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| July 2009 |
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One last croak? Frogs meet their match in climate changeBy Jeanne Tarrant
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 Taking a swab to test for chytrid from a Critically Endangered Micro frog, Kennilworth, Western Cape. |
Frogs may have survived the extinction of the dinosaurs 65 million years ago, but their luck is now running out.
Frogs are ideal bio-indicators of the health of the environment because of their pivotal position in the food chain as both predator and prey. The fact that as many as half of the approximately 6300 amphibian species worldwide are experiencing declines and, in some cases disappearing altogether, is a sure sign that something is fundamentally wrong with Earth’s ecosystems.
Dramatic declines in amphibian populations have been observed over the past three decades. Currently amphibians are the most threatened class of animal on the planet. The rate of extinction of amphibians is up to 2700 times higher than it’s been in the 360 million years that this diverse group has inhabited Earth. Amphibians survived the extinction of the dinosaurs 65 million years ago, but now face a similar fate. This extinction crisis is one of the biggest challenges currently facing conservationists.
The causes of amphibian declines are complex and often insidious. Loss of habitat, chemical pollution and invasive species are the primary threats to amphibians; however, these threats are working synergistically with other more subtle threats such as emerging infectious diseases and global warming, resulting in declines and mass mortalities even in relatively pristine environments. In particular, the emerging infectious disease chytridiomycosis (chytrid), caused by the fungus Batrachochytrium dendrobatidis has been identified as one of the most important drivers of the amphibian crisis. The disease is one of, if not the most, virulent pathogens of amphibians and is responsible for population declines and extinctions throughout the world, especially in the tropics. The disease can entirely eliminate entire frog populations within a few months, and rescue operations such as those taking place in Australia and Panama, whereby frogs are removed from the wild and bred in captivity, appear to be the only way these species can be saved. Scientists in these regions are hoping to discover more about resistance to the disease and ultimately to release the animals back into the wild. As such, preservation of habitat is key to the long-term survival of many species.
Another insidious threat is that of global climate change, which also affects
otherwise “untouched” regions. Climate change can have potentially disastrous
consequences for amphibian communities as a result of altered temperature and
rainfall patterns. Amphibians are particularly sensitive to such changes due to
their dependence on water for reproduction. Times of breeding seasons may also
change, with knock-on negative effects. A further impact of climate change is
its affect on interactions between species. In particular, the relationships
between pathogens and their hosts are likely to be impacted, since temperature,
rainfall patterns and humidity levels influence pathogen activity. Furthermore,
climate change is likely to facilitate and accelerate the spread of disease and
increase host susceptibility. Each of these threats is significant, but together
they are creating the perfect storm for amphibian extinction.
Threatened
In South Africa, 15% of the 135 described frog species are considered Threatened according to the Atlas and Red Data Book of Frogs. Fortunately, this percentage of threatened species is below world averages, however it is imperative that action is taken to conserve habitat and, where necessary, to implement ex-situ (i.e. captive) breeding programmes for severely threatened species whose wild habitats are unlikely to recover in a reasonable time frame. Habitat loss and fragmentation through the processes of urban expansion, agricultural activities, spread of alien vegetation and chemical contamination of both water resources and terrestrial habitats are the main threats to South Africa’s frogs. South Africa’s water resources are becoming increasingly polluted as a result of mining and industrial waste, pesticide run-off from agricultural lands and increased pressure from growing urban sprawl.
.jpg) The Endangered Kloof frog, which has lost much of its habitat along the KwaZulu Natal coast of South Africa. |
The study of chytrid in South Africa is being led by the African Amphibian Conservation Research Group of North-West University. Although the disease is present across South Africa, it appears that most frog populations are able to persist without major declines. However, significant die-offs have been observed in frog species that occur at high altitudes, in particular the Phofung River Frog (Amietia vertebralis) which occurs in a restricted region of the Drakensberg. The Critically Endangered Table Mountain Ghost frog has also been found to host a high level of infection. This is worrying considering that only small populations of this species remain, and much of its habitat has already been lost. Further research into the ecology of the disease in South Africa is essential to understanding why some amphibian populations seem unaffected by the disease, while others appear to be more susceptible.
There is increasing evidence that amphibians become more susceptible in fragmented and damaged habitats and that pollutants may also interfere with the frogs’ ability to resist the pathogen. Climate change is also a key influence on pathogenicity. The effect of these factors on chytrid remains unclear, and may mean that South African species could potentially still face the threat of deadly outbreaks of the disease.
The plight of amphibians is receiving growing attention around the globe. Preservation of habitat, especially wetlands will be integral to stemming the amphibian extinction crisis. On a positive note, both national and regional conservation bodies in South Africa are incorporating frog conservation plans into their overall biodiversity management plans. In terms of ex-situ conservation, 2008 saw the globally coordinated “Year of the Frog” campaign successfully bring public awareness to the plight of amphibians through Zoos, Aquaria and Botanical Gardens.
Frogs play an essential role in many ecosystems. Tadpoles are an important food source for other animals and feed on algae and plant detritus keeping water systems clean. As both predator and prey frogs are responsible for keeping insect populations, including pests such as mosquitoes, in check as well as providing food for a large array of predators. Their removal from the system would therefore result in significant disturbance of the food chain, with far-reaching consequences. Furthermore, many species of frogs are important to medical research due to pharmaceutical chemicals in their skin secretions. Current research includes potential advances in cures for both HIV and cancer. The loss of any frog species is thus potentially a loss to human medicine. Aside from these direct benefits, frogs are aesthetically interesting creatures and often culturally important, and allowing the continued loss of this 360 million year-old group would be morally irresponsible of humans. The current massive decline in amphibian biodiversity should be a wake-up call to humans that there is something severely wrong with the global environment.
More information:
Jeanne
Tarrant - jeannetarrant@ymail.com
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