OFFICIAL13 Why does the professor say this?

[00:00.00]NARRATOR: Listen to part of a lecture in an ecology class.
[00:05.17]FEMALE PROFESSOR: So—uh, continuing our discussion of ecological systems, whole systems…the main thing to keep in mind here is the interrelationships. [00:14.84]The species in a system, uh—and even the landscape itself—they're interdependent. [00:20.74]Let's take what you read for this week, and see if we can't apply this interdependence idea. Mike?
[00:26.59]MALE STUDENT: Well, um, how about beavers—ecosystems with beavers and waterways.
[00:32.51]FEMALE PROFESSOR: Good good. Go on.
[00:34.12]MALE STUDENT: Like—well you can see how it's so important, ‘cause if you go back before Europeans settled in North America, like before the 1600s, back when Native Americans were the only people living here—[00:47.07]well, back then there were a lot of beavers. [00:49.84]But later on, after Europeans—
[00:52.69]FEMALE PROFESSOR: OK, wait—I see where you're 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? [01:03.87]Think about what it was like before the European settlers came. [01:07.37] We'll come back to where you were headed…
[01:10.66]FEMALE STUDENT: OK, well, beavers live near streams and rivers… And they block up the streams and rivers with like, logs, and sticks, and mud, [01:20.43]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.
[01:29.31]FEMALE PROFESSOR: And that creates wetlands. [01:31.19]Ok, tell me more.
[01:32.60]FEMALE STUDENT: Well, with wetlands it's like there’s more standing water, more still water around. [01:37.32]And that water's a lot cleaner than swiftly flowing water because the dirt and sediment and stuff has a chance to sink to the bottom.
[01:44.54]FEMALE PROFESSOR: More important for our discussion: Wetland areas support a lot more varieties of life than swiftly flowing water. [01:52.28]For example, there are more varieties of fish, of insects, lots of frog species. [01:57.92]And then species that rely on those species start to live near the wetlands, too.
[02:02.72]FEMALE STUDENT: Yes.  Like birds and mammals that eat the fish and insects. [02:06.64]And you get trees and plants that begin to grow near the standing water that can’t grow near running water. [02:12.34]Oh, and there's something about wetlands and groundwater, too…
[02:16.75]FEMALE PROFESSOR: OK. Good. [02:18.28]Wetlands have a big effect on groundwater—the amount of water below the surface of the land. [02:24.88]Think of wetlands as…um, like a giant sponge. [02:29.32]The earth soaks up a lot of this water that's continually flooding the surface, which increases the amount of water below. [02:36.53]So where there are wetlands, you get a lot of groundwater. [02:40.39]And groundwater happens to be a big source of our own drinking water today. [02:44.99]All right. So back to the beavers. [02:47.89]What if the beavers weren't there?
[02:50.55]MALE STUDENT: You'd just have a regular running stream, 'cause there's no dam. [02:57.28]So the ecosystem would be completely different—there’d be fewer wetlands.
[03:01.66]FEMALE PROFESSOR: Exactly. So now let's go back where you were headed before, Mike. [03:06.20]Uh, you mentioned a change that occurred after Europeans came to North America…
[03:10.90]MALE STUDENT: Yeah—well, there used to be beavers all over the place…um, something like 200 million beavers, just in the continental United States. [03:19.17]But when Europeans came, they started hunting the beavers for their fur…‘cause beaver fur, it's really warm—and it was really popular for making hats in Europe. So the beavers were hunted a lot, overhunted—they were almost extinct by the 1800s. [03:34.37]So…that meant fewer wetlands—less standing water.
[03:38.65]FEMALE PROFESSOR: And what does that mean for the ecosystem? Kate.
[03:41.84]FEMALE STUDENT: Well if there's 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.
[03:55.72]FEMALE PROFESSOR: Precisely. [making a key point]So the beaver, in this ecosystem, is what we call a keystone species. [04:02.59]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.  [04:14.72]Well, that's what a keystone species does in an ecosystem—it's the crucial species that keeps the system going.
[04:23.27]Now, beaver populations are on the rise again, but there's something to think about: consider humans as part of these ecosystems. [04:33.84]You've probably heard about water shortages or uh restrictions on how much water you can use, especially in the summertime, in recent years. [04:42.88]And remember what I said about groundwater.  Imagine if we still had all those beavers around, all those wetlands. What would our water supply be like then?