TOEFL Listening: ETS-TOEFL听力机经 - VNZE6F96SK1U16R2

Lecture: M. Sabuleti Ants: Narrator: Listen to part of a lecture in an environmental science class. Professor: It's hard to predict how successful a conservation project will be. Many early efforts to reestablish extinct or nearly extinct species have been unsuccessful. This is especially true for insects. We now know that extinction rates for insects are much higher than those for other animals, or even plants. There are a couple of reasons for this. Insects reacts rapidly to the environmental change. And they tend to rely on specific interactions with other species, much more than other animals do. So their survival depends on the survival of the whole ecosystem, and this is something that we are much more aware of today. Let's look at a specific example, the large blue butterfly. Populations of large blues are found throughout the European mainland. And they were fairly common in southern England until about 100 years ago. But by the end of the 1800, conservationists were concerned that they were disappearing, basically on account of overly zealous butterfly collectors. Butterfly collecting was a popular hobby. So in the 1930s, collecting large blues was banned in certain areas, but populations continued to decline. Over the next 40 years nine separate grassland sites were declared conservation areas, but nothing seemed to slow the butterflies' extinction. And that really puzzled biologists. See there are a couple of things they knew about these butterflies. First they knew that young large blue butterflies, like to feed on the flower of the thyme plant. That's THYME. And second, they knew that the caterpillars also spent months living underground in ants' nest. Student: Living with ants? how bizarre! Is this one of those what you call it, mutualistic type relationships, that they both benefit from, somehow? Professor: No. Actually quite the opposite. See when the large blue caterpillar hatches, it eats thyme flowers for a few days, then it dropped to the ground, and after that it has to survive off of it and of grabs. So it actually tricks these ants by secreting a fluid and by singing to them. It makes them think it's their queen. The ants take the caterpillar home for the winter and it has all the food it needs to become a butterfly. Student: Wow. Professor: Anyway researchers would go out to this nature reserves. See plenty of time flowers and ants, but no butterflies. And they couldn't figure out why. Many will give up that point. But a small group decided to do a more detailed study of the butterfly to see if some unsuspected aspect of their biology was responsible for the decline. And what they found has really changed the way we approach conservation. See, turns out there's only one type of ant that a large group could successfully fool, the M. sabuleti. And these M. sabuleti ants have certain specific requirements as well. They can survive only within a very narrow range of ground temperature. And this ground temperatures only maintained when the grass is covering the area at certain length. In other words when they are kept short by animals grazing on them, because the grasses even a centimeter too long, the ground will be too cool and the M. sabulet will die. And if there are no M. sabulet ants, well, need I say more? So it turns out that while the researchers may have seen ants in the nature preserves, they are not particular ants that the large blues rely on. Why? Because during the same time period that efforts were being made to bring the large blues back, the numbers of grazing animals in the United Kingdom, sheep, cattle, wild rabbits, were also decreasing. And of course the last place animals will be grazing within the special nature preserves that have been satisfied and protected. So paradoxically even though researchers thought they were doing the right thing by establishing these conservation sites, they were actually dooming their efforts to failure today. Large blues have been successfully re introduced into the United Kingdom through careful management of preserves to ensure that grasses are maintained at the right length. And this idea of habitat management has been extended to the conservation of other insect species as well.