DAR ES SALAAM, Tanzania and NEW YORK — When it came to a choice between a hydropower dam that would provide electricity to one-third of Tanzania and a tiny endangered toad, the dam won.

But the Kihansi toad lives on thanks to the efforts of Tanzania and two American zoos.

One hundred of the critically endangered Kihansi spray toads, bred in laboratories in the United States, were returned to Tanzania last week where they will live in the area for the first time in 10 years.

The return of the rare toads to their homeland is a significant development in an expensive and complicated conservation effort that has spanned nearly 15 years, beginning with the discovery of the toads in 1996.

Biologists found the new species while conducting an environmental impact assessment of a hydropower project, financed by the World Bank and the development agencies of Norway, Sweden, and Germany. They quickly realized that the Kihansi spray toad existed in one of the world’s smallest endemic habitats — the spray zone of a single waterfall in the Udzungwa Mountains of southern Tanzania.

About the size of nickel or a quarter, the spray toads are also unique in that they give birth to fully developed babies rather than laying eggs which then go through a tadpole stage.

Concerns about the effect of the hydropower generators on the toads were raised by biologists and international conservation groups, but the project was not stopped. The plant is expected to provide 180 megawatts of electricity to the country.

“Really that’s only enough for half a borough of New York City. In Tanzania, it’s enough to provide one third of the electricity in the entire country,” said William Newmark, a prominent American conservationist and environmental consultant to the World Bank on Kihansi.

Currently 10 percent of Tanzania’s population has access to power, though the number falls to 2 percent in rural areas.

When the hydropower plant was commissioned in 2000, it significantly reduced the toads’ habitat.

“At that stage it was obvious that if something wasn’t done fast the chances were they were going to be extinct in no time,” said Peter Hawkes, a South African biologist who was involved in the environmental impact assessment. “Just about all of the frogs that were alive were clustered around the base of the falls wherever they could get wet.”

Although artificial irrigation systems designed to mimic the spray of the falls helped to replenish the toad population over the next two years, it was then depleted over a two week period by an outbreak of the amphibian disease known as chytrid fungus, which has severely diminished affected frog populations around the globe, particularly in Central and South America.

Luckily, in November 2000, American biologists travelled to Tanzania with the government’s permission and airlifted 500 of the toads out of the Kihansi gorge, bringing them back to the Bronx Zoo and the Toledo Zoo to be kept in bio-secure “arks.”

“When the captive population was transferred to the States, there were considerable problems maintaining them, due to parasites that the population was housing,” said Newmark. At one point, the world population of spray toads was down to approximately 70 before zookeepers found ways to mitigate disease and improve their artificial habitat through ultra-violet lights and water purification systems.

“I remember when we had like 12 and now we have almost 4,000,” said Timothy Herman, a zookeeper at the Toledo Zoo who has worked for years with the spray toads. “It’s been quite a ride.”

In a twist of fortune, today the Kihansi spray toad constitutes one of the largest populations of a critically endangered species on the globe. Plans to return a small sample of the toads in America back to Tanzania have been in the works for several years and a group of international conservationists, population management experts, herpetologists, and zookeepers have met in Dar es Salaam twice to carefully plan every aspect of the reintroduction..

After flying to Tanzania last week accompanied by zookeeper Alyssa Borek of the Bronx Zoo, the spray toads were sent to a bio-secure facility at the University of Dar es Salaam campus where a team will monitor them carefully. The hope is that in a few years time, a population of toads can be returned to their original habitat, though no one is sure if they will be able to survive in the wild anymore.

“I’m hoping that this is going to be a very positive thing to get the toads back,” said Borek. “Having them there, having them on the grounds of the university will allow people to see them and become real, not just something that is costing a lot of money.”

Anna Maembe, a senior staff member of Tanzania’s National Environmental Management Council, said she believes the Kihansi spray toad has led to greater capacity within the country to deal with possible future environmental crises as the country develops.

“Even the construction of the facility at UDSM is one of the indicators that we have learned a lot from the discovery of this small toad,” said Maembe. “I would like to say that we might look at this as just a tiny toad but it has led to the growth of scientific knowledge in Tanzania and elsewhere.”