A. To explain the government's role in regulating assisted migration
B. To discuss ways in which plants and animals adapt to climate change
C. To discuss a controversial approach to conserving plant and animal species
D. To describe a recently discovered consequence of global warming
NARRATOR:Listen to part of a lecture in a conservation biology class.
FEMALE PROFESSOR:One consequence of global warming is extinction… there’s compelling evidence that global warming will be a significant driver of many plant and animal extinctions in this century.So, we’re considering various strategies to help some threatened species survive this unprecedented, this warming trend, which, as you know, is caused mainly by greenhouse gases produced by the burning of fossil fuels.Um, the most radical strategy being debated among conservation biologists is “assisted migration.”
Assisted migration means picking up members of a species—or members of a group of interdependent species— and physically moving, or translocating them…
…um, translocating threatened species to a cooler place, to higher latitudes or higher elevations, for example.
[Backtracking]Now, migration’s a natural survival strategy.Over the past two million years, colder glacial periods have alternated with warmer interglacial periods, and so…um, in-in response to these gradual climatic swings, some species have shifted their ranges hundreds of kilometers.So, perhaps you’re wondering: Why not let nature take its course now?Well, we can’t.The main problem is today’s fragmented habitats.During previous interglacial periods, when glaciers retreated, they left behind open land in their wakes.Today, human development has paved over much of the natural world.
Ecosystems are fragmented.Housing developments, highways, and cities have replaced or sliced through forests and prairies.There’re few corridors left for species to migrate through—without help.
So, conservationists are trying to save as many species as possible.Now, assisted migration could become a viable part of our rescue strategy, but there are a number of uncertainties and risks.Without more research, we can’t predict if assisted migration will work for any given species.A translocated species could die out from lack of food, for example.At the other extreme, we might successfully translocate a species, but within five or ten years, that species could proliferate and become an invasive species.Like a nonnative plant that chokes out native plants by hogging the nutrients in the soil.Translocated animals can become invasive, too.It happened in Australia.The cane toad was introduced back in 1935 to control an insect pest that was destroying Australia’s sugarcane plantations.But the cane toad itself became a pest and has destroyed much of the wildlife on that continent.
Also, many species are interdependent, intimately connected to one another, like animals that eat a certain plant, and that plant relies on a certain fungus to help it get nutrients from soil and on a certain insect for pollination.We’d probably have to translocate entire networks of species, and it’s hard to know where to draw the line.And, in addition to all that, it’s not even clear that assisted migration, or any migration for that matter, will help, at least for some species.Earth was already in one of its warm, interglacial periods when we started burning fossil fuels.And in the twenty-first century, global temperatures are expected to rise 2 to 6 degrees.That rate of heating’s far greater than during the last glacial retreat some 12,000 years ago.
Um, whether to use assisted migration… this debate is mostly within the biology community right now.But the ultimate decision makers— in the United States at least— will be the government agencies that manage natural resources.Assisted migration really needs this level of oversight—and soon.Currently, there’s no public policy on using assisted migration to help species survive climate change.People aren’t even required to seek permits to move plants or invertebrate animals around, as long as they’re not classified as pests.In one case, a group of conservationists has already taken it upon itself to try, on their own, to save an endangered tree, the Florida torreya tree, through assisted migration.
There’s only about a thousand individual Florida torreyas left, and global warming’s expected to significantly reduce or eliminate this tree’s habitat.So, this conservation group wants to translocate seedlings, Florida torreya seedlings, 500 kilometers north in order to expand the species’ range.The group believes that its effort is justified.[little skeptical, concerned]But I and many other biologists will be watching very closely how this maverick group makes out because, like I said, there could be unintended consequences.
原文定位：Now, assisted migration could become a viable part of our rescue strategy, but there are a number of uncertainties and risks. Without more research, we can't predict if Assisted Migration will work for any given species. A translocated species could die out from lack of food, for example. At the other extreme, we might successfully translocate the species, but within five or ten years that species could proliferate and become an invasive species.
文章前面一开始教授说到extinction，然后说到radical strategy 对Assisted Migration进行了定义，随后分别详细介绍了viable and risk. 最后说会有意想不到的结果
C选项即为文章核心关键词viable and risk的同义替换， 是对全文主旨最准确的概括