托福听力TPO22听力文本(Conversation+Lecture)

　　TPO22 Lecture 3

　　Zoology (The Sixth Mass Extinction)

　　Narrator

　　Listen to part of a lecture in a zoology class.

　　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 have 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 are witnessing a sixth mass extinction, but unlike the others, the current loss of bio-diversity 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 over-hunting, habitat destruction and habitat fragmentation, pollution and other unnatural human causes.

　　So, as a way of repairing some of that damage, a group of conservation biologists has proposed an ambitious-or some might say-a radical plan, involving large vertebrates, or, mega fauna. Mega fauna include elephants, wild horses, big cats, camels, large animals. Eh, actually, the proposal focuses on a particular subset of mega fauna, the kind that lived during the Pleistocene epochs.

　　OK. The Pleistocene epoch, most commonly known as 11,500 years ago. In the Americas, many mega fauna began disappearing by the end of the Pleistocene

　　So here's the biologists' idea. Take a select group of animals, mega fauna from places like Africa and Asia, and introduce them into other systems 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 mega fauna by providing new refuges, new habitats for them. The other is to restore some of the evolutionary and ecological potential that has been lost in North America.

　　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 presumably6 on other prey animals stopped. So it is conceivable' that the pronghorn antelope would have 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 mega fauna with local flora, in particular, plants that rely on animals to disperse their seeds. Like Pleistocene rewilding could spark the re-emergence of large seeded American plants, such as the macular tree. Many types of macular used to grow in North America, but today, just one variety remains and it is found in only two states. In the distant past, large herbivores like mastodons dispersed macular 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 macular 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 mega fauna might devastate plants and animals that are native to the western United States. In the year since the Pleistocene epoch, native species have adapted to the changing environmental there, plants, smaller animals, they have 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 do when they graze the prairies of the America 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 is based on good intentions, but you know, it probably would just be more of the same thing.