[00:01.00]Listen to part of a lecture in an environmental science class.
[00:06.00]Professor:Today we are going to begin discussing ecosystems.[00:09.50] One important point I want to emphasize in the reading is that there are many interactions that take place within an ecosystem, interactions between animals, interactions between living and non-living 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 plants to eat. [00:36.10]And the competition influence is 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 are 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. [01:03.70]Um, specifically the role of salmon.
[01:07.70]Ok, as you probably know, salmon are born in fresh water streams, they might ran to oceans where they spent 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, graveled streambed.
[02:10.50]Bred:I read that salmon also help keep stream healthy.
[02:13.60]Professor:Right. [02:14.60]Salmon contribute important nutrients like carbon and phosphorus. [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.
[03:12.75]Maria:Why did 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]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]Bred: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 probably eat everything I can catch.
[03:48.80]Professor:Well, certain parts of a 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 as 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 forest is important. [04:28.00] Forest near streams with salmon actually reach maturity faster than other forests.
[04:34.30] Ok, so, why’s all these 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’ve 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.