ATHENS, Ga. -- A new study by wildlife researchers at the University of Georgia could mean better protection for endangered sea turtles along the Atlantic Coast. The research is the first to show that wire screens secured over the buried nests are the most effective way to keep out raccoons, the primary predator of eggs layed by the majestic but threatened sea turtle.
Scientists from UGA's Warnell School of Forest Resources found that screening nests is more effective than routine killing of raccoons at turtle-nest sites, a method wildlife managers have tried with mixed results. Still, the complex threats to sea turtles and the high costs of nest screening could slow widespread use of the technique.
"Under natural conditions, sea turtle nests should exist in such concentrations that natural predators like raccoons wouldn't decimate populations," said Dr. Robert J. Warren, a wildlife ecologist at UGA. "But human development along the coast has removed so much of the nesting environment that every nest is critical."
The study, conducted at Florida's Canaveral National Seashore and published in the April 1997 issue of the Journal of Wildlife Management, was the first to compare lethal control of raccoons with alternatives like nest screening.
A practical solution is urgent. The Loggerhead Turtle Recovery Plan mandates that lands within the National Park Service achieve a 60 percent sea turtle hatching rate. And while lethal control of raccoons is effective and permanent, both managers and researchers worry that killing off predators could upset the natural balance in fragile coastal ecosystems.
"An ecosystem approach [to wildlife management] includes the preservation of all native species, including raccoons," said Warren. "We know the removal of predators can have lasting secondary effects beyond the desired effects."
In fact, Warren said the coastal ecosystem has already been disrupted by the elimination of red wolves and pathers, which once preyed on raccoons.
Researchers also looked at conditioned taste aversion, which involves the use of estrogen-spiked chicken eggs designed to "sour" raccoons on turtle eggs. The method has shown promise at other sites, but was not effective in this study, "because the raccoon population on Canaveral National Seashore is so high, it simply isn't feasible," according to Warren.
Though slightly more expensive and labor-intensive than lethal control, nest screens were the only method to control wide-spread nest depredation. The 3 ft.x 3 ft. mesh screens have openings about 2 inches x 9 inches -- small enough to discourage raccoons, but large enough to let turtle hatchlings emerge. Researchers stretched the screens over the nests, securing them with 3 ft. rebar pounded into the sand.
Nests in the lethal removal and conditioned taste aversion trials were four times more likely to be plundered by raccoons than screened nests.
To verify that raccoons are the problem, researchers set up cameras at 10 random nest sites on the island. Seventy-four photographs, tripped with a wire, revealed nest looting by raccoons (80 percent), but also to a lesser degree by crows, bobcats, rats and opossums.
While screening in this study kept turtle egg losses to less than 8 percent, earlier trials were far less successful, with losses between 32 and 64 percent. Warren and colleagues Mary Ratnaswamy, Monique Kramer and Michael Adam showed the effectiveness of nest screens depends largely on the effort expended.
Still, nest screening deprives raccoons of a natural food source, which researchers say could shift their feeding behavior to other prey. And screening, while effective in protected areas like Canaveral National Seashore where raccoons thrive, may not be a good management option at other sites.
"Different beaches have ecological factors that influence [sea turtle] mortality and raccoon feeding behavior," said Warren. "Costly screening programs should not be implemented before determining whether or not raccoons contribute significantly to that mortality."
Journal
Journal of Wildlife Management