Listen to part of a lecture in a Marine Biology class.

We've been talking about the decline of coral reefs in tropical areas all over the world... um... how natural and man-made stresses are causing them to degrade, and in some cases, to die.

So now let's focus on a specific example of a natural predator that can cause a lot of damage to coral reefs - The Crown of Thorns, or Cot starfish.

The Cot starfish is found on coral reefs in the tropical Pacific Ocean and it eats coral.

Now, in small numbers, the starfish don't affect coral reefs dramatically.

But periodically the starfish population explodes.

And when that happens, the reefs can become badly damaged or even destroyed, something we are trying very hard to prevent.

For example, during the 1960, there was an outbreak of Cot starfish in the Great Barrier Reef, off the east coast of Australia.

Luckily, the Cot starfish population gradually declined on its own and the reefs recovered.

But we were left wondering - what cause the population to increase so suddenly?

Well, over the years, we've come up with a few hypotheses, all still hotly debated.

One hypothesis is that it's a natural phenomenon, that the starfish naturally undergo population fluctuations following particularly good spawning years.

There are also several hypotheses that suggest some sort of human activities are partly responsible, like fishing.

There are fish and snails that eat starfish, particularly the giant triton snail, which is main predator of the starfish.

These fish and snails have themselves experienced a decline in population because of overfishing by humans.

So with a decline in starfish predators, the starfish population can increase.

Another hypothesized human-related cause is fertilizer runoff.

People use fertilizer for their crops and plants and a lot of it eventually makes its way from land into the seas.

It's fertilizer, so it has a lot of nutrients.

These nutrients have an effect on the starfish, because they cause an increase in the growth of phytoplankton.

Phytoplankton are microscopic plants that grow in the ocean.

Larval Cot starfish eat phytoplankton in their first month of life, so more fertilizer in the ocean means more phytoplankton, which means more starfish, bad for the reefs.

Now, the final hypothesis has to do with the storm events.

If some reefs are destroyed by storms starfish populations that inhabited those reefs would have to condense and concentrate on the reefs that are left.

So this can cause a kind of mass feeding frenzy.

So we have ideas, but no real answer.

And because we aren't sure of the causes for starfish population increases, it's difficult to prevent them.

I mean, some progress has been made.

For example, new survey techniques have enabled us to detect population increases when the starfish are quite young, so we can be ready for them.

But meaningful progress requires much better evidence about the cause.

On the bright side, in all the research being done on causes, we have discovered something related to how starfish populations might affect coral reef diversity.

We think that when reefs are damaged, after a few years, the fastest-growing corals repopulate the areas.

And these fast-growing species can grow over the slower-growing species of coral, denying them light and preventing them from recovery.

However, the faster-growing coral species are the preferred food of Cot starfish.

So when an outbreak of Cot starfish occurs, they thin out the fast growing coral and may give the slower ones a chance to reestablish.

So without the outbreak, the diversity of coral would be reduced.