TOEFL Listening: ETS-TOEFL听力机经 - WL4GET3544193MD4Q$

Lecture: Mutualistic relationship: Narrator: Listen to part of a lecture in a biology class. Professor: OK. So far we've been studying interactions of populations in nature that belong to a category of negative. Urn ... we've discussed predators that feed on the prey, like wolf and rabbit, corn root worms that feed on the corn and so forth. This type of relationship is characterized by the fact that one organism actually harms the other. Today I wanna begin talking about some positive interactions. First of all, let's look at mutualism and the classic mutualistic relationship. A mutualistic relationship is when two organisms of different species "work together," each benefiting from the relationship and do not harm each other. Some mutualisms are so close that interacting species actually can't survive without each other. One good example of this is the mutualistic relationship between the yucca plant and yucca moth, which you read about in your textbook for today. Tom, why is it that yucca plant can't live without the yucca moth? Tom: Well, it needs the yucca moth as a pollinator, right? Urn ... the yucca moth collects and carries the pollen from one flower to another in different areas. The yucca plant will produce seeds that get carried off by the wind and eventually germinate and grow into new plants again. Professor: Great. And interestingly, only the yucca moth can pollinate the yucca plant. Most people think of bees when they talk about a pollinator. But even bees can't pollinate the yucca plant because they can't move the yucca pollen. Yucca plant. Most people think of bees when they talk about a pollinator. But even bees can't pollinate the yucca plant because they can't move the yucca pollen. Yucca pollen is really heavy and sticky, so it can't be scattered by wind or by casual contact with other insects. But the yucca moth has special mouthparts that enable it to carry the pollen. Uh ... Jennifer? Why can't the yucca moth live without the yucca plant? Jennifer: Well, what the female yucca moth does is ... urn ... when she pollinates the yucca plant, she also lays her eggs. And then, when the eggs hatch, the larvae feed on the yucca plant's seeds. They can't eat any other kind of food. I mean, they will die from hunger unless they eat the yucca seeds. Professor: Very good! So, this is the perfect kind of relationship. The yucca moth pollinates the yucca plant, the yucca plants make seeds, and in return, the yucca moth gets food for its young from the yucca plant. Tom: Excuse me, professor. Urn, I seem to he missing something here. I mean, I thought you said, in mutualism, neither species is harmed. But the yucca moth larvae eat the yucca plant's seeds. Doesn't that mean the yucca moth actually harms the yucca plant? Professor: That's a good question, Tom. Actually it's not that much of a problem because there're plenty of seeds for the larvae to eat without harming the yucca plant supply. Also, the yucca moth doesn't lay too many eggs on one flower. She usually lays about 1 to 3 per flower. So most of the seeds will still be safe when the eggs hatch. But it raises an interesting question. What if a female yucca moth plays unfair? What if she plays an unfair game and lays more eggs on each yucca flower than the usual one to three. The moth would certainly be benefited this way because more offspring could be produced. But it wouldn't be very good for the plant. Well, one scientist has observed that the yucca plant produces a huge array of flowers and then drops a large number of flowers including many that have been pollinated and have eggs on them. This dropping of flowers is a matter of pure chance. Uh ... you never know which flowers will fall off. So, if the yucca moth lays lots of eggs at a few flowers, those eggs may all be dropped, and she may produce no young at all. But if the yucca moth lays fewer eggs on lots and lots of flowers, there's a greater chance that more caterpillars will hatch and survive among the flowers that have remained on the plant. According to the observation, perhaps the yucca plant is in a sense forcing the moth to lay eggs thin and wide.