Endangered Species Habitat: Still A Big Deal

The peninsular bighorn sheep, threatened by disruption of its habitat | Photo by Chris Clarke

A much-ballyhooed study on habitat and extinction may mean increased peril for California desert species -- not because of the science, but because of how it's being reported.

The study, entitled "Species-area relationships always overestimate extinction rates from habitat loss" and published May 19 in the prestigious peer-reviewed journal Nature, looked at the way in which ecologists estimate the number of species likely to go extinct as a result of destruction of a given amount of habitat.

The authors, Stephen Hubbell of UCLA and Fangliang He of Sun Yat-Sen University in China, concluded that a common method used to project extinction rates -- the "species-area relationship" (SAR) -- in fact leads to overestimates of the number of species likely to be lost, by as much as 160 percent. The mathematics underlying the study is rather complex, but the basic concept is simple enough. Scientists using the SAR method to predict extinctions essentially calculate the number of new species found when a certain kind of habitat is mapped, and then use that figure to determine how many species will be lost if a similarly sized piece of habitat is paved over.

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Paul Voosen, who reported on the controversy over the paper at the New York Times' Greenwire blog, put it this way:

The Nature paper published today stems from a simple idea. Both SAR and EAR [Endemic-Area relationship] can be pictured as circles spreading over a forest from a fixed point. When building a SAR, the diversity count rises by one for each new species encountered — say, a white pine — as the circle expands; each additional white pine won't increase this count. By contrast, an EAR counts up only at the last white pine found in its circumference.


If the pines were randomly distributed across a forest, then the SAR and EAR would be identical. But that is not how nature works. Animals and plants clump together, and when that is the case, extinction estimates derived from SAR methods will, as He and Hubbell boldly put it, "always" be above EAR estimates.

Many other scientists have taken issue with the paper, pointing out that Hubbell and He's methodology ignores the deleterious effects of habitat fragmentation, edge effects and other factors.

Almost all scientists in the field agree that species are now going extinct at at least a hundred times the normal rate, and this study doesn't challenge that. In fact, He and Hubbell take pains to point out that they're not saying species endangerment isn't a big problem. As Hubbell told National Geographic,

"I think [scientists and conservationists] are right in saying that we're really on the cusp of a sixth mass extinction or that it's actually in progress. We certainly don't disagree with that assessment."

The scientific dispute here, then, is about mathematical models rather than the actual threat of extinction, and we can be confident that the scientists will argue until they come to a rough consensus.

The federally Threatened desert tortoise | Chris Clarke photo

The problem for the desert — were you wondering when I'd get around to talking about the desert? — is in the way Hubbell and He's work has been covered in the press. Let's look at some representative headlines. From the Los Angeles Times, we have:

Global species extinction isn't quite so dire, study finds

A rather unsurprising take from Fox News:

Species Extinction Rates Grossly Overestimated, Scientists Say

In the Wall Street Journal, capping off what has to be the worst piece of science reporting I've seen in a long long time:

Habitat Loss Isn't the Villain of Species Extinction

And a typical offering from the anti-environmental blog sphere:

Species extinction rates being exaggerated to raise funds

This sort of misrepresentation of the science takes its toll on protection of the desert's own struggling species.

Consider the Mojave Desert. A short drive away from anywhere in Southern California's megalopolis, the Mojave is nonetheless widely considered to be North America's most intact ecosystem, 25,000 square miles of habitat spread across four states.

That's not to say we haven't done some damage to the Mojave: we have. Sprawling towns like Palmdale and Victorville and Vegas, military training over the course of the last century, rampant off-road vehicle use, mining, grazing, and a burgeoning renewable energy industry have all left marks on this ecosystem.

Mojave milkweed, Ivanpah Valley. This rare plant has since been buried under solar energy development. | Photo by Jim André

But there's a lot left, and much of what is there isn't well known, even to the experts. Jim André, director of the Sweeney Granite Mountains Desert Research Center, estimates that between five and ten percent of the species living in the Mojave Desert are unknown to science. The situation is likely similar in the adjacent Colorado Desert, California's seven-million-acre portion of the Sonoran Desert. 37 percent of California's known plant species are found in the deserts, and André expects another couple hundred to be discovered in the next century. And where there's plant diversity, animal diversity follows. André's colleagues have documented about 500 plant species just in the Granite Mountains, and those plant species support at least 150 migrant and resident bird species, more than 30 lizard and snake species, at least 42 species of mammals and close to 400 species of insects and other invertebrates.

These totals reflect only those found to date. The desert's known biodiversity rivals that of the redwood forests just in terms of species per square mile, and we're still adding to the total. André tells me that he finds something new on just about every field trip he takes into the desert: a new species, or an already-described species not formerly known to exist in that location. Many plant species bloom only when a precise combination of rainfall and temperature patterns give them a nudge: fall rains after cool summers, for instance. They may spend decades as dormant seeds waiting in the soil — being missed by biologists who walk right past them —only to emerge, bloom, set seed and die.

The region's topography and climate contribute to what we know and don't know about its biodiversity. The majority of species records in the Mojave were documented within a mile of a paved road. Species that occur many miles off pavement are less likely to be documented by even the most intrepid biologists, especially if those species are most visible when it's 120 degrees out, or live halfway up forbidding cliff faces, or both.

To top it off, that biodiversity is extraordinarily fragile. You can famously see the ruts in desert soil from Patton's WWII tank training exercises almost 70 years after the damage was done. Some vegetative communities in the Mojave may take millennia to recover from disturbance. If it ever does.

So we've got an essentially intact, astonishingly biodiverse ecosystem that is also astonishingly fragile and slow to recover from damage, surrounded by a growing population of millions of human beings. Much of that human population already cares less about the welfare of endangered species than it ought to, and sees those species' existence as standing in the way of their own comfort.

The desert's biodiversity depends on people wanting to save it, in other words, and misinformation that persuades people to the contrary doesn't help. The last thing desert species needed was a raft of news headlines saying scientists have decided that protecting their habitat isn't as important as the enviros say it is. Especially when that's not what the scientists in question are saying at all.

Chris Clarke is an environmental writer of two decades standing. Author of Walking With Zeke, he writes regularly at his acclaimed blog Coyote Crossing and comments on desert issues here every Wednesday at 10 a.m. He lives in Palm Springs.

About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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