In what’s being called a “miraculous conservation success,” critically endangered giant tortoises on the Galapagos island of Española have bounced back from the brink of extinction. Because of historical exploitation, Española tortoise (Chelonoidis hoodensis) numbers in the wild dwindled down to just 15 by the 1960s. Now, nearly 1,000 of them are breeding on their own. The findings were published in PLOS ONE this week.

Between 1963 and 1974, the last of these giants (12 females, 3 males) were taken into captivity. Galapagos National Park Service began releasing their captive-raised offspring in 1975, and by 2007, a total of 1,482 tortoises were reintroduced. As of a few years ago, about half are still alive.

“The population is secure," SUNY’s James Gibbs says in a news release, but the ecosystem they depend on still needs to recover. “Population restoration is one thing but ecological restoration is going to take a lot longer.”

Attempts to restore locally extinct species using captive breeding typically rely on population viability to evaluate success -- but this doesn’t measure the restoration of ecological functions and interactions. Turns out, these tortoises are also ecosystem engineers, like dam-building beavers or herbivore-managing carnivores in Yellowstone. Just by eating plants (then spreading the seeds), these tortoises direct the distribution and abundance of wildlife communities.

Since their reintroduction, the giant tortoises have been restoring some of the ecological damage caused by (now-eradicated) feral goats brought to the island by sailors in the late 19th century. Most of these invaders were killed by gunmen on foot, and high-tech interventions cleared out the last of them in the 1990s, Washington Post reports.

To see how the population is faring now, Gibbs and colleagues analyzed some 40 years of data from marked tortoises who’ve been periodically recaptured for measurements and monitoring. They estimate that 864 tortoises were still alive as of 2007.

However, while stable, the population is unlikely to increase until more of the landscape recovers from a century of goat-inflicted damage. With all the grassy vegetation devoured, more shrubs and small trees grew. Chemical analysis of the soil shows a major shift from grasses to woody plants on the island in the last 100 years.

Not only does this obstruct the movement of animals ranging from the tortoises to the endangered waved albatross, it also hinders the growth of cactus. Their dropped pads are vital to the tortoise diet. What’s more, the goats had even learned to munch on the tallest cacti, eating roots and chewing away at the bark until the cacti toppled -- "an incredible buffet of maybe 500 to 1000 years of cactus growth, demolished in a week or two," Gibbs tells BBC.

Here’s an adult male resting beneath an arboreal prickly pear cactus (Opuntia megasperma) surrounding by woody plants on Española in 2010:

Lonesome George was the last Pinta Island tortoise (Chelonoidis abingdoni). Repeated attempts to mate him with females from closely related species have failed, and he died in 2012. However, Pinta tortoise hybrids were recently discovered on another island, where researchers are hoping to replicate the Española restoration success.

Images: James Gibbs/SUNY-ESF (middle), J.P. Gibbs from 2014 Gibbs et al., PLOS (middle)