NARRATOR：

Listen to part of a lecture in an environmental science class.

FEMALE PROFESSOR：

Today we’re going to begin discussing ecosystems.

One important point I wanna emphasize in the reading is that there’re many interactions that take place within an ecosystem…interactions between animals, interactions between living and non-living things and so on.

Now these interactions can be fairly simple and straightforward. Ah, there are certain species of ants and rodents sharing a desert ecosystem in Arizona, and they compete for the same plant seeds.

And the competition influences is not only the size of the ant and rodent populations, but also the number of eventual plants.

Now this interaction is easy to see, right?

However, there’re many other interactions within ecosystems that are not so apparent and require closer examination.

And the example from your reading was the forest ecosystem along the Pacific coast of North America…um… specifically the role of salmon.

OK. As you probably know, salmon are born in fresh water streams, they migrate to oceans where they spend most of their lives, and then they return to the same streams where they were born to reproduce… or spawn.

In order to spawn, salmon need cold, clear streams to ensure the survival of their eggs… and trees in the surrounding forest play an important role here.

Their leaves provide shade from the sun. When logging removes the trees, the streams are open to the sun and the water becomes warmer.

When the water warms up, the concentration of dissolved oxygen in the water decreases… and this reduces the chance that the salmon eggs will survive.

And the trees also help keep the soil on the banks of the stream in place.

Salmon cannot spawn in streambeds clogged with sediment, dirt, from the surrounding area… they need a clean, gravel streambed. Brad?

MALE STUDENT：

I’ve read that salmon also help keep streams healthy.

FEMALE PROFESSOR：

Right. Salmon contribute important nutrients like carbon and phosphorous, and these nutrients promote diversity in the stream environment.

OK. Um, so salmon need trees to successfully reproduce.

But surprisingly, trees also need salmon… and bears play an important intermediary role.

So in the autumn, bears are busy putting on extra weight as they prepare to hibernate.

Each bear catches an estimated 700 fish during the 45 days that the salmon are spawning.

The bears catch the salmon in the streams, and then they carry them back into the forest to eat… sometimes as much as 800 meters from the streams.

And since the bears only eat about half of each fish they catch, other animals like eagles, crows, and insects feed on the leftovers. Maria?

FEMALE STUDENT：

Why did the bears bring the salmon so far into the forest? Why not just eat the fish near the streams?

<-FEMALE PROFESSOR:-> Well, imagine several hungry bears looking for salmon.

When one bear catches a fish, it’s not uncommon for another bear to try stealing it.

These confrontations can be pretty intense, so it’s safer to bring it back into the forest… to a place where the bear can eat undisturbed.

MALE STUDENT：

Um, you said that the bears only eat half of each fish they catch?

I mean if I were a bear preparing to hibernate, I probably eat everything I could catch.

<-FEMALE PROFESSOR:-> Well, certain parts of salmon are more nourishing… fattier than others.

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.

OK. So after the scavengers have eaten the leftovers, only the fish’s skeleton remains.

Now, salmon contain nitrogen, so their decomposing bodies and skeletons provide a lot of nitrogen to the surrounding forest.

Plants absorb this nitrogen, which they need to grow, so the transfer of this nitrogen to the forests is important. Forests near streams with salmon actually reach maturity faster than other forests.

OK. So, why’s all this important? Well, salmon are in trouble.

Some of their populations have gone extinct, and most of the remaining populations have been significantly reduced by overfishing and environmental challenges.

Now, conservationists can try to prevent overfishing, but, well, I mean you can see the interconnections within this ecosystem.

We've already talked about the importance of trees to salmon, and the negative effect that something like logging can have.

So you can see that protecting this ecosystem is going to take a broad effort.