certainly some of the those are nonnative or rely on nonnative habitat. Despite the reasonable explanation by Dr. Shapiro that quantifying biodiversity is not a meaningful measure I would be interested to know how the removal of healthy eucalyptus would achieve biodiversity in Glen Canyon, per such claims by the Natural Areas Program, and which of the 100 species would suffer and whether the gain in new species would be worth the effort or even sustainable within the current staffing and budget allocated for the park. The Natural Areas Program has been around long enough that there should be benchmarks for what is success. In fact, meeting a threshold should be a requirement for funding.

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