Saving Tiny Toads Without a Home
This is a story about a waterfall, the World Bank and 4,000 homeless toads.
Maybe the story will have a happy ending, and the bright-golden spray toads, each so small it could easily sit on a dime, will return to the African gorge where they once lived, in the spray of a waterfall on the Kihansi River in Tanzania.
The river is dammed now, courtesy of the bank. The waterfall is 10 percent of what it was. And the toads are now extinct in the wild.
But 4,000 of them live in the Bronx and Toledo, Ohio, where scientists at the Wildlife Conservation Society and the Toledo Zoo are keeping them alive in hopes, somehow, of returning them to the wild. This month, the Bronx Zoo will formally open a small exhibit displaying the toads in its Reptile House.
Meanwhile, though, the toads embody the larger conflicts between conservation and economic development and the complexity of trying to preserve and restore endangered species to the wild. Their story also raises questions about how much effort should go to save any one species.
These issues are particularly pressing for frogs, toads and other amphibians, whose populations are plunging worldwide in the face of factors like habitat loss, climate change and disease. Jennifer B. Pramuk, the curator of herpetology at the Bronx Zoo. said at least 120 species vanished in recent years.
“It’s probably much higher than that,” said Dr. Pramuk, a leader in the toad effort. “There are areas of South America where all the amphibious fauna are wiped out.”
The spray toads, Nectophrynoides asperginis, were unknown to science until 1998, when they were found living on less than five acres, perhaps the smallest known range of any vertebrate. They are unusual in that they do not lay eggs. The baby toads emerge fully formed, each one small enough to fit on the head of a pin.
When the toads were first described, as many as 20,000 lived in the misty waterfall tract on the Kihansi, climbing mossy plants and feeding on small insects. But the government of Tanzania, with a loan from the World Bank, was already planning a dam upstream.
When the dam opened in 2000, the flow of water to the dam fell by 90 percent, and mist-dependent native plants gave way to invasive species. Within months, the toad population plummeted. When the survivors contracted a fungal disease called chytrid, the toad population fell again.
The species was in imminent danger of disappearing. So the conservation society responded by sending in Jason Serle, a wild-animal keeper at the time, and Tim Davenport, a field programs director in Tanzania. Along with Tanzanian scientists and conservation officials, they spent a day at the gorge, collecting 499 toads and putting them in plastic bags with damp moss. The bags were placed into coolers for the flight back to the Bronx.
“It was get on the plane, collect them, get back,” said Jim Breheny, the director of the Bronx Zoo.
The problem then was how to keep them alive. The Bronx Zoo sent toads to five other zoos in the United States, but only one of them, the Toledo Zoo, managed to keep them alive, as did the Bronx Zoo.
“No one had kept anything in that genus in captivity,” Dr. Pramuk said. “It was very difficult for us to figure out what they needed.”
The crucial factors, not surprisingly, turned out to be water, light and food — very carefully prepared water, light and food.
Jason Wagner, a life-support specialist at the Bronx Zoo, assembled a system of tanks, pipes, filters, aerating vats and other equipment in the warm damp behind the scenes in the reptile house. The system produces 1,500 gallons a day of pure mist to be sprayed into the toad tanks. The system is necessary because the treatments that help make city water safe for people would be lethal for the toads.
Halogen bulbs provided the best light; the Toledo Zoo figured that out. And Alyssa Borek, a zookeeper in the Bronx, produced a safe food supply by breeding tiny bugs like fruit flies, wood lice and weevils in plastic shoeboxes and other containers filled with cocoa matting, beans and alder leaves that she gathers on the zoo grounds.
Ms. Borek raises the insects for several generations to make sure they are disease-free before she feeds them to the toads, who, except for the 60 or so on exhibit, live in 26 aquarium tanks in two clean rooms at the zoo. Even so, she said, an outbreak of chytrid in one of her tanks killed half of that population within days. The rest died in less than a week, she said, “even with aggressive treatment by our veterinary staff.” She still does not know how the disease erupted.
Ms. Borek also called in zoo vets to perform a “C-section” when a pregnant toad died. The babies, delivered from their dead mother’s eggs, were born as tadpoles. Ms. Borek kept them in petri dishes, but after a few weeks they too had died.
The overall effort, however, was a success. By trial and error, the zoos kept the spray toads alive.
Ms. Borek learned so much that she wrote a husbandry guide for the species; Dr. Pramuk said it would be useful for anyone raising frogs or toads. In fact, working with the Association of Zoos and Aquariums, the Bronx and Toledo Zoos will offer their third course on toad husbandry at the Toledo Zoo in April.
As the effort of raising the toads in the zoos progressed, their numbers in Tanzania declined until last November, when the International Union for Conservation of Nature and Natural Resources, which maintains listings of endangered species worldwide, declared the toad extinct in the wild.
That finding presented the next hurdle: reintroducing the toads to the wild.
There is “at least the potential for a viable restoration program,” Mr. Breheny, the Bronx Zoo director, said, but a lot depends on conditions and the operation of the dam. The World Bank has established an artificial mist system there, and workers have dug out invasive plants, but it is unknown whether these efforts will be enough.
The scientists said they did not blame the dam-builders for all this trouble. “Tanzania is a real poor country and they needed a source of electricity,” Dr. Pramuk said. “When people weighed their options, it was for them an easy decision.”
Nonetheless, Dr. Pramuk is not troubled by the money spent to preserve the toad — “I’m guessing it’s in the millions,” she said.
“Either you lose the species or you do something about it,” she said.
Dr. Pramuk said efforts to breed other amphibians in captivity and reintroduce them had met with some success, with 13 of 21 species reintroduced into the wild breeding for multiple generations. Of the rest, five showed some breeding, and three have at least survived after being released.
So the rescue work proceeds. The toads destined for Tanzania must be screened, to make sure they will not bring alien pathogens with them. Meanwhile, scientists at the zoos and the University of Dar es Salaam are developing ways to keep the reintroduced toads in pens in the gorge to track their mortality and monitor their reaction to their new environment.
Dr. Pramuk said researchers would gather in Tanzania later this month to develop guidelines for this work with colleagues at the University of Dar es Salaam and the Sokoine University of Agriculture. And Mr. Breheny said that if all went well, the conservation society would begin returning the spray toads to their Tanzanian home this year.
The society will use knowledge gained in the process in efforts to sustain threatened amphibians elsewhere. “Amphibians tend to be small, they produce a lot of offspring and generally have a short generation time,” Mr. Breheny said. “We can raise them in small spaces and get numbers up and consider restoration if the environment is safe for them.”
But they do not want to move too fast. “We don’t want to lose it on the last leg of the journey,” he said.
Mr. Breheny conceded that given the small number of spray toads, their minuscule range and their extreme vulnerability to environmental disturbance, it was possible that the researchers might one day have to conclude that returning them to the wild was impossible. Then they would have to evaluate the merits of continuing to keep them alive at all.
“We have talked about it,” he said. “What would be the point of maintaining these toads if there was no hope of restoring them to the wild? We don’t know if you would maintain them in that situation. But right now. ...”
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