Posted by seahunt on January 13, 2003 at 14:48:00:
In Reply to: Boring Subject "Sea Urchin" posted by Steve on January 13, 2003 at 13:22:18:
Not everything is conclusive on this and it is complicated by the fact that there are different urchin species in the equation and many different species of kelps effected.
This issue started at Palos Verdes when it was believed that the purple urchins, Strongylocentrotus purperatus, were destroying the macrocystis kelp beds (brown algaes). As far as I know, ultimately, the culprit reducing the kelp beds was believed to be pollution.
Still.... When you see the occasional massive purple urchin populations and realize that their predators (especially sheepheads and otters) have been more than decimated, you have to wonder if they are actually limiting recovery of the kelp, let alone smaller red and green algae. (But what about Picnopodia. They are still there and healthy. How many urchins do they eat?) Really, looking at everything, I suspect that urchins are not the whole problem and maybe not most of it. Consider when El Nino comes along and the kelp is basically gone. It then recovers, regardless of the urchins, but never to what it was 30 years ago. That suggests that it is an environmental problem more than just urchins. Also, you can go places where there are a zillion urchins that you can see are eating the kelp, but it is still there.
Then again, in terms of red urchins, Stronglylocentrotus franciscanus, they used to coat the bottom, but after the urchin industry started harvesting, they are reduced to maybe 5% of their former population, yet the kelp density hasn't changed.
Then there are the urchin barrens that no one has figured out (as far as I know so far). I will break purple urchins in urchin barrens as I swim.
Look at frontside Catalina with it's thick bottom kelp. It isn't heavily attacked by urchins. Then look at the west end of San Nicolas Island. There are so many small urchins you can't believe it and if a kelp leaf hits the rocks, it is immediatly consumed and there are none of the smaller leafy kelps. Still, the urchins do not eat or climb the kelp (brown algae) stipes.
In the central coast where there are no urchins to speak of, the large kelp (brown algaes) is relatively (considering water temp and some other factors) the same as in the south where there are lots of urchins, but the smaller bottom kelps, reds and greens, are far thicker.
Really, I can't say what all the answers to your question are, but I suspect that the urchins are less of a problem for the larger kelps than are environmental factors, but have more of an effect on the smaller, leafy bottom kelps. You asked about the kelp forest, which implies Macrocystis. I don't think the urchins dominate them like they do the smaller leafy kelps (reds and greens). Still, when the bigger kelps are stressed by other environmental factors, the urchins may finish the job.
I'm fairly current, but not completely, so the data may be out there, but I suspect not.
Enjoy the diving, seahunt