托福TPO13 Lecture2

TPO 13 Lecture 2 Ecology Narrator: Listen to part of a lecture in an ecology class. Professor: So, continuing our discussion of ecological systems--- whole systems. The main thing to keep in mind here is the interrelationships. The species in a system uh…. and even the landscape itself, they are interdependent. Let’s take what you read for this week and see if we can apply this interdependence idea. Mike? Student: Well, um…, how about beavers--- ecosystems with beavers and waterways. Professor: Good, good, go on. Student: Like, well, you can see how it's so important, ‘cause if you go back before European settled in north America, like before the 1600s, back when native Americans were the only people living here, well, back then there were a lot of beavers, but later on, after Europeans… Professor: OK, wait, I see where you are heading with this, but before we go into how European settlement affected the ecosystem, tell me this--- what kind of environment do beavers live in? Think about what it was like before the Europeans settlers came, we’ll come back to where you were headed. Student: OK, well, beavers live near streams and rivers and they block up the streams and rivers with like logs and sticks and mud. You know, they build dams that really slow down the flow of the stream. So then the water backs up, and creates like a pond that floods the nearby land. Professor: And that creates wetlands. OK, tell me more. Student: Well with wetlands, it's like there is more standing water, more still water around, and that water is a lot cleaner than swiftly flowing water, because the dirt and sediment and stuff has a chance to sink to the bottom. Professor: More important for our discussion, wetland areas support a lot more varieties of life than swiftly flowing water. For example, there are more varieties of fish, or insects, lots of frog spices, and then species that rely on those species start to live near the wetlands too. Student: Yes, like birds and mammals that eat the fish and insects, and you get trees and plants that begin to grow near the standing water that can't grow near the running water. Oh, and there's something about wetlands and groundwater too. Professor: OK, good. Wetlands have a big effect on groundwater, the amount of water below the surface of the land. Think of wetlands as, Umm, like a giant sponge, the earth soaks up a lot of this water that's continually flooding the surface, which increases the amount of water below. So where there’re wetlands, you get a lot of groundwater, and groundwater happens to be a big source of our own drinking water today. Alright… So, back to the beavers, what if the beavers weren't there? Student: You just have a regular running stream, because there is no dam, so the ecosystem would be completely different, there would be fewer wetlands. Professor: Exactly, so, now let's go back to where you were headed before, Mike. You mentioned a change that occurred after Europeans came to North America. Student: Yeah, well, there used to be beavers all over the place, something like 200 million beavers, just in the continental United States. But when Europeans came, they started hunting the beavers for their fur, because beaver fur is really warm, and it was really popular for making hats in Europe. So the beavers were hunted a lot, overhunted, they are almost extinct by the 1800s, so… that meant fewer wetlands, less standing water. Professor: And what does that mean for the ecosystem? Kate? Student: Well if there is less standing water, then the ecosystem can’t support as many species, because a lot of insects and fish and frogs can't live in running water, and then the birds and animals that eat them, lose their food supply. Professor: Precisely, so the beaver in this ecosystem is what we call a keystone species. The term keystone kind of explains itself. In architecture, a keystone in an archway or doorway is the stone that holds the whole thing together, and keeps it from collapsing. Well, that's what a keystone species does in an ecosystem. It's the crucial species that keeps the system going. Now, beaver populations are on the rise again, but there is something to think about. Consider humans as part of these ecosystems, you've probably heard about water shortages or restrictions on how much water you can use, especially in the summer time, in recent years. And remember what I said about groundwater; imagine if we still have all those beavers around, all those wetlands. What would our water supply be like then?