91av

SERGEY ZIMOV is no Steven Spielberg, but if this hardy Russian biologist gets his way, he could one day achieve something the film director has only dreamed of. He is trying to recreate a landscape not seen on Earth for nearly 10,000 years, since the end of the last ice age – complete, if possible, with woolly mammoths.

The project is called Pleistocene Park, but it’s not a tourist attraction. Zimov wants to answer some fundamental questions about the impact early modern humans had on the environment, and in the meantime he might just help to save the planet.

During the Pleistocene epoch, from 1.6 million to 10,000 years ago, great sheets of ice repeatedly advanced and retreated across North America and Eurasia, in places extending as far south as the 40th parallel. That’s the same latitude as Denver in Colorado, or Madrid in Spain. The end of the epoch, the period most often associated with the term “ice age”, was marked by ecological upheaval, a mass extinction of large mammals and the rise of modern humans.

Some time between 20,000 and 10,000 years ago, dozens of species of large animals went extinct throughout northern Eurasia, the Americas, Australia and Madagascar. North America was the most severely hit, with 70 per cent of large mammals disappearing abruptly around 11,000 years ago.

The cause of these extinctions has long been a matter of debate. The prevailing view is that the glacial retreat at the end of the Pleistocene caused drastic shifts in climate and vegetation. What had been cold, arid grassland gave way to warmer and wetter tundra and forest. Large herbivorous mammals couldn’t cope with the ecosystem changing under their feet, and died out. And with large herbivores gone, the large carnivores that preyed on them swiftly followed suit, including sabre-toothed tigers and cave lions.

The alternative theory places the blame at the door of Homo sapiens. Proponents of this “blitzkrieg hypothesis” say it is no coincidence that the extinctions followed hot on the heels of human migration into the affected areas. These intelligent newcomers with their stone spear points and coordinated attacks would have decimated the slow-moving herds. The wipe-out would have been particularly pronounced in North America, where human hunters crossed the Bering land bridge 10,000 to 15,000 years ago to find a continent full of unsuspecting meals on the hoof.

Zimov, who is director of the Russian Academy of Sciences’ Northeast Science Station in Cherskiy in the Republic of Sakha, counts himself firmly among the blitzkrieg group. He is particularly interested in the extinction in north-eastern Siberia at the end of the Pleistocene. And he thinks that he can prove experimentally that the swiftly changing climate was not to blame for the extinctions.

During the late Pleistocene, north-eastern Siberia was part of the world’s largest ecosystem, stretching from western Europe across the Bering Strait to Canada and from the Arctic to northern China – half the land area of the planet. But while most of the area was repeatedly scoured by glaciers, north-eastern Siberia remained largely ice-free. The result was a vast expanse of cold, arid steppe and rolling grassland that teemed with huge herds of herbivores – mammoths, bison, elk, woolly rhinos, yaks, saiga, horses, reindeer, musk oxen and moose. Zimov calls this ecosystem the “mammoth steppe”.

Around 10,000 years ago the mammoth steppe abruptly disappeared. Most of the large animals died out – only reindeer and moose survived – and the grassland that supported them also disappeared, replaced by mossy forests and tundra.

What caused this catastrophic change? Zimov claims that it was the animals that disappeared first, hunted to oblivion by humans. The disappearance of the animals then caused the landscape to change. In other words, the herbivores were sustaining the mammoth steppe, rather than the other way around. By grazing, fertilising the soil with their manure and trampling down moss and shrubs, the herds kept mosses from taking over. Zimov points to today’s central African savannahs as an example of how large herds of herbivores can maintain a grassland ecosystem (Science, vol 308, p 796).

“We have neither witnesses nor direct evidence that humans exterminated large animals in northern Siberia,” Zimov says, “but the climate there has a reliable alibi.” Considering how many species humans have destroyed, he says, “we can suspect that the mammoth-steppe ecosystem was also destroyed by our ancestors”.

Clinging to life

Zimov says there are lots of reasons to believe that climate change alone could not have put paid to the mammoth ecosystem. For one thing, the climatic events at the end of the Pleistocene were not a one-off. Similar shifts from cold and dry to mild and moist occurred numerous times during interglacial periods, but none caused the vegetation to alter so catastrophically that the herbivores died out.

What’s more, recent radiocarbon dates show that some large mammals survived well beyond the end of the Pleistocene. Mammoths lived on Wrangel Island in the East Siberian Sea until about 4000 years ago, which also happens to be the time that humans first visited the island. In Alaska, wild bison survived long after the end of the Pleistocene before being wiped out by human hunters. “Native elders still tell stories that chronicle the taste of bison meat,” says Zimov.

Bison have since been successfully reintroduced into Alaska. Similarly, in the past century or so musk oxen have been transferred from the cold, arid islands of the Canadian arctic to warmer, wetter Alaska – and thrived. All of this, says Zimov, suggests that the Pleistocene animals could have prospered in a warmer, moister climate if only they hadn’t been killed.

