OFFICIAL22 What does the professor mean when she says this:

[00:00.00]NARRATOR: Listen to part of a lecture in a zoology class.
[00:04.15]FEMALE PROFESSOR: A mass extinction as when numerous species become extinct over a very short time period. [00:10.22]Short, geologically speaking that is, like when the dinosaurs died out 65 million years ago. [00:16.89]And the fossil record, it indicates that in all the time that animals have inhabited Earth, there have been five great mass extinctions, dinosaurs being the most recent. [00:27.52]In each of the others up to half of all land animals and up to 95 percent of marine species disappeared.
[00:34.88] Well, today, we're witnessing a sixth mass extinction, but unlike the others, the current loss of biodiversity can be traced to human activity. [00:44.41] Since the Stone Age, humans have been eliminating species and altering ecosystems with astounding speed? [00:51.37]countless species have disappeared due to overhunting, habitat destruction and habitat fragmentation, pollution, and other unnatural, human causes.[01:02.13] 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.[01:15.59]Megafauna include elephants, wild horses, big cats, camels—large animals. [01:21.70]Uh, actually, the proposal focuses on a particular subset of megafauna—the kind that lived during the Pleistocene epoch.
[01:29.76]OK. The Pleistocene epoch, most commonly known as the Ice Age, stretched from 1.8 million to 11,500 years ago. [01:39.74]In the Americas, most megafauna began disappearing by the end of the Pleistocene.
[01:44.94]So here is the biologists?idea: [01:47.61]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. [01:59.82]They call their plan Pleistocene rewilding.
[02:03.90]Now, the advocates of Pleistocene rewilding cite two main goals. [02:08.77]One is to help prevent the extinction of some endangered megafauna by providing new refuges, new habitats for them. [02:16.16]The other's to restore some of the evolutionary and ecological potential that's been lost in North America. [02:22.74]Wh-what do I mean by "restore evolutionary potential"?[02:27.01]Well, as you know, the evolution of any species is largely influenced by its interactions with other species.
[02:33.58]So, during the Pleistocene epoch...let's take the now-extinct American cheetah, for instance. [02:40.61]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. [02:53.89]When the American cheetahs disappeared, their influence on the evolution of pronghorn, and presumably on other prey animals, stopped.[03:02.29]So, it's conceivable that the pronghorn antelope would've continued to evolve, get faster maybe, if the cheetahs were still around. [03:10.97]That't what't meant by "evolutionary potential."[03:14.38] 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.
[03:26.35]Another example is the interaction of megafauna with local flora, in particular, plants that rely on animals to disperse their seeds. [03:35.41]Like Pleistocene rewilding could spark the re-emergence of large seeded American plants, such as the maclura tree. [03:42.99]Many types of maclura used to grow in North American, buy today, just one variety remains and it is found in only two states.
[03:51.10]In the distant past, large herbivores like mastodons dispersed maclura seeds, each the size of an orange in their droppings. [03:59.47]Well, there aren't any mastodons left, but there are elephants, which descended from mastodons. [04:05.42]Introduce elephants into that ecosystem and they might disperse those large maclura seeds, like their ancestors did. [04:12.14]Get the idea?[04:13.30]Restoring some of the former balance to the ecosystem? [04:16.48]But as I alluded to earlier, Pleistocene rewilding is extremely controversial.[04:22.39]A big worry is that these transplanted megafauna might devastate plants and animals that are native to the western United States.[04:29.86]In the years since the Pleistocene epoch, native species have adapted to the changing environmental there, plants, smaller animals, they have been evolving without megafauna for millennia. [04:41.66]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 do when they graze the prairies of the America West during the Ice Age. [04:58.44]Granted, the climate today is not as cold as it was in the Pleistocene.
[05:02.96]But winters on the prairie can still get pretty harsh today. [05:06.33]And there are many more considerations. [05:09.17]Well, you see how complex this is.[05:11.46]If you think about it though, the core problem with this sixth mass extinction is human interference. [05:19.09]Pleistocene rewilding is based on good intentions, but you know, it probably would just be more of the same thing.