Toad on brink of extinction, scientists race to study amphibian for bioactive compounds
June 29, 2005
Wildlife Conservation Society works to save species.
Under the bright florescent lights of the reptile house in the Bronx Zoo of New York, a colorful exotic toad makes its final stand. Once gathering by the thousands at the waterfalls of the Kihansi Gorge of Tanzania, the population of the Kihansi Spray Toad now stands at less than 200 individuals. The hasty construction of a desperately needed dam, built with good intentions by the World Bank, has relegated this species to the edge of existence.
A decade ago the Kihansi Spray Toad thrived in its thoroughly unique habitat, the waterfalls of the Kihansi River, part of ecosystem that is one of only 25 Global Biodiversity Hotspots on the planet (Hotspots are regions noted for their extensive range of species in a very small area). The gorge is located in the Southern Udzungwa Mountains of South Central Tanzania, which possess the greatest biodiversity in all of Tanzania.
Prior to the construction of the dam, the Kihansi River descended 700 meters through the gorge in a spectacular series of cascades. The spray from the falls maintained an almost constant temperature and humidity to the area and its specifically adapted inhabitants including the Kihansi Spray Toad.
The Kihansi Spray Toad, Nectophrynoides asperginis, is a dwarf toad, with adults reaching no more than three quarters of an inch long. The diminutive, mustard-colored amphibian notably bypasses the tadpole stage of development and gives birth the purple-hued toadlets.
That the Kihansi Spray Toad possesses this particular developmental feature — though not exclusive to the species — may or be a function of the ephemeral nature of its environment. The Kihansi Gorge environment is so specialized that the toad has not been found in any surrounding wetlands of gorges. The toad had one of the smallest geographic ranges — approximately two hectares — of any four-legged vertebrate species in the world. Due to the unique conditions of the spray zone habitat, other endemic species have made adaptations to survive. Another local amphibian, the Torrent Frog, has modified suction mouth parts that enable it to cling to the slick rocks at the base of the falls.
Trouble for the toad, then unknown to science, began in the mid 1980s when the Tanzanian power authority took notice of the falls and its potential as a significant power source for the energy-strapped region. In July of 1994, the Tanzanian government began construction of the 180 mega-watt Lower Kihansi Hydropower Project (LKHP) in order to meet growing electricity demands from mining and tourist industries. The $275 million project was jointly funded by the World Bank and several international development agencies. A year into construction, researchers carrying out the first environmental survey discovered the Kihansi spray toad along with two endemic plant species.
Wild Kihansi Spray Toad in the gorge. Today, the Kihansi Spray Toad may now be extinct in the wild.
PHOTO BY TIM DAVENPORT / WCS.
Despite the findings, construction continued uninterrupted and late in 1999, dam operators began diverting water to produce badly needed electricity. Within six months the original flow of the Kihansi River was reduced to 25 percent, having an immediate impact on the Kihansi Spray Toad. In the absence of the spray, the critically endangered amphibian and at least two endangered plant species, including a type of wild coffee that grew only in the waterfall spray zone, suddenly sat on the brink of extinction.
The decreased river flow caused the spray zone wetlands to dry out and Kihansi Spray Toads congregated by the tens of thousands on rocks at the base of the falls where the minimal bypass flow created a negligible amount of mist. Over 90 percent of the gorge’s unique spray zone habitat was destroyed and the toad counts began to plummet.
To address the collapsing toad population, from July 2000 to March 2001, a sprinkler system was installed over a limited section of the original spray zone wetlands to simulate the cascade’s former mist, but the system suffered chronic clogging of sprinkler heads from river silt. In the fall of 2000, more aggressive efforts to rescue the toads commenced, primarily through an ex situ or captive breeding program, developed and led by the Wildlife Conservation Society (WCS), a consortium of zoos in the United States. After finalizing an agreement with the Tanzanian Ministry of Natural Resources and Tourism in late November 2001, WCS reptile keeper Jason Searle collected 500 toads which were transported to the Bronx Zoo in New York. 238 of the animals were then transferred to the Detroit Zoo’s National Amphibian Conservation Center.
The captive breeding program immediately suffered a series of setbacks and over the next six months, the newly captive wild-caught toads fell prey to lungworm infections. By May 2001, the original 500 member captive toad population had dropped to a scant 124 amphibians. Meanwhile, the wild toad population fell from an estimated 17,000 to fewer than 2,000 animals, suffering from the loss of the precious mist that provided ecosystem stability and afforded protection from predatory safari ants.
By the end of 2001, things started to improve for the captive populations of Kihansi Spray Toads. After a scientist from the Detroit Zoo unit devised an effective treatment to control the lungworms, live births increased and by mid-2002, the Detroit Zoo was inundated with toad offspring.
Researchers at Cornell
The zoo tried to send surplus toads to other institutions but failed to get permission from the Tanzanian government. Keepers turned off the misters to curtail breeding and the captive population fell back to carrying capacity and then below as toads began to suffer from a range of ills that are common in captive toads, including metabolic bone disease, hypovitaminosis A, short tongue syndrome (associated with the hypovitaminosis), bacterial sepsis and chronic bloating linked to renal disease. Intensifying the problem, the Kihansi Spray Toads proved to breed best in large groups, so as the population dropped, so did the odds for successful breeding. As of spring 2004, the captive toad population was at 70.
