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African frogs: Threats and discoveries

Alan Channing

Frogs are a widespread group of animals found in almost every part of Africa, from deserts to forests. Most lay eggs that develop in water, becoming tadpoles that feed on the abundant algae, growing as fast as possible. The tadpoles grow legs, as part of a transition process called metamorphosis, that leads to the development of the adult frog. Metamorphosis is a complex process involving changes to the skin, blood and digestive system, all needed for a life on land.

Frogs and tadpoles are known to be susceptible to environmental disturbance, and are useful as indicators of change. If frogs are unable to survive, then the environment is probably not safe for us humans either.

Frog populations have been shown to be disappearing in many parts of the world. The causes have been suggested to implicate many factors, from increasing levels of UV light, pesticides in the environment, climate changes affecting breeding cycles, and the rapid spread of a pathogen, known as the chytrid fungus. These and other factors may be acting in concert, magnifying the effects. The chytrid fungus is found on the mouthparts of tadpoles, where it feeds on the hard keratin. This prevents the tadpoles from feeding normally, and may lead to their death. Adult frogs may also be infected, but the fungus lives and feeds on the skin, apparently interfering with the frog's ability to take up oxygen. The fungus requires water to live, so frogs that live in or near aquatic habitats are at risk.

Although the fungus has been present in Africa since the 1930s, (it was found on a museum specimen collected then), no large-scale population losses have been attributed to the disease. The more immediate threats in Africa result in loss of habitat. These range from clearing of forests, excessive seasonal burning, to the building of the urban sprawl, often in low-lying flat areas where the wetlands used by frogs once were.

The Atlas and Red Data Book of the Frogs of South Africa, Lesotho and Swaziland, published in 2004 presents the results of an effort to map the distribution of all the frogs in southern Africa, and to consider the conservation status of each of the more than 115 species listed. The results of the detailed study show that 33 species, or nearly one third of the total, are under some level of threat. The most serious levels of threat result in species being classified as Endangered, or Critically Endangered. 11 species, about 10% of the total, fall into these two categories.

One of the paradoxes of research on African frogs, is that every year many new species are discovered and named. This is a reflection of how poorly the frog fauna is known, but it is a concern that there are probably many threatened species of frogs that have not yet been described. In the last 10 years, since 1996, 13 new (or previously confused) species have been recognised, and there is every reason to believe that many other new species await discovery.

More information:

  Professor Alan Channing is with the Biodiversity and Conservation Biology Department, University of the Western Cape

www.uct.ac.za

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