by Tony Fitzpatrick
Abiologist at Washington University and colleagues at the University of California, Davis were recipients of a windfall -- literally -- when Hurricane Lili struck the Bahamas in October 1996, according to an article just released in Science magazine.
Fate had handed them a unique chance to study phenomena and record results that previously only had been hypothesized. In the wake of the storm, the study that had taken them there metamorphosed into a completely different one, dramatically revealing how natural forces periodically play with an ecosystem's populations and tip the so-called "balance of nature."
The scientists published the results of their study in the July 31 issue of Science.
Jonathan Losos, Ph.D., associate professor of biology in Arts and Sciences, and biologists David A. Spiller and Thomas W. Schoener of the University of California, Davis, had just finished taking censuses of lizard and spider populations on 19 tiny islands in the Bahamas when Hurricane Lili hit the area Oct. 19, 1996.
The trio had introduced lizards to the islands in 1993 to conduct an experiment about 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 on the effect of natural catastrophe on island organisms.
Eleven of the islands -- all about one-third the size of an American football field -- were on the southwest side of Great Exuma and directly in the path of Lili's 110-mile-per-hour winds; eight other islands on the northeast of Great Exuma also sustained direct hits after Lili 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 catastrophically hit islands to the southwest, whereas populations of lizards were reduced approximately by one-third and those of spiders nearly 80 percent on the moderately damaged northeastern islands. Vegetation was affected, but to a much smaller degree.
Proving principles
The group found proof of several ecological principles. One is that the recovery rate of different organisms increases significantly with their ability to disperse. For instance, spiders, which produce a silk string to which they cling and get blown into areas by 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.
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 that were bereft of spiders and lizards. These discoveries 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.
"We had data on the island ecosystems for the three years preceding 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 went back to the islands six weeks after the hurricane and at regular intervals to take censuses of populations and observe vegetation regrowth.
"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 added. "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."
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