OFFICIAL52 Why does one of the students say this:

[00:01.00]NARRATOR: Listen to part of a lecture in an environmental science class.
[00:06.00]FEMALE PROFESSOR: Today we're going to begin discussing ecosystems. [00:09.50]One important point I wanna emphasize from the reading is that there’re many interactions that take place within an ecosystem… interactions between animals, interactions between living and nonliving things, and so on. [00:23.60]Now these interactions can be fairly simple and straightforward.
[00:27.50]Ah, there are certain species of ants and rodents sharing a desert ecosystem in Arizona, [00:33.25]and they compete for the same plant seeds…[00:36.10]and the competition influences not only the size of the ant and rodent populations, but also the number of eventual plants. [00:45.10]Now this interaction is easy to see, right? [00:49.10]However, there're many other interactions within ecosystems that are not so apparent and require closer examination. [00:57.50]And the example from your reading was the forest ecosystem along the Pacific coast of North America…um… specifically the role of salmon.
[01:07.70]OK. As you probably know, salmon are born in freshwater streams, they migrate to oceans where they spend most of their lives, [01:16.70]and then they return to the same streams where they were born to reproduce… or spawn. [01:22.15] In order to spawn, salmon need cold, clear streams to ensure the survival of their eggs…[01:29.50]and trees in the surrounding forest play an important role here. [01:33.60]Their leaves provide shade from the sun. [01:36.60]When logging removes the trees, the streams are open to the sun and the water becomes warmer. [01:42.65]When the water warms up, the concentration of dissolved oxygen in the water decreases…[01:49.03]and this reduces the chance that the salmon eggs will survive.
[01:52.87]And the trees also help keep the soil on the banks of the stream in place. [01:57.90]Salmon cannot spawn in streambeds clogged with sediment—dirt— from the surrounding area…[02:05.00] they need a clean gravel streambed. [02:08.10]Brad?[02:10.50]MALE STUDENT: I've read that salmon also help keep streams healthy.[02:13.60] FEMALE PROFESSOR: Right. [02:14.60]Salmon contribute important nutrients like carbon and phosphorous, [02:18.80] and these nutrients promote diversity in the stream environment.[02:23.30]OK. Um, so salmon need trees to successfully reproduce. But surprisingly, trees also need salmon…
[02:33.90]and bears play an important intermediary role. [02:37.55] So in the autumn, bears are busy putting on extra weight as they prepare to hibernate. [02:43.70]Each bear catches an estimated 700 fish during the 45 days that the salmon are spawning.
[02:51.30]The bears catch the salmon in the streams, and then they carry them back into the forest to eat…[02:57.70]sometimes as much as 800 meters from the streams. [03:01.60] And since the bears only eat about half of each fish they catch, other animals like eagles, crows, and insects feed on the leftovers.
[03:11.77]Maria?[03:12.75]FEMALE STUDENT: Why do the bears bring the salmon so far into the forest? [03:16.30] Why not just eat the fish near the streams?
[03:19.15]FEMALE PROFESSOR: Well, imagine several hungry bears looking for salmon. [03:23.80]When one bear catches a fish, it's not uncommon for another bear to try stealing it. [03:30.00]These confrontations can be pretty intense, [03:33.12]so it's safer to bring it back into the forest… to a place where the bear can eat undisturbed.[03:39.70]MALE STUDENT: Um, you said that the bears only eat half of each fish they catch? [03:43.44]I mean if I were a bear preparing to hibernate I'd probably eat everything I could catch.
[03:48.80]FEMALE PROFESSOR: Well, certain parts of salmon are more nourishing… fattier than others. [03:53.60]It's actually more efficient for a bear to only eat some parts of the fish, and then try catching another one, instead of eating the whole fish.[04:02.70]OK. So after the scavengers have eaten the leftovers, only the fish's skeleton remains. [04:09.60]Now salmon contain nitrogen, [04:12.20]so their decomposing bodies and skeletons provide a lot of nitrogen to the surrounding forest. [04:19.10]Plants absorb this nitrogen, which they need to grow, [04:23.40]so the transfer of this nitrogen to the forests is important—[04:28.00]forests near streams with salmon actually reach maturity faster than other forests.[04:34.30]OK. So, why's all this important? [04:37.50] Well, salmon are in trouble. [04:40.15]Some of their populations have gone extinct, [04:43.20] and most of the remaining populations have been significantly reduced by overfishing and environmental challenges. [04:50.50] Now conservationists can try to prevent overfishing, but, well, I mean you can see the interconnections within this ecosystem. [05:00.40] We already talked about the importance of trees to salmon, and the negative effect that something like logging can have. [05:07.20] So you can see that protecting this ecosystem is going to take a broad effort.