Review Of PBS NATURE Documentary Thin Green Line
By Robert Deyes
For those who take issue with the rather blase application of evolution to every aspect of biology, Public Television's recent showing of Thin Green Line is bound to have raised hackles. Few nature documentaries that I have seen begin with quite the same density of evolutionary suppositions. We are told for example that amphibians were the first of our ancestors to venture out of the water and that they have since evolved 'into an explosion of species'. By the same token frogs are made out to be evolutionary gems that over the millennia adapted to live alongside dinosaurs, survive asteroid impacts and withstand the rigors of the ice age. Yet herein lies the irony. For despite all their supposed evolving and adapting, amphibians today have been unable to keep up with the more recent pace of environmental change.
Indeed evolutionary just-so stories aside, Thin Green Line provided important yet deeply troubling details about a tragedy that is unfolding beneath our very eyes- one that is unprecedented in its sheer scale. A third of all amphibian species across the globe are currently in decline and half of all amphibian species may eventually disappear altogether. Like many environmental tragedies, human activity is partly to blame. Spade-Foot toads in Cape Cod for example are being edged out by an increase in road construction while the Mountain Yellow-Legged frogs of America's Yosemite National Park have only recently recovered from a hard-fought battle against fish that were introduced by recreational fishers in the early 1900's (Ref 1).
As the demand for new housing continues to rise across the world, amphibians are facing survival challenges on every front. Yale University's David Skelly claims that 21% of frogs in suburban ponds across the United States suffer from reproductive deformities (Ref 2). Synthetic oestrogens released into US water ways are thought to be at the heart of the problem. Tyrone Hayes from UC Berkeley is likewise adamant that agricultural run-offs are responsible for causing immunosuppression in tadpoles (Ref 3). But these issues by no means provide the full story. Dr Roland Knapp from the Aquatic Research Laboratory in Sierra Nevada has shed light on a much more insidious problem- one brought about by a fungus called Chytrid (Ref 1).
As Thin Green Line documented, Chytrid attacks the skin of amphibians depriving them of much of their oxygen. Considered by many as an amphibian equivalent to 'Yellow Fever', the resulting fungal disease has reached global epidemic proportions and is today decimating frog populations across much of Central and South America as well as Australia. Conservation biologist Dr Karen Lips was the first to raise the alarm in Costa Rica when she noticed a mysterious die off of frogs in local nature reserves (Ref 4). Since then local ecosystems have been thrown off-balance with insect and snake populations being the worst affected. In central Panama emergency action has been taken to rescue dwindling frog species from an otherwise certain extinction by placing them under the care of a Noah's Ark-style facility called 'El Valle Amphibian Conservation Center' (EVACC).
Desperate times require desperate measures. Indeed scientists in Australia have turned their faith towards the limited powers of natural selection in their bid to stem the spread of Chytrid. In the Australian Alps, Kosciuszko National Park remains an ailing sanctuary for the yellow Corroboree frog. In fact there are now fewer than 200 of these beautiful creatures left within the confines of the park. Herpetologist Gerry Marantelli and his wife have taken it upon themselves to try and save them by building their own make-shift 'ark' out of disused shipping containers. Their approach seems simple enough- release captive frogs back into the wild and hope that in the process they somehow drive evolution in favor of Chytrid-resistant individuals.
Such attempts to select for specific traits in animal populations are nothing new. In 1997 scientists published data on guppy populations showing how the presence or absence of predators affected the time taken for guppies to reach sexual maturity (Ref 5). More dramatic results are seen when commercial fisheries set minimum allowed sizes for caught fish. In such cases selection favors smaller fish simply because these lie outside catch size limits and are therefore less likely to get caught (Ref 5). Nevertheless major problems can arise when animals kept in captivity are released into the wild where selective pressures are quite clearly very different from anything that they have been accustomed to (Ref 5).
One line of research is providing reason for hope in the fight against Chytrid. Microbiologists have discovered a bacterium- Janthinobacterium lividum- that lives naturally on frogs' skin and prolongs the lives of Chytrid-infected animals (Ref 6). If further tests prove positive, we could start seeing the application of Janthinobacterium in the wild sometime in the next few years (Ref 6). Still, today Chytrid continues to pose a severe threat to the well being of amphibian wildlife and anything less than its complete eradication could be catastrophic. The stakes are high. In short, without swift action we could be kissing the frog prince goodbye.
1. See Roland Knapp's Current Research at http://vesr.ucnrs.org/pages/knapp/research/research.html
2. "Sex And the Suburban Frog", See Skelly Lab Website at http://www.cbc.yale.edu/people/skelly
3. Tyrone Hayes, Biologist/Herpetologist, See http://www.nationalgeographic.com/field/explorers/tyrone-hayes.html
4. Kelly Blake (2009), Biologist Karen Lips Investigates Amphibian Extinction Mystery, See http://www.newsdesk.umd.edu/scitech/release.cfm?ArticleID=1856
5. Carl Zimmer (2003), Rapid Evolution Can Foil Even the Best-Laid Plans, Science, Vol. 300, p.895, See http://www.sciencemag.org/cgi/reprint/300/5621/895.pdf?ck=nck
6. Richard Black (2008), Bacteria could stop frog killer, BBC News, See http://news.bbc.co.uk/2/hi/science/nature/7438205.stm
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