by C. Mlot
In 1996, biologists counted 2,600 manatees lolling in the coastal waters of Florida -- a record number. They also counted a record number of carcasses: 415, including about 150 poisoned after the bloom of a toxin-producing marine microbe, or red tide, last spring.
This year, after the mild winter and fewer reported carcasses, researchers expected to find an even bigger population. Instead, the aerial count of the endangered animals dropped to 2,229.
The difficulties of counting the elusive manatees have prompted biologists to use computer models to fashion a picture of the population's dynamics. One such model, described in the April Conservation Biology, is the first to project the animals' future in the long haul. If birth and death rates and other conditions hold steady, the population should remain stable over the 1,000-year time frame of the model. Just a 10 percent increase in the usual death rate of about 150 per year, however, could gradually eliminate manatees in the United States.
That translates into only about one additional death per year in each of 13 key counties in the state, says zoologist Thomas J. O'Shea of the Biological Resources Division of the U.S. Geological Survey in Fort Collins, Colo. At that rate, "the cumulative impact could be extinction." O'Shea developed the analysis with Miriam Marmontel and Stephen R. Humphrey of the University of Florida in Gainesville.
They used data on the age and reproductive status of about 1,200 carcasses recovered between 1976 and 1991. The model then generated a pattern of growth for the population. By altering birth or death rates and factoring in hard-to-predict events like hurricanes or red tides, the researchers projected what might happen to the manatees under a variety of conditions.
"There's a certain amount of guesswork involved," says biologist Chip Deutsch of USGS' manatee research program in Gainesville. More important than the model's absolute numbers or dates, he says, is its identification of the factors that have the biggest effect on the direction of the population changes. Death rates among adults turn out to drive the changes, and accidents involving motorboats or other waterway contraptions account for about half of these deaths.
Despite the uncertainties in predicting natural catastrophes, one thing is clear about the Florida manatees' future. They will be jostled by an increasing population of people and boats. The problem of collisions with manatees "is only going to get worse," says Deutsch. The quality of the manatees' habitat will undoubtedly be at risk as well.
O'Shea notes that the manatees can survive, provided motorboat speed limits and other conservation efforts in the key counties stay in place and are enforced.
Maromontel, M., S.R. Humphrey, and T.J. O'Shea. 1997. Population viability analysis of the Florida manatee (Trichechus manatus latirostris), 1976-1991. Conservation Biology 11(April):1.
O'Shea, T.J., B.B. Ackerman, and H.F. Percival, eds. 1995. Population Biology of the Florida Manatee. U.S. Department of the Interior National Biological Service Information and Technology Report 1. Washington, D.C.
Biological Resources Division
412 N.E. 16th Avenue
Gainesville, FL 32601
Thomas J. O'Shea
Biological Resources Division
U.S. Geological Survey
4512 McMurry Avenue
Fort Collins, CO 80525
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