But Zimov was not content with dreaming up ways to explain that the climate was not to blame. He wanted to put the blitzkrieg idea to the test. After all, if the wipe-out was caused by hunting, then it ought to be possible to reverse it. And if grazers helped maintain the mammoth steppe, then bringing them back might resurrect it as well.

In 1989 Zimov set out to do exactly that by setting up Pleistocene Park. It sits in 160 square kilometres of meadows, forests and willow shrublands in the Kolyma river basin, about 30 kilometres south of Cherskiy (see Map). This remote corner of Siberia, once a final destination for exiled Soviet dissidents, is a sparsely populated region where scattered native groups herd reindeer and horses. The reserve is owned and administered by a not-for-profit corporation, and is surrounded by a 600-square-kilometre buffer zone in which development is prohibited.

But despite the support of the regional government, progress has been painfully slow. “Our scientific station is rather small – only two families,” says Zimov. “It is rather difficult to do science, build fences and communicate with bureaucrats simultaneously.” At the moment Pleistocene Park itself is rather modest – a 50-hectare paddock with six horses and three moose. “It is difficult to call this an ecosystem,” admits Zimov. But he says the team is constructing a 1600-hectare paddock surrounded by high-tensile fencing, built to house 300 to 400 animals.

If all goes to plan, later this year the park will move into the next phase – stocking the new paddock with the surviving large herbivore species of the mammoth ecosystem, or their closest living analogues. In the first instance that means adding 20 horses and 20 reindeer. Zimov is also working on importing wood bison from Canada, the closest living relatives to the extinct steppe bison. Musk oxen and moose will come next.

As the population density of herbivores increases, the boundary will gradually be extended. If all goes well, the buffer zone will become part of the reserve, and predators such as wolves, bears, lynxes and wolverines – all of which already live in northern Siberia but are vanishingly rare – will be reintroduced. Eventually Pleistocene Park could support a population of Siberian tigers, which are the closest living analogue to the extinct European cave lion, the top predator in the mammoth ecosystem.

And what of mammoths themselves? Russian and Japanese scientists have been attempting to clone these giants using elephant eggs and DNA extracted from mammoth remains found in permafrost. But this remains in the realm of science fiction, says Adrian Lister, an expert on Pleistocene mammals at University College London. “Based on the poor, fragmented preservation of DNA recovered so far, I don’t think it will work.” Zimov is more hopeful, but realistic about the timescale. “If somebody in the future – in tens of years at best – will be able to recreate mammoth, then Pleistocene Park will be the best place for the animals,” he says.

“If anyone is ever able to clone the mammoth, Pleistocene Park will be the best place for it”

Despite its glacial progress, the project is producing results. Zimov says that the vegetation in the paddock is changing, with mosses in decline and grasses increasing. Zimov is planning to set up test enclosures containing different mixes of animals, plus areas from which all animals are excluded, to see what happens to the vegetation.

The project may even help fight global warming. The soil of the former mammoth steppe holds about 500 gigatonnes of sequestered organic carbon – 2.5 times as much as all the rainforests on Earth. At present this is locked into the permafrost, but as the climate warms it will be converted into CO₂ and methane by bacteria.

According to Zimov, restoring the mammoth steppe will help maintain the permafrost. Grassland reflects more sunlight than tundra and so keeps the soil cold. And herds of herbivores trample the winter snows, which exposes the surface to cold air.

One supporter of the experiment is ecologist Terry Chapin of the University of Alaska Fairbanks (UAF), who has collaborated with Zimov in the past. “It’s a valid hypothesis, because the same kind of [extinctions] occurred on virtually every continent, regardless of climate,” he says. And he applauds the conservation goals. “It’s a really exciting idea, taking a more proactive approach to conservation rather than just trying to minimise ongoing rates of extinction.”

But not everyone thinks that Zimov’s experiment will work. Zoologist R. Dale Guthrie, also of UAF, calls it an exciting idea, but says its theoretical basis wilts under scrutiny (Quaternary Science Reviews, vol 20, p 549). Although grazers can influence vegetation, Guthrie believes late-Pleistocene herbivores would have been too sparse to disturb soils to such an extent. What is more, huge herds of caribou migrate across the region but have not converted the wet tundra into a steppe.

Lister is even more sceptical. “There is an extremely plausible climatic model and insufficient evidence that human population sizes were large enough to have carried out overkill on a massive scale,” he says. “We know that large mammal ranges were seriously altered and reduced by climate change acting through vegetation.” He says it is possible humans could have hunted out small populations already squeezed by this process, but were unlikely to be a major cause of the extinctions.

Guthrie and Lister both think that the comparison with Africa is misleading. Savannah elephants probably exist in much higher densities than mammoths did, says Guthrie. “In other parts of the world, elephants and rhinos live in, but do not remove, the forest,” Lister points out.

But whatever Pleistocene Park does for our understanding of the great ice-age extinctions, it could still be a success story, by helping to protect threatened species such as wood bison and Siberian tigers. The reserve may eventually become a national park, drawing adventurous tourists from around the world. Big-game hunting might even be allowed one day, Zimov jokes: “But only with stone weapons you make yourself here in the park.”