OFFICIAL31 What is the lecture mainly about?

[00:00.00]Narrator: Listen to part of a lecture in a Marine Biology class.
[00:05.14]Professor: 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.
[00:18.31]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. [00:30.14]The Cot starfish is found on coral reefs in the tropical Pacific Ocean and it eats coral.[00:38.45]Now, in small numbers, the starfish don't affect coral reefs dramatically.
[00:43.14]But periodically, starfish population explodes.[00:46.97] And when that happens, the reefs can become badly damaged or even destroyed, something we are trying very hard to prevent. [00:54.44]For example, during the 1960s, there was an outbreak of CoT starfish in the Great Barrier Reef, off the east coast of Australia. [01:03.16]Luckily, the CoT starfish population gradually declined on its own and the reefs recovered.
[01:09.32]But we were left wondering – what cause the population to increase so suddenly? [01:14.30]Well, over the years, we've come up with a few hypotheses. [01:18.38]All still hotly debated.
[01:20.54]One hypothesis is that it's a natural phenomenon, that the starfish naturally undergo population fluctuations following particularly good spawning years.
[01:30.60]There are also several hypotheses that suggest some sort of human activities are partly responsible, like fishing. [01:38.07]There are fish and snails that eat starfish, particularly the giant triton snail, which is the main predator of the starfish. [01:46.73]These fish and snails have themselves experienced a decline in population because of overfishing by humans. [01:53.75]So with a decline in starfish predators, the starfish population can increase.
[01:58.96]Another hypothesized human-related cause is fertilizer runoff. [02:04.11]People use fertilizer for their crops and plants and a lot of it eventually makes its way from land into the seas. [02:10.93]It's fertilizer, so it has a lot of nutrients.[02:13.81] These nutrients have an effect on the starfish, because they cause an increase in the growth of phytoplankton. [02:20.42]Phytoplankton are microscopic plants that grow in the ocean. [02:24.70]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.
[02:36.88]Now, the final hypothesis has to do with storm events. [02:42.55]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. [02:51.16]So this can cause a kind of mass feeding frenzy.
[02:54.64]So we have ideas, but no real answer.[02:58.26] And because we aren't sure of the causes for starfish population increases, it's difficult to prevent them. [03:04.34]I mean, some progress has been made. [03:06.86]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.[03:15.58]But meaningful progress requires much better evidence about the cause.
[03:20.16]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. [03:29.79]We think that when reefs are damaged, after a few years, the fastest-growing corals repopulate the areas. [03:35.94]And these fast-growing species can grow over the slower-growing species of coral, denying them light and preventing them from recovery.
[03:44.03]However, the faster-growing coral species are the preferred food of the CoT starfish. [03:49.41]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. [03:57.58]So without the outbreak, the diversity of coral would be reduced.