Kihansi spray toads bred at the Wildlife Conservation Society’s Bronx Zoo. (JULIE LARSEN MAHER/Wildlife Conservation Society.)

Two American zoos plan to release hundreds of Kihansi spray toads back into their native habitat in Tanzania this week, in what would mark a major comeback for a species declared extinct in the wild three years ago.

The Wildlife Conservation Society’s Bronx Zoo and the Toledo Zoo in Ohio have spent the past dozen years working with the government of Tanzania and the World Bank to breed the tiny animals in captivity after a major hydropower dam diminished the toad’s habitat.

Scientists discovered the species in 1996 living in an area spanning just five acres, bathed in the mist of nearby waterfalls in the Kihansi Gorge. A hydroelectric dam financed by the World Bank reduced the size of the Kihansi falls by 90 percent, reducing the spray on which the toads thrived.

The toad was last seen in the wild in 2004, and in 2009, the International Union for the Conservation of Nature declared it extinct in the wild. Researchers believe a combination of habitat loss and the spread of the chytrid fungus, which now threatens amphibian species, drove the species extinct.

As the population began to plummet, WCS scientists and Tanzanian officials collected an “assurance colony” of 499 toads from the gorge. While several American zoos tried to raise the animals in captivity, only two were able to consistently breed and care for them.

In 2010, 100 Kihansi spray toads returned to Tanzania, where the University of Dar es Salaam established another assurance population.

The reintroduction of the Kihansi spray toad to the Kihansi Gorge in Tanzania is a momentous achievement in conservation. It clearly shows how zoos play an important role in conservation,” said Cristian Samper, president and chief executive of WCS. “This project shows that through partnership and science, wildlife can prevail.”

Juliet Eilperin