Years ago, when I first studied academic ecology in the 60's, there was a belief that ecological systems went through a series of changes in species composition (called succession) with each group of species preparing the way for the next. Eventually the system reached a stable state known as the "climax" stage. At this point, species composition remained relatively constant assuming no significant climatic or other change. You could tell which successional stage you were in by looking at the species that occupied the habitat at that time.
As you read this, I will be somewhere in south Florida diving between Ft. Lauderdale and Key West. It's time to return for another visit with my parents. However, first I'll be meeting my dive buddy Andrea for a few days of diving. Then the day she flies back to California; Stephanie, another dive buddy, will fly in from Texas to dive a few days with me. I'm looking forward to being in warm water again and seeing tropical and subtropical critters, as well as visiting with my dive buddies.
As I plan for the upcoming trip, my thoughts return to my previous attempt to dive the Florida Keys. You may remember that I made the mistake of flying in during hurricane season. First Ivan, then Frances hammered the state with forceful winds and crashing waves. The Keys were evacuated and my plans to dive them went up in... a whirlwind. This trip has been carefully planned so it coincides only with the appearance of Andrea and Stephanie, and not with the storm season!
So what does hurricane season have to do with "climax" ecosystems? A massive hurricane is a very large disturbance event in the marine (and land!) environment. Coral reefs that have taken centuries or millennia to grow are devastated within days by the powerful waves. Fish and invertebrates are driven out to sea, or tossed up on dry land. Ecologists now realize that such disturbance events are an important factor in dispelling the "myth" of a climax community.
Disturbance can enter an ecological system in many different ways. It may be caused by large storms, by excessive short-term changes in temperature (such as during an El Nino), or even the appearance of invasive species such as the crown-of-thorns starfish on the coral reefs of the Pacific, or the feral pig on Catalina. The timing and scale of disturbance events may create a patchwork in the affected habitat. Certain areas will remain relatively unaffected and resemble the "climax" community, others will be severely affected and damaged. The areas that survive unimpacted are often referred to as refugia, refuges from the "storm" (or other disturbance event).
One of the ecological issues I looked at in my Ph.D. research was the effect of disturbance on the kelp beds around Catalina. By looking at kelp distribution over a 60 year period, I found that disturbance events differed on our leeward and windward coasts. As might be expected, kelp on the exposed windward side was frequently disturbed by winter storm events. Kelp in the mature forests would be "uprooted" (the holdfasts detached from the rock) and either drift out to sea or be tossed on the sandy beaches. This would create open spaces in the kelp canopy which allowed light to penetrate to the bottom and stimulate growth of the tiny young kelp. This is something like weeding out some of the old growth in a forest of trees, either through selective cutting or a storm event. Disturbance is often a natural and healthy part of the ecological cycles, giving rise to new individuals and genetic material in the system.
On the leeward side, disturbance was caused by a different set of factors. Yes, the occasional nor'easter storm might create the same kind of disturbance event on this protected coast, but these are less frequent. Temperature was more likely to be a cause of disturbance here. A long-term ocean warming trend that began about 1976-77 decimated the kelp in many areas of the leeward side. Here temperatures are somewhat warmer than on the windward side, and any increase in water temperature brought the water closer to the maximum temperature kelp can survive in. Add to this the shorter-term El Nino events over the last few decades, and kelp on the leeward side was "disturbed" (albeit on a less frequent basis) by temperature (thermal) changes.
On my upcoming trip to Florida, I hope to avoid any form of disturbance whatsoever. I'm there to film the marine life and bring home footage to share with my viewers! To ensure such stresses are not a factor while I'm diving with these two lovely ladies, I'm going to make sure I hang the "DO NOT DISTURB" sign on the door knob of the hotel room. That way I may have a better chance of experiencing the climax community... in my dreams! Just teasing folks... they are my friends, not my lovers (sigh).
© 2005 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.
The massive disturbance of a hurricane; smaller scale
disturbance of a warm water event on
Catalina's leeward side; giant kelp holdfast torn loose by storm disturbance;
disturbance on grassy slope in Bulrush Canyon caused by feral pigs.
This document maintained by
Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia