Colourful indicators of doom?

2009-11-12 00:00

“HARMLESS, colourful, melodious and ecologically vital,” — that’s the verdict­ on frogs from Louis du Preez and Vincent Carruthers, co-authors of A Complete Guide to the Frogs of Southern Africa.

Frogs are found everywhere — mountains, rainforests, deserts — and on all the continents barring Antarctica­, a ubiquity married to a unique physiology that has seen them become unwitting barometers of environmental damage all around the planet.

“The decline and extinction of species­ is occurring at a disturbingly high rate,” say the two authors, “indicating widespread environmental malaise.”

Worldwide there are about 5 200 identified species of frogs (new ones turn up every year) and 157 of these are found in southern Africa where 13 frog families occur, 13 out of the 32 families worldwide. This new field guide describes the species found in the region south of the Zambezi, Okavango­ and Cunene rivers. Each species gets a double-page spread featuring key identification points, description, habits, plus a range map and detailed photographs. There is also a CD with the calls of 146 species.

Du Preez and Carruthers’s book really is the complete guide as it says it is. There is everything you need to know about frogs and, in case you were wondering, a toad is a frog.

“Toads are one of the frog families, along with the Rain Frog family, Ghost Frogs, Reed Frogs and others,” say the authors. “It is therefore incorrect to separate frogs from toads.”

From an evolutionary point of view we have a lot to thank frogs for.

“Amphibians were the first vertebrates­ to emerge from the water and exploit the terrestrial landscape more than 350 million years ago. The early amphibians were the common ancestors of all terrestrial vertebrates — mammals, birds, reptiles and modern­ amphibians.”

The evolution of amphibians from an aquatic to a terrestrial environment and the subsequent exploitation of widely divergent habitats was achieved through a variety of remarkable physical and functional adaptations. Not least the skin. This is semi-permeable and aids in respiration and moisture regulation. While oxygen intake is mainly through the lungs, carbon dioxide is lost primarily through the skin. Frogs do not drink water, absorbing it by osmosis through their skin when submerged in water or sitting on wet surfaces. In some species the skin secretes unpleasant-tasting fluids to deter predators.

Another remarkable frog trait is the noise they make. David Attenborough, in his documentary series Life on Earth, mused on the significance of that distinctive sound.

“It is intriguing to speculate, as you stand in a swamp listening to this astounding and deafening chorus, that, although much must have changed in the millions of years since the first amphibians appeared, it was nonetheless an amphibian voice that first sounded over the land which, until then, had heard nothing but the chirps and whirrs of insects.”

Frogs make a number of sounds. The most common is the advertisement calls produced by males to attract females. These are amplified by an expanded vocal sac but not all male frogs advertise their presence.

“Occasionally a silent male, referred to as a satellite, stations himself close to a robust caller and intercepts the female as she approaches.”

Mating is a hectic affair. “Competitiveness at the breeding site and the need to mate urgently drive males to mounting females as soon as they are detected. Often other males, frogs of other species and even bits of flotsam on the water are mistaken for females and assaulted.”

Frogs’ other appetites are correspondly large. They are “voracious predators of invertebrate life and themselves important prey for a wide diversity of predators. In both capacities they influence a wide ecological spectrum.”

Which is why the fact that frogs on several continents have suffered sudden, high mortality rates, resulting in some species becoming extinct and others experiencing massive population reductions, is cause for alarm.

“Globally, amphibians are now the most threatened class of vertebrate ... Because of the ecological­ importance of frog populations in terrestrial and aquatic ecosystems, these losses may bring about complex and far-reaching complexes.

“It is only relatively recently that scientific research has led to an understanding of the great ecological importance of frogs in many ecosystems around the world.

“Our current knowledge of their critical position in the food chain as predators and prey, their bioindicator status in determining wetland and river­ health, and their role in keeping down the numbers of insects — many of them vectors of human diseases — continues to grow. It is now thought that frogs are a critically important group of animals to study in connection with issues such as global climate change.”

So what is killing frogs? While there is no consensus, the prime culprits look to be the destruction of habitats, climate change, which affects reproduction, and disease. The fungal pathogen­ Batrachochytrium dendrobatidis, or amphibian chytrid, may also be a major contributor to declines and extinctions.

While there have been no major declines in southern Africa,­ the large number of the International Union for the Conservation of Nature and Natural Resources (IUCN) red-listed species may be at risk from disease in the future.

Also impacting on frogs is the increased exposure to ultraviolet rays as the result of the depletion of the stratospheric­ ozone. This affects the development of frog eggs and tadpoles. Their porous skin also makes them susceptible to chemical pollution, although some species are remarkably resistant.

They also get eaten by humans. The use of frogs as a food source in southern Africa is confined to a few rural communities although the illegal­ export of Giant Bullfrogs for food and as pets persists. Elsewhere they are a popular food item: as many as 200 million frogs legs are exported annually from Indonesia to the rest of south Asia.

As frogs fall silent around the world, their declining numbers stand as a warning to us all. They were in at the beginning; does their retreat signal the end? •  A Complete Guide to the Frogs of Southern Africa by Louis du Preez and Vincent Carruthers is published by Struik.


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