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An endangered giant tortoise population is finally stable, thanks to some aggressive goat eradication

An Espanola tortoise. (James Gibbs)

After dwindling to a population of only 15, the tortoises of the Galapagos island of Española have reached stable levels. A study published Tuesday in PLOS ONE reports that efforts to reintroduce the species have been successful, with some 1,000 tortoises now breeding without human help.

This is the result of a conservation effort that started 40 years ago, when those 15 remaining Española tortoises were captured and bred in captivity. Their 2,000 offspring were released on the island in 1975, but this is the first comprehensive look at how the reintroduced tortoises have fared.

"To me, it's been a quiet but remarkable success story in conservation, mostly because nobody's ever taken the time to synthesize the results of this massive effort," said James P. Gibbs, a professor of vertebrate conservation biology at the SUNY College of Environmental Science and Forestry and lead author of the paper.

Gibbs and his colleagues crunched 40 years worth of data and found that half of the 2,000 tortoises dropped in the 1970s are still alive. With 1,000 tortoises now living and breeding on their own, he said, "we can step out of the process and let them take care of themselves."

The success of the effort, which was lead by the Galapagos National Park Service, owes an awful lot to a dedicated team of goat exterminators. To understand why, it helps to know a bit of the region's history: Sailors used to catch and eat the giant tortoises, which proved to be easy prey. They also left goats behind on the Galapagos islands — an animal not native to the area — which created a devastating invasive species problem.

After 80 years of goats chowing down on the islands' vegetation, many parts of the Galapagos had lost their natural ecosystems — and the tortoise populations that had once been such a vital component of them.

"These goats were destroying literally everything, turning [Española] into a dust bowl," Gibbs said. Most of the goat invaders were killed by gunmen on foot, but some high-tech interventions cleared out the last of them from the islands in the '90s.

Without that Herculean goat-killing effort, the tortoises wouldn't have had a chance. But Gibbs credits the tortoises themselves for their success, too.

"They're hard-wired, tough reptiles," Gibbs said. "They don't need to learn how to survive from their parents — you can just release them, and they know what to do."

Next year, the small island of Santa Fe — one that's lost all of its tortoises — will become host to 200 Española tortoises. "This success is going to beget another success on Santa Fe," Gibbs said.

Unfortunately, while Española's tortoises will continue to thrive without our help, they're not going to be able to magically restore theses islands to their pre-human condition. The invading goats destroyed a lot of the island's cacti, which tortoises use for food and shade, and woody vegetation thrived in its place. Gibbs and his colleagues found that the island's cactus population is still low, though the tortoises seem to be helping it along.

"The tortoises will eventually recover everything themselves, but it's going to take a very long time," Gibbs said. If the Galapagos National Park Service wants the island restored fully in less than a few hundred years, they may need to go in and clear out some of the woody vegetation themselves.

And the success of  Española provides a sad contrast to Pinta, another island in the Galapagos. Pinta tortoises and Española tortoises are closely related, but not the same species. Lonesome George was the last of Pinta's kind, and he died in captivity while researchers tried desperately to breed him. Now researchers are hoping to replicate their Española restoration using Pinta tortoise hybrids that have been found on another island.

It wouldn't have taken much for Española to go the way of Pinta. When the Galapagos National Park Service was founded, the Española tortoise was already classified as extinct. If they'd waited any longer before going looking for survivors, they probably wouldn't have found the 15 they did — and the fewer the tortoises, the less likely they'll breed successfully in captivity.

"There's only so much you can do with one tortoise," Gibbs said.