Back in Tanzania, the wild Kihansi Spray Toad population had its own ups and downs. In late June 2003, with the implementation of an improved sprinkler system in the Kihansi Gorge, the wild toad population had climbed back to near 20,000 individuals. Then in July the population was hit by the devastating introduction of chytrid fungus, the same pathogen that has been decimating amphibian populations around the world. The population collapsed to a mere 40 toads in early July 2003. Recent reports back from the gorge all but suggest the toad is extinct outside of captivity. The outlook for potential wild survivors is not good: once the chytrid fungus arrives in an area, it never leaves. The only way to treat the fungus, which kills amphibians by invading layers of their skin, is with medicinal baths for individual toads — there is no way to bathe an entire ecosystem.
No one knows the origin of chytrid fungus in Tanzania — the fungus had never previously been documented in Tanzania before the dam’s construction. There were three exotic frog species accidentally introduced during construction, as well as trout, rumored to have been introduced in the Kihansi River upstream from the dam. Both are possible candidates for bringing the fungus. More likely however, the pathogen stowed away on an inadequately sterilized boot of a visiting biologist who had come from an infected area. The project employed biologists from both Australia and South Africa, countries with serious outbreaks of the fungus.
Attempts to revive the species through controlled releases of water through the dam have probably done more harm than good. The sudden increase in water flow during the summer of 2003 may well have swept away remaining toads and washed down trace amounts of deldrine, a pesticide used in agricultural areas above the dam, that may have contaminated the toad habitat.
Captive populations of the Kihansi Spray Toad have rebounded in the past few months. At the time of my visit to the Bronx zoo in May 2005 there were 52 toads — 15 males, 16 females, and 21 unidentified juvenile frogs, while another 160 or so reside at the Toledo Zoo in Ohio, where the toads were moved after their stay in Detroit. There have been efforts to preserve the species by creating living cell-lines of the toads’ genetic material. Using new technologies to extract cells from recently deceased animals and establish living cell lines, Coriell Institute scientists aimed to preserve cellular matter for possible cloning in the future and eventual release of toads into the wild — assuming there is habitat for them. Unfortunately, in the disaster-stricken odyssey of the Kihansi Spray Toad, these well intentioned efforts have paid few dividends and all the cell lines died out through bacterial infection. Currently, the Bronx and Toledo facilities are functioning as literal arks for the species because recent surveys in the original habitat have showed dismal results. The last sighting of any toads in the gorge was last spring and it seems unlikely that there will ever be a place in the wild for these creatures again. In a goodwill gesture, the Tanzanian government recently gave $30,000 to aid in funding captive programs at the breeding institutions and has pledged another $60,000. This money ultimately comes from the World Bank which is now paying for the ecological damage created by the dam which currently supplies 25 percent of the country’s power demand and generates $63 million a year in revenue.
With this sad fate in prospect, many groups have been quick to lay blame on various parties for the toad’s seemingly unavoidable demise as well as the general mismanagement of a fragile ecosystem. The International Rivers Network (IRN) accuses the World Bank of negligence in failing to require a thorough Environmental Impact Assessment (EIA) before it financed the hydroelectric project. The IRN notes that the bank also neglected to modify the project design, implementation and operation after the environmental impacts became more clearly apparent as the gorge’s endemic species were discovered in December 1996. Construction continued on uninterrupted after the discovery and the Tanzania Electricity Supply Company was not informed of these discoveries until 1998. This critical omission violated Bank policies and betrayed their commitments under the International Convention on Biological Diversity.
Baby Kihansi Spray Toads in captivity. Young toadlets are purple in color, while adults are yellow.
Whatever the fate of the Kihansi Spray Toad and its irrevocably altered gorge habitat, this story is one that scientists have heard before and are likely to hear again. This outcome is indicative of a much larger and very grave trend in amphibian species on a global level. Because amphibians are particularly susceptible to disturbances in the habitats, they continue to disappear in disproportionate numbers. Nearly one third of the world’s amphibian species are threatened and almost half are in decline. Since 1980, more than 100 of these animal species have gone missing and are assumed to be extinct. If the Kihansi Spray joins the ranks of these fallen species, will people start listening?
In late May 2005, a very few Kihansi Spray Toads were found in the upper wet zone of the Kihansi Gorge. Thus the species is clinging on and not officially extinct in the wild yet.
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Special thanks to Sam Lee and Tim Davenport of the Wildlife Conservation Society (WCS) for their assistance with the preparation of this article.
- Sam Lee is a herpetologist formerly with WCS. He is still involved with Kihansi Spray Toad conservation efforts.
- Tim Davenport is a WCS biologist based in Tanzania. He was recently a co-discoverer of a new species of monkey in Tanzania, the Highland Mangabey, and leads the The WCS Southern Highlands Conservation Program.
This article is an abridged version of a longer, more technical article to be published by Tina Butler later this year.