Toad on brink of extinction, scientists race to study amphibian for bioactive compounds
Tina and Rhett Butler, mongabay.com
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.
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.
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.
The last refuge for the Kihansi Spray Toad: an off-exhibit terrarium in the Bronx Zoo.
Researchers at Cornell University will determine if any bioactive compounds, such as alkaloids and bufadienolides, are present in the skins of wild and captive born Kihansi spray toads. Medically valuable compounds have been derived from frog secretions in the past.
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.
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.
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.|
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.
- - -
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.
The WCS Southern Highlands Conservation Program
The Wildlife Conservation Society