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An endangered
population of giant tortoises has recovered
on the Galapagos island of
Espanola. Photo Credit:
James P. Gibbs, SUNY-ESF.
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Some 40 years after the first captive-bred tortoises
were reintroduced to the island by the Galapagos National Park Service, the
endemic Española giant tortoises are reproducing and restoring some of the
ecological damage caused by feral goats that were brought to the island in the
late 19th century.
"The global population was down to just 15
tortoises by the 1960s. Now there are some 1,000 tortoises breeding on their
own. The population is secure. It's a rare example of how biologists and
managers can collaborate to recover a species from the brink of extinction,
" said James P. Gibbs, a professor of vertebrate conservation biology at
the SUNY College of Environmental Science and Forestry (ESF) and lead author of
the paper published in the journal PLOS ONE.
Gibbs and his collaborators assessed the tortoise
population using 40 years of data from tortoises marked and recaptured
repeatedly for measurement and monitoring by members of the Galapagos National
Park Service, Charles Darwin Foundation, and visiting scientists.
But there is another side to the success story: while
the tortoise population is stable, it is not likely to increase until more of
the landscape recovers from the damage inflicted by the now-eradicated goats.
"Population restoration is one thing but
ecological restoration is going to take a lot longer," Gibbs said.
After the goats devoured all the grassy vegetation and
were subsequently removed from the island, more shrubs and small trees have
grown on Española. This hinders both the growth of cactus, which is a vital
piece of a tortoise's diet, and the tortoises' movement. Chemical analysis of
the soil, done by Dr. Mark Teece, an ESF chemistry professor, shows there has
been a pronounced shift from grasses to woody plants on the island in the last
100 years.
The shrubs and trees also inhibit the movements of the
endangered waved albatross that breeds on the island. Gibbs said the plants
make it difficult for the ungainly sea birds to take flight.
"This is a miraculous conservation success
accomplished by the Galapagos National Park Service," said Gibbs, "
but there is yet more work to fully recover the ecosystem upon which the
tortoises and other rare species depend."
Citation
Gibbs JP, Hunter EA, Shoemaker KT, Tapia WH, Cayot LJ. 2014. Demographic Outcomes and
Ecosystem Implications of Giant Tortoise Reintroduction to Española Island,
Galapagos. PLoS ONE, 2014; 9 (10): e110742 DOI: 10.1371/journal.pone.0110742