11th May, 2011. Headlines in newspapers and websites blared out the headlines this week: “IPCC wrong again: species loss far less severe than feared.” (IPCC is the Nobel Prize winning international group that assesses climate change and its consequences.) “IPCC’s species extinction hype fundamental flawed” it continued.
“Species extinction rates wildly overstated” went other headlines.
“The International Union for the Conservation of Nature (IUCN) “Red List” of endangered species — a benchmark for policy makers — is now also subject to review” went another, quoting the two authors of a paper that appeared in this week’s journal of Nature.
The paper’s title was emphatic enough: “species-area relationships always overestimate extinction rates.” With modesty, they told the media that it had taken eight years of hard work to come to that stunning conclusion.
The Species-Area Relationship
It took me eight seconds to know the paper was a sham — and I am slow reader. So let me explain why this paper fails so obviously to get at the truth. Along the way, I’ll tell you about those “species-area relationships.”
“Species-area relationships” are about predicting how many species will survive in “forest islands” like these in the coastal forests of Brazil. When forest is cleared, some species will be killed off immediately because they lived only in places that now lack trees. But many more will die slowly, because the patches of forest that remain are too small to hold populations viable in the long term. Photo: Stuart Pimm
First, the Red List. The Red List compiles a list of species that are threatened with extinction. Some species are already extinct and some survive only in zoos. Others range from being critically endangered — often meaning only a handful of individuals — though “endangered,” “vulnerable,” “near-threatened,” to those “of least concern.”
Go to your nearest zoo and look at the labels on the exhibits to see examples!
There are rigorous criteria for deciding into which category a species should fall. Many thousands of experts around the world evaluate the species they know and decide into which class each species should fall.
What do this have to do with those, species-area relationships?”
Absolutely nothing. The authors’ remark was entirely gratuitous.
The authors didn’t present any estimates of how many species are likely to go extinct, making their media claims particularly egregious in other ways.
Scientists have long studied why endangered species got that way. There are very many causes, but a very important one is the loss of habitat.
The less area of habitat, the fewer species it will contain. That’s the “species-area relationship” at the core of the problem.
To see that there’s more than one relationship, imagine your favorite patch of woodland. I’ve selected Rock Creek Park, just outside of Washington, DC, for my example. It was the site of the first National Geographic — National Park Service bioblitz a few years back.
How many bird species live there? Well, download the checklist from the National Park Service!
It’s a large list, with four species of woodpecker, three species of owls, and many others.
A World with One Forest
Imagine this nightmare scenario. Suppose all the forest in eastern North America was cut — except Rock Creek Park. How many species would go extinct? There are several answers.
Suppose all those forests were cut within months — metaphorically “overnight.” If you think that’s unlikely, consider how fast the world’s tropical forests are being cut!
How many species would be extinct “the following morning?” The answer is “not many” — because Rock Creek has many species. That’s the number that the Nature paper calculates. It does so with complex, mathematical formulas, and with considerable elegance. That’s why it took the authors eight years.
It’s not the relevant answer, however.
How many species would eventually become extinct? The answer is very much higher. The populations of the species that survived the initial deforestation elsewhere would eventually die out.
Those species are part of what scientists call the “extinction debt.” They are the species that are fatally wounded, taking a while to die. The populations are too small to be viable in the long-term.
One pair of (say) pileated woodpeckers or great-horned owls may have plenty of food in the Park. But, if one pair produces just one pair of young that survive to adulthood then you don’t need elegant mathematics to work out the answer. There’s a 50-50 chance that both those young will be the same sex — both female or both male.
Even if they are different sexes, they will be brother and sister.
Simply, small populations suffer from the vagaries of sex and death and, on top of that inbreeding, that doom them in the long term.
The Florida panther became seriously inbred when isolated from panther populations elsewhere. It was in a slow decline toward extinction until rescued by the introduction of panthers from Texas. Many endangered species have no such chances of being rescued.
Do we know how many species will die this slow death? Well, yes!
Islands provide a model of small populations that get few, if any, immigrants that might rescue a foundering population. There’s a species — area relationship for islands. Many studies of the numbers of species on islands show that, for example, that if island “A” has half the area of island “B” then the former will have only about 85% of the species that A does.
Eastern North America tests exactly that prediction for “forest islands”. By about 1870, it had about half the forest cover it had in 1620. (Most of the forest had been cut, but not all at once.) So, we expect 100 – 85% = 15% of the species to have become extinct according to this island species-area curve. Apply that to the roughly 30 species of bird that live only in the forests of Eastern North America and one predicts 4.5 species will go extinct.
Four have. And another one is endangered, which is in strikingly good agreement.
It was when the Nature paper used that as an example to hype its own results, by asserting that species area methods always failed that I knew it was sham. The eastern North America example, in fact, is only one of a dozen or more studies from around the world that show how well the island species –area relationship works.
Wording matters. It always does.
In writing to me about the fuss his paper had caused, author Fangliang He, an ecologist at Sun Yat-sen University in China, said:
“I have followed up some of the media and felt there is a danger of misinterpreting our work, which I would like to clarify here. … All we have said is that the backward SAR is flawed and overestimates extinction rates, not anything more than that.”
Well, of course, that wasn’t what the paper said and it wasn’t what the authors said to the media. If the paper had had “backward SAR” in the title, the media wouldn’t have commented. And one wonders whether Nature would have published it.
I checked the websites that carried this story. Most allow comment, except the ones in China. Professor He had not bothered to provide them with the clarification he provided me.