The big dying

Extinction down under

Sizing up size

Smilodon, the most famous saber-toothed cat, is extinct. It's the second commonest fossil in California's La Brea Tar Pits.
University of California Museum of Paleontology.

Until recently, controversy surrounded the notion that an extra-terrestrial snowball or rock wasted the mighty dinos, and while the case is convincing, it's hardly airtight.

Even fiercer disputes swirl around two extinctions that occurred just yesterday, geologically speaking.

About 12,000 years ago, the New World was populated by a zoo-ful of large, strange critters -- mastodons, saber-tooth tigers, mammoths, giant ground sloths, long-horned bison, even giant beavers.

And guess what? They're all gone. Dead. Kaput. Extinct. History.

A larger die-off struck Australia about 50,000 years ago. Fully 28 vertebrates genuses, comprising 55 species, dove off the deep end, including 300-kilogram, claw-footed kangaroos and a 100-kilogram flying bird (see "Mass Extinctions..." in the bibliography).

Effects so grand just beg for causes. What extinguished these animals?

Old death in New World
Most of the New-World extinctions occurred over a span of 1,000 to 2,000 years, when two suspicious characters walked the land. One was a warming trend at the end of the Ice Age that so changed the climate across North America that The Why Files can be produced in post-glacial Wisconsin, now a thousand miles from the nearest glacier. Proponents of climate change say the warming trend changed vegetation radically enough to make the now-extinct animals go hungry.

The second theory attributes the killing to the humans who strode across the temporarily dry Bering straits sometime before 12,000 years ago, and, within 2,000 years, reached the tip of South America. During roughly the same period, 73 percent of large herbivore species apparently went extinct in North America.

Did people eat the big mammals all the way into geological history?

A stormy debate on whether weather or people caused the extinctions has raged since 1967, when Paul Martin of the University of Arizona proposed that people had exterminated large prey, "blitzkrieg" style.

The idea that people had so effectively and counterproductively hunted their prey out of existence rankled archeologists and Native Americans. This June, mathematical ecologist John Alroy of the University of California at Santa Barbara published evidence that, he says, closes the case.

Hunters, he says, prefer large prey. "Hunting a mammoth is like robbing a bank, it's a much better deal than whitetail deer." Certainly, archeologists have found mammoth fossils associated with spear points, indicating that the ancient elephant relatives were destined for the table -- or should we say the groaning board...

Mortal model
The math in Alroy's model of extinction (see "Multispecies Overkill..." in the bibliography) is way beyond us mortals. But computer models are worth considering because they are handier than, say, rerunning the Ice Age. Let's take a gander at his method and results.

Essentially, Alroy set up imaginary cells covering the 48 contiguous United States and filled them with animals and hunters. Then he put in plausible numbers for hunting efficiency (the chance that in a given year, with a given density of humans and prey, that human would run into prey and kill it) and population change (for people and animals).

Then, without allowing climate to change, Alroy pushed the "start" button and ran the model down the centuries.

The results, as you can see from the graph, were unambiguous. The big winner was Homo sapiens, and the losers included weird giants like the mammoth, the mastodon and the giant sloth. In many of the simulations, the modeled patterns of extinction closely matched the actual ones.

As humans grew more numerous in the ancient United States, other animals went extinct, according to this computer simulation.
Courtesy John Alroy

Completing the job
Before we listen to critics of the simulation, how could humans cause so many extinctions? After all, predators seldom cause their prey to go extinct.

You might credit brains for our impressive hunting ability, but there are other reasons. Advocates of overkill say that New-World animals had never seen humans, and so were unafraid of hunters.

A second factor reflects the fact that humans are more cosmopolitan than the average predator. When wolves depend on moose for food, wolves will get hungry and eventually become scarce once they snarf up too many moose. Then, with predators scarce, the prey can recover -- making a self-regulating cycle.

People are different because they can change prey, says Alroy. When one prey species -- say mammoths -- is scarce, the hunters will hunt other species. Eventually, by accident, they may find and hunt the last mammoth in the course of bagging, say, sloths or saber-toothed tigers.

