Climate change is melting the poles, and the Deepwater oil disaster is killing countless animals in the Gulf. It feels like an environmental apocalypse is brewing. Is the planet about to undergo a mass extinction? We asked an extinction biologist.
Ross MacPhee is a paleontologist and curator at the American Museum of Natural History, who has researched mass extinctions for most of his career. Though there is some debate over the definition of "mass extinction," MacPhee says "large scale losses" must be involved. He adds:
The definition [of mass extinction] I like is where you have numerous phylogenetically distant organisms involved in losses at the same time. For example, 65 million years ago there was a mass extinction. Dinosaurs were lost, but what made it a mass extinction was that all kinds of other species went down at the same time - all large marine reptiles and primitive types of birds, as well as many groups of plankton and other one-celled creatures. It was a large scale extinction that affected apparently all ecologies on the planet, from moderately deep marine to high altitude terrestrial ones. These were dramatic losses. By contrast, the End Pleistocene [about 12,000 years ago] doesn't stack up as a mass extinction. There were losses of large mammals [like woolly mammoths and mastodons], and some small. There were bird losses in the scavenger/raptor category. But then the loss picture drops off to nothing. There's no evidence for large extinctions among reptiles or fishes. Nor for plants.
MacPhee has come up with a novel hypothesis to explain the End Pleistocene extinctions: He calls it the "hyperdisease theory." Though he's published about hyperdisease in major scientific journals, he also laughingly describes it as a "half-baked idea, for which there's ultimately not enough evidence." All the same, the hyperdisease idea goes a long way toward explaining why, 12,000 years ago, mammoths and other megafauna began dying off in droves. A hyperdisease is a fast-spreading sickness that's "100 percent lethal and smacks down all populations in a species. And then transfers to other species." A hyperdisease might explain how diverse megafauna populations died quickly in America during the End Pleistocene. "Whatever it was, was in a position to infect other mammals and burn through them as well," MacPhee muses.
The mainstream scientific explanation for this dramatic event, however, is that humans hunted these creatures to extinction, essentially killing as many animals as possible as quickly as possible. MacPhee doesn't think that people alone could have caused it. "There's no ethnographic parallel that we know of that makes it plausible that people ever acted that way," he says. It's hard to believe that humans could have killed enough megafauna fast enough to trigger total extinction:
One thing we do know is that an extinction has to be accomplished rapidly. If it's not rapid, then most species recover. That makes sense from Darwinian perspective: You don't get a bunch of weak sisters in evolution. Species are built to take heavy hits. The only way you keep them down is to hit them hard and fast. And one thing in nature that does that is a novel infectious disease. Not people spreading out across a continent.
The biggest objection to the hyperdisease idea is that there are no diseases on the planet like the one MacPhee describes taking down a wide range megafauna. But now we're witnessing the emergence of a possible hyperdisease within a single group: An infectious fungus called a chytrid, which is causing a massive collapse in amphibian populations. MacPhee explains:
But why would a disease behave this way? Killing off your host organism is no way to evolve. MacPhee says, "This amphibian disease has made us rethink that idea. As long as there are other hosts out there, it doesn't matter what you waste as you go along."
Though he's keen to identify a hyperdisease in the making, MacPhee is certain that we're not in the midst of a mass extinction. He's very concerned about the environment, but he bridles at alarmist claims that we're facing a mass extinction comparable to the one 65 million years ago, or ones before that, where large percentages of animal and plant species died out.
I hate it when people say "becoming extinct" when there's no basis for it. If you have individuals left, the species isn't gone. People talk about "extinction" of local populations. But if you take a Darwinian view, local populations are shock troops – some take hits or disappear, but the species as a whole keeps marching on. If you talk about the "disappearing" Florida panther, it's ridiculous. These panthers are in fact a population of pumas, also known as mountain lions. Pumas are all over the Americas from Yukon to Tierra Del Fuego. The puma isn't going to disappear – local populations drop out, but that's not the same thing as extinction. All the extinctions that we know of that affected animals in last 500 years mostly occurred on islands. There have been only one or two extinctions of mammals on the American continents during that time. What this means to me is that we've buggered up the planet and we should be restrained in what we're permitted to do. Whether our behavior ineluctably leads to mass extinctions in all groups and places – well, that remains to be shown.
He's particularly concerned about the misinformation that comes from a widely cited back-of-the-napkin calculation that E.O. Wilson made in his book Amazonia, that three species per hour are disappearing in the rain forest:
He knew the size of the rainforest, and the average rate of depletion due to Brazilians cutting it up and so forth. He also had a rough idea of how many species you'd have in a given area. So he put that together with the rate of the forest being cut, and came up with three species disappearing per hour. But my point is this: What three species? Do they have names? Can we check on this scientifically? No.
He points out that Wilson's numbers are just a generalization, and don't account for species having overlapping distribution, or living beyond the range of the areas Wilson's calculated.
I'm very favor of being careful with the planet. But I'm an extinction biologist and for me to listen you have to have facts on the ground. I need the facts and the body count. My point is that people are speculating about extinction with arm-waving. I don't see evidence of mass extinction or extinctions of large effect. Doubtless humans are endangering species – but how many gravestones are we talking about? I don't know, and I don't think anybody else does either.