Now listen to part of a lecture on this topic in an ecology class.
Okay, this happened a few years back. There was this healthy coral reef in the Pacific Ocean. You know how gorgeous coral reefs are with all the red, pink and orange. And it had a huge variety of species, um, of fish and plants, that it protected and provided with food. Then the sea water got polluted because the ships built a large amount of diesel fuel. The pollution caused a species of fish to die out. And this fish that died out used to eat a particular type of ocean plant called algae.

Now, of course, if there aren’t any fish to eat algae, the algae will grow out of control and there will be too much algae which is very bad for coral. But that did not happen to this coral reef. Know why it didn’t happen?

Because there were still many other species left that ate that same algae. So even though the algae increased at first, so did the population of the other fish species who ate the algae, because there was more algae for them to eat. The other species took over for the fish that died out and eventually the coral reef looked about the same as before.

Now think about what would’ve happened to this coral reef if there had not been a wide variety of different fish species. If that one species of fish that ate algae had died off, well, you could imagine soon the whole coral reef would’ve been covered in algae.