The Pleistocene slaughter would have been even worse, Alroy observes, but bison, Canadian elk, moose and musk ox migrated to Canada, closer to the melting glaciers and further from the hunters.

Why die?
Mathematical ecologists assume that each species has an "extinction threshold." When numbers dip below the "minimum viable population," the species can easily disappear. Translated into everyday English, this jargon means that scarce animals are prone to becoming extinct animals.

When populations plummet, chance becomes an enemy. If, say, musk oxen were hunted until only 100 remained, they could be wiped out by a disease, especially if they all hang out in one valley.

But that's not all. Small groups tend toward genetic uniformity, so it's more likely that all the animals would be susceptible to the disease. And lacking a backup population in another valley, extinction could ensue.

Disease is one of what ecologists call the "stochastic," or random, factors, that can cause extinction. Other factors include fires, floods, storms, droughts and the appearance of new predators.

This bit about stochastic threats is not just idle theory. Conservationists are using ultralight airplanes to teach whooping cranes to use a new migratory pathway.

The idea is to avoid placing all crane eggs in one basket.

For geographic and genetic reasons, a population of 1,000, spread into 10 valleys, is much more resistant to extinction through disease.

A nice script, but a true one?
The human quest for food that's big and meaty may have contributed to the extinction of larger mammals like this woolly mammoth.
(c) Taylor Studios, Inc. 2001.

Proving cause and effect, even "just" 10,000 years ago, can be elusive. If, for example, the animals that died off were already declining before humans arrived, then hunters might have delivered the coup de grace rather than the main blow.

To Alroy, the case is pretty well closed. "It's still debated, there's controversy in some disciplines but not in others. I've never met an ecologist who seriously believes that anything else could have caused this extinction."

One person who begs to differ about the die-offs is named Richard Slaughter (honest!), assistant director of the geology museum at University of Wisconsin-Madison. For one thing, he questions the dating. "For a significant number of megafauna, the timing of the extinction in poorly known, maybe there's one date. I'd like to date more examples from other localities."

A second problem concerns the mode of death. "For the vast majority of Ice-Age fauna that went extinct, there is no evidence that humans preyed on them," Slaughter says. "It's not like we find spear marks and cut marks on all these animals." Although there is evidence that mammoths were speared and butchered, he cautions against generalizing to other large critters and other sites.

Slaughter also questions Alroy's assertion that "anthropogenic extinction is unavoidable," meaning that no matter what numbers were used as input, "I do not find realistic parameters that don't lead to mass extinction. You'd have to pretend that humans don't need 2,200 calories per day to not get mass extinction."

Strange. Yes. True. Maybe
The idea that extinction is inevitable under almost any combination of conceivable circumstances does seem odd. Why, for example, would large mammals go extinct if human populations grew very slowly, or if people had the hunting prowess of Homer Simpson?

In fact, Slaughter and colleague Joseph Skulan have written about what they see as flaws in Alroy's overkill model. They noted that all of the prey species in his computer simulations were given "pitifully low" reproductive abilities. For example, living populations of African elephants reproduce faster than the number given to the white-tailed deer in the model. "In a more ecologically realistic version of Alroy's model, human predation could not account for the full spectrum of megafaunal extinction," they concluded (see "Extinction Model..." in the bibliography).

In a counter-letter, Alroy stood by his results, while admitting a "minor programming error. ... The reason for the almost unchanged results is feedback. Larger and more rapidly-growing prey populations merely make life easier for hunters, thereby fueling the strong, indirect interspecific competition that generates most extinctions."

But if hunting didn't cause the extinctions, then what did? The obvious alternative is the warming climate that ended the Ice Age, raising temperatures and changing rainfall patterns.

The debate matters, of course, because animals are still going extinct, hunting is still occurring, especially to feed loggers and miners in forested regions. And climate is changing.

To Slaughter, the case remains open, and troubling. "I'm frightened at the idea that If we can't figure out this extinction that occurred 10,000 years ago, how good a job are we doing about extinctions from 100-million years ago?"

Hold on. What caused the wave of extinctions in Australia 50,000 years back?