July 31, 1998 ST. LOUIS, Mo., July 31, 1998 -- A biologist at Washington University in St. Louis and colleagues at the University of California, Davis were literal recipients of a natural windfall in October 1996 during Hurricane Lili.
Through a quirk of fate, the biologists saw one study metamorphose into a completely different one that graphically reveals how natural forces periodically play with an ecosystem's populations and tip the so-called "balance of nature."
Jonathan Losos, Ph.D., associate professor of biology at Washington University in St. Louis, and biologists David A. Spiller and Thomas W. Schoener at the University of California, Davis, had just finished censusing lizard and spider populations on 19 tiny islands in the Bahamas when Hurricane Lili hit the area on October 19. The trio had introduced lizards to the islands in 1993 to conduct an experiment, "the effect of predators on island ecosystems." The day after the hurricane blew through the large island of Great Exuma, where they were staying, the biologists quickly took to their boats to re-examine the islands for a suddenly different study, "the effect of natural catastrophe on island organisms." Fate had handed them a marvelously unique chance to record results that previously had only been hypothesized.
The scientists published the results of their study in the July 31, 1998 issue of Science magazine.
Eleven of the islands -- all about one-third the size of an American football field -- experienced 110-mph winds; eight other islands on the northeast of Great Exuma also were directly hit by Lili after it had passed over Great Exuma. Location made a difference in the fate of organisms. Spiders and lizards were completely wiped out and vegetation greatly damaged on the 11 southwest, or catastrophically hit, islands, whereas populations of lizards were reduced approximately by one-third and those of spiders nearly 80 percent on the northeast, or moderately damaged, islands. Vegetation was affected, but to a much smaller degree.
The group found proof of several ecological principles. One is that the recovery rate of different organisms increases strongly with their ability to disperse. For instance, spiders, which produce a silk string to which they cling and get blown into areas courtesy of wind ( a phenomenon called "ballooning"), rebounded quickly on islands where they had been wiped out, unlike lizards, which don't have such "high-tech" dispersal abilities.
Another is that larger organisms, lizards in this case, are more resistant to the immediate impact of moderate disturbance than smaller organisms. On the moderately disturbed islands, lizard populations were less affected by the hurricane than were spider populations, but the spiders rebounded much more quickly because of their more prolific breeding capability.
A third is that the risk of extinction is related to population size when disturbance is moderate but not when it is catastrophic. In relation to this, the biologists uncovered perhaps the first concrete evidence of how hurricanes wreak devastation on low-lying island organisms. It's not the wind so much as the water. The biologists found a starfish on top of one southwest island, and sand deposits on many of the islands, which were bereft of spiders and lizards. This indicated that a tidal surge as high as 15 to 20 feet -- a response to the lower air pressure caused by the hurricane -- inundated the islands, which are about five feet above sea level.
"All of the study islands are within several miles of Great Exuma, which is at most one mile wide," explained Losos. "While hurricanes slow down over land, a mile width is not enough to substantially slow down a hurricane. Thus, the wind speed of the hurricane probably was the same for all of the islands. However, the indications are that the southwest islands were immersed in water for a while. The northeast ones weren't because the size of Great Exuma is substantial enough to halt the impetus of a storm surge. The effect is that with the whole ocean at a higher level for several hours, everything that wasn't stripped away when the surge hit the island was drowned or carried off by water."
Losos said there are several unique aspects to the study.
"We had data on the island ecosystems for the three years preceeding the storm," he said. "Many times scientists go into an ecosystem and study the affects in the aftermath of a disturbance, but they don't know the situation beforehand. Moreover, we had information not just on past populations, but on populations immediately before the event and immediately afterward. We know exactly what effect the hurricane had on the islands because we had been there just days before and then we repeatedly visited the sites in the following months to see how the ecosystem recovered."
The investigators came back to the islands six weeks after the hurricane and during regular intervals up to one year to census populations and observe vegetation regrowth.
"Moreover, it has long been a hypothesis that the reason you don't find these common lizards on the small islands is that hurricanes keep coming in and wiping them out," Losos said "And because lizards don't get from one island to another very readily, once they're wiped out, they don't come back. Well, now that hypothesis is documented."
Over the past 20 years there has been increasing discussion and debate over the role of natural disturbances -- flood, fire, winds -- in structuring ecosystems. The question is: do they play a major, or transient role?
The results of the Spiller, Losos, Schoener study lean toward a conclusion that rare catastrophic events may plan an important role and have a long-lasting effect on an ecosystem's content.
"To my knowledge, this is one of the best documented studies of the effect of catastrophic disturbances," said Losos. "We have before-and-after data, a set of islands that were devastated as opposed to others that were moderately damaged, and multiple islands from which we can deduce general principles. We were very happy that there were no serious injuries on Great Exuma during the hurricane, but we also know how lucky we were to be there when it happened to come up with a study such as this."
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