Impact-induced (hammer smashing) population explosion

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Posted by Wayne on January 14, 2003 at 20:40:01:

In Reply to: Re: Hmmmmm... posted by Steve on January 14, 2003 at 11:08:48:

My understanding of the legend about the impact induced reproductive acceleration goes like this.

Most of this is based on the PURP (Palos Verdes Underwater Restoration Project) in which divers descended on PV with Hammers in the '70s. The idea was to cause a significant temporary reduction in the total number of urchins to allow the ecology to go to its so called "natural" state. It caused a population explosion instead of a decline. I know of two versions of the explanation of the unanticipated and unintended outcome.

Version # 1. When the urchins were smashed, there was an unprecidented amount of eggs and sperm released into the water. The fish could not keep up with the amount of this food being released by hundreds of divers over an period of a couple of days. This caused an unusually high amount of fertilized eggs to survive which created a spike in the population. Therefore, killing an urchin and feeding it immediately to a garibaldi does no harm.

Version # 2. The mass killing of all those urchins caused a natural species-survival response that caused all urchins to immediately spawn. Many marine animals will emit a chemical trigger when they are killed. An example that I know of is the conch. When you kill one, the trigger is sent into the ocean and all nearby (close enough to be affected by the chemical) conchs will broadcast their sperm/eggs into the sea to ensure that reproduction happens, even though there might be a preditor that might quickly clean out a local adult population. Those silly broadcast-spawners have to have some way to synchronise, and some seem to trigger on distress of a fellow critter.

My personal favorite is version # 2. Some Kelp Protectors get rid of urchins in areas where they are planting and/or researching the kelp forests. The ones I know do not ever smash them because of the risk of massive repopulation. Instead they gather them into nets and move them out to sea. There they will either find something else to eat, or more likely starve to death without triggering mass reproduction.

Since some critters naturally eat urchins and do not trigger population explosions, I think that a single urchin being eaten by a garibaldi does not trigger the orgy. Based on that, I still occasionally feed the fish. But clearly there is enough annecdotal evidence of the problems associated with mass extermination.


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