NARRATOR：

Listen to part of a lecture in a zoology class.

FEMALE PROFESSOR：

A mass extinction is when numerous species become extinct over a very short time period- short, geologically speaking, that is. Like, when the dinosaurs died out 65 million years ago.

And the fossil record- it indicates that in all the time that animals have inhabited Earth, there've been five great mass extinctions, dinosaurs being the most recent.

In each of the others, up to half of all land animals and up to 95 percent of marine species disappeared.

Well, today, we're witnessing a sixth mass extinction, but unlike the others, the current loss of biodiversity can be traced to human activity.

Since the Stone Age, humans have been eliminating species and altering ecosystems with astounding speed... countless species have disappeared due to overhunting, habitat destruction and habitat fragmentation, pollution, and other unnatural, human causes.

[making main point finally]So, as a way of repairing some of that damage, a group of conservation biologists has proposed an ambitious- some might say a radical-plan involving large vertebrates, or megafauna.

Megafauna include[tossing out examples] elephants, wild horses, big cats, camels-large animals.

Uh, actually, the proposal focuses on a particular subset of megafauna- the kind that lived during the Pleistocene epoch.

OK. The Pleistocene epoch, most commonly known as the Ice Age, stretched from 1.8 million to 11,500 years ago.

In the Americas, most megafauna began disappearing by the end of the Pleistocene.

So here is the biologists' idea: Take a select group of animals- megafauna from places like Africa and Asia, and introduce them into other ecosystems similar to their current homes, beginning in the western United States.

They call their plan Pleistocene rewilding.

Now, the advocates of Pleistocene rewilding cite two main goals.

One is to help prevent the extinction of some endangered megafauna by providing new refuges, new habitats for them.

The other's to restore some of the evolutionary and ecological potential that's been lost in North America.

Wh-what do I mean by "restore evolutionary potential"?

Well, as you know, the evolution of any species is largely influenced by its interactions with other species.

So, during the Pleistocene epoch... let's take the now-extinct American cheetah, for instance.

We believe it played a pivotal role in the evolution of the pronghorn antelope- the antelope's amazing speed, to be exact, because natural selection would favor those antelope that could outrun a cheetah.

When the American cheetahs disappeared, their influence on the evolution of pronghorn, and presumably on other prey animals, stopped.

So, it's conceivable that the pronghorn antelope would've continued to evolve, get faster maybe, if the cheetahs were still around.

That's what's meant by "evolutionary potential."

Importing African cheetahs to the western United States could, in theory, put the pronghorn back onto its, uh, natural evolutionary trajectory, according to these biologists.

Another example is the interaction of megafauna with local flora- in particular, plants that rely on animals to disperse their seeds.

Like, Pleistocene rewilding could spark the reemergence of large-seeded American plants, such as the maclura tree.

Many types of maclura used to grow in North America, but today, just one variety remains, and it's found in only two states.

In the distant past, large herbivores, like mastodons, dispersed maclura seeds, each the size of an orange, in their droppings.

Well, there aren't any mastodons left, but there are elephants, which descended from mastodons.

Introduce elephants into that ecosystem, and they might disperse those large maclura seeds, like their ancestors did.

Get the idea?

Restoring some of the former balance to the ecosystem?

But, as I alluded to earlier, Pleistocene rewilding is extremely controversial.

A big worry is that these transplanted megafauna might devastate plants and animals that are native to the western United States.

In the years since the Pleistocene epoch, native species have adapted to the changing environment there... plants, smaller animals-they've been evolving without megafauna for millennia.

Also, animal species that went extinct 11,000 years ago, uh, some are quite different genetically from their modern-day counterparts.

Like elephants don't have thick coats like their mastodon ancestors did when they grazed the prairies of the American West during the Ice Age.

Granted, the climate today is not as cold as it was in the Pleistocene, but winters on the prairie can still get pretty harsh today.

And there are many more considerations-well, you see how complex this is.

If you think about it, though, the core problem with this "sixth mass extinction" is human interference.

Pleistocene rewilding's based on good intentions, but, you know... it probably would just be more of the same thing.