Sea turtle conservation has too many eggs in one basket.

13/08/2007 00:00:00 Conservation assessments of endangered Caribbean sea turtles are overly optimistic, says Loren McClenachan of the Scripps Institution of Oceanography. Conservation efforts over the last 30 years have significantly helped raise green and hawksbill turtle numbers nesting on protected beaches. However, McClenachan argues that declining turtle numbers on many historically important nesting sites are overlooked by conservation studies that concentrate on a few large remaining nesting sites. The study, ‘Conservation implications of historic sea turtle nesting beach loss,’ appears in the August issue of Frontiers in Ecology and the Environment.

The researchers have created the first maps of historical nesting populations

Hunted for many hundreds of years for food and decoration, turtle numbers have been greatly reduced by humans. Using records from over 160 historic sources in 4 time periods across 20 Caribbean regions, McClenachan and colleagues mapped the historic nesting areas of green and hawksbill turtles, and used density descriptions and harvest data to categorize ‘major’ and ‘minor’ nesting sites. In the past large nesting populations existed throughout the Caribbean. The researchers estimated that there were 59 green turtle nesting sites, and 55 hawksbill turtle sites. Based on their results, 20% of historic nesting beaches have been lost due to land development and turtle exploitation, and another 50% of the remaining beaches have been reduced to dangerously low populations.

‘The loss of even a single nesting site makes a permanent, irreversible dent in the sea turtle population,’ says McClenachan.
The scientists estimate that today’s population of roughly 300,000 turtles is all that is left of an estimated 6.5 million adult turtles that lived on the Cayman Islands in the 1800s, with something like to 91 million green turtles living throughout the Caribbean at that time. For Hawksbill turtles, the researchers estimate that numbers have crashed from 11 million to less than 30,000.

McClenachan and colleagues suggest that the loss of complete nesting sites and nesting populations has compounded the population crash for the turtles, and see the reestablishment of lost sites as ‘extremely unlikely.’ Their research points to hunting having destroyed 24 nesting sites and also severely reducing turtle numbers at the remaining beaches.

Impact on Ecosystems
Aside from the loss of the turtles themselves, sea turtles are also ecosystem engineers in the Caribbean, shaping the environment in which they lived.

According to the researchers, ‘The ecological extinction of green turtles transformed an ecosystem with diverse species of seagrass dominated by large herbivores into a detritus-based ecosystem dominated by overgrown monocultures of one species of grass.’
Green turtles predominantly eat turtle grass, while hawksbill turtles mostly eat marine sponges.

‘The decline in green turtles has led to a loss of productivity available to the animal food chain – including commercial reef fishes – reducing the protein-rich food available for the Caribbean people,’ say the researchers.

When hawksbill numbers were larger they ate more toxic sponges. Results now suggest that the turtles are eating more non-toxic sponges. The changing ratio of sponge species will ultimately affect the landscape of coral reefs.

The scientists say more protection is needed for all the sea turtle nesting sites.

‘Protecting more nesting beaches is not a politically or socially simple endeavour, but it is the only way to avoid the risk of putting all turtle eggs in a very few baskets.’

Conservation implications of historic sea turtle nesting beach loss
Loren McClenachan, Jeremy Jackson and Marah Newman (Scripps Institution of Oceanography)
Frontiers in Ecology and the Environment 2006; 4(6): 290-296