IT'S A RAINY SPRING NIGHT in the Tully River Gorge, and I'm hunting frogs. Swatting ineffectually at a swarm of mosquitoes, I slowly wade up a tiny unnamed creek as it tumbles down from Queensland's Atherton Tablelands to the rushing river below.
In the highlands above me, the rich forests of Wooroonooran, Girringun, Daintree and Koombooloomba have been evolving in relative isolation since the break-up of Gondwana tens of millions of years ago. Now they are fortresses of biodiversity, protecting a host of unique fauna: tree kangaroos, cassowaries, quolls and bandicoots, as part of the vast Wet Tropics World Heritage Area.
But while the combined clout of the U.N. and the Australian Department of the Environment has preserved the unique birds and mammals of the Australian rainforests, it has done little for the frogs.
Systematically exterminated by a water-borne fungus known as chytridiomycota (or 'chytrid') that scoffs at national borders, park boundaries and conservation areas, the frogs and toads of the world's high altitude forests are disappearing.
Since 1980 up to eight species have gone extinct in Australia, with many more in peril around the world. According to a 2006 paper by J. Alan Pound et al. in the British journal Nature, some 73 species have died out in the forests of Central America alone. Declines and disappearances are occurring in Africa, Europe, and the Andes of South America, and reports of infection from New Zealand and across North America hint at a pathogen that has reached across the entire planet.
I should have been in class that week I spent hunting frogs, but I'd been lured from my ecology courses at James Cook University (JCU) by a PhD student named Jodi Rowley (see "Portrait", Cosmos 15, p9). I knew her from her Townsville JCU office, hunched over her computer in a freshly laundered T-shirt and the ubiquitous Queensland thongs.
Now, eyes twinkling and clad in well-worn dive boots and shredded Gore-Tex, she held a rain-soaked court on the stream bank. We field assistants scoured the vegetation, sneaking up on frogs and grabbing them with plastic-bag-encased hands. Rowley weighed each one, swabbed its belly, tied a tiny radio transmitter around its waist and told us to return it to its original location.
At first glance, only a backward mind chooses night-time to look for tiny amphibians in the convoluted foliage of a rain-drenched jungle. But frogs are more active at night, and their eyes shine in the beam of a headlamp. Despite having to distinguish the minute red gleam of pea-sized frog eyes from the jumbled reflections of raindrops, insect eyes, and mica crystals in the river rock, frogging at night is far more efficient than waiting for daylight to search for their diminutive bodies.
The next day, though, Rowley's beacons led us straight to each one, and an infrared thermometer gave us their temperature. Rowley asked the question why, even as chytrid destroys some species in a given stream with horrifying efficiency, it leaves others alone.
Her results were intriguing. When she compared the temperature of each frog's daytime refuge with the amount of fungus it carried, the difference between life and death was just a few degrees Celsius. The fungus begins to die above 30°C, and species that preferred the warm forest foliage were staying alive, while those just metres away in the cooler water were dying. "Even though they're in the same genus, they're the same size, and they inhabit exactly the same stream, they can experience totally different micro environments," Rowley says.
PATTERNS LIKE THIS stumped ecologists when the first frogs began disappearing from the mountains west of Brisbane in the late 1970s. Back then, the idea that a pathogen was the culprit wasn't the leading theory; an environmental cause such as drought or pollution seemed far more likely.
"There were all sorts of things, like gold mining, that were in the area," remembers Keith McDonald, now chief ranger of Queensland Parks, a frog ecologist who has monitored the declines since they began. "It was a puzzle. A real puzzle."
McDonald and his colleagues had good reason to suspect an environmental cause. Chemical pollution, increased UV radiation and habitat loss had all hit frog populations hard in recent years.
According to Andrew Blaustein, an amphibian researcher at Oregon State University in Corvallis, USA, "43 per cent of all amphibian species are in decline, 32 per cent are threatened, and probably over 100 species of amphibians have gone extinct since 1980." Compared to other groups of vertebrates, frogs are in crisis.
But unlike habitat loss and pollution – often worst near human settlements – chytrid specialises in exterminating species from pristine high-altitude areas. Therefore, it can have a disproportionately large impact on some of the world's highest conservation-value species.
A case in point is the gastric brooding frog (Rheobatrachus sp.). In 1979, it was the disappearance of the southern species of this frog from the entirety of its small, protected range in southeast Queensland that first put researchers on alert to mysterious die-offs.
Amphibians have evolved a host of wondrous reproductive strategies since they left the seas of Earth and colonised the land some 350 million years ago: they lay their eggs in giant frothy bundles; they abandon them in streams; they carry them around in pouches on their backs like confused marsupials. However, the gastric brooding frog was arguably the strangest of them all.
Blunt-nosed and bug-eyed, the fully aquatic, mottled brown frogs spent their days flitting about their mountain stream homes. Sometime in the distant past, the females of these species evolved a novel but effective way of safeguarding their developing young – in their stomachs. They began by carefully swallowing their newly hatched young, then sat patiently for six weeks while the babies developed in their guts.
As the 20 or so young slowly metamorphosed from tadpoles into frogs, they would push out the stomach walls until the organ filled the mother's entire body. When the time was right, she contracted her stomach muscles and launched them into the world with a life-giving heave.
Just seven years after their discovery, during a few months in 1979, the southern species of this frog was completely wiped out. The northern species followed into oblivion in 1985, having only been discovered 18 months previously. "Over a three-month period, it just disappeared – went completely, along with some other frogs," remembers McDonald.
UNFORTUNATELY FOR GASTRIC brooding frogs, their aquatic lifestyle made them ideal targets for chytrid, whose two-stage lifecycle depends on access to water.
The fungus begins life as a sperm-shaped cell called a zoospore, swimming around for up to seven weeks in streams and damp soil while it searches for a plot of frog skin. If it lands well, it burrows down just a few cells, and begins breaking down the keratin-rich outer layer of its host's skin.
Still only visible under high magnification, it constructs a shell of chitin around itself and begins producing more zoospores inside it. After a few days to a few weeks, the chytrid pokes a tube out between the skin cells and shoots its new crop of zoospores out into the environment.
What the fungus does during this period that kills its frog host is still a mystery, but current research focusses on the changes it causes in the infected frog's skin.
Ordinarily only two or three cells thick, the outer layer of amphibian skin is very permeable and performs the role of a lung - shuttling oxygen, carbon dioxide, and water between the body and the outside environment. The fungus apparently disrupts this vital outer layer, perhaps so badly that the frog cannot survive.
But back in the 1980s and early 1990s, a fungus hadn't even been considered as the culprit. While attention remained fixed on environmental factors, the die-offs moved north in an alarming wave of extinctions, eventually reaching Big Tableland near Cooktown in Far North Queensland.
For 14 species of high-altitude stream-dwelling frogs, the pattern was the same: explosive die-offs with 90 to 100 per cent mortality, while frogs that lived away from the streams – and all species at lower elevations – remained virtually untouched. For as many as eight of those 14 species, whose population was found exclusively at high elevations, that pattern meant extinction.
After years of fruitless searching for some change in the environment that could link the die-offs, in 1993 McDonald enlisted the help of Rick Speare, a friend from JCU.
Speare, a wildlife veterinarian, reviewed the evidence: an unknown disturbance was moving quickly northward, killing species of frogs that lived in high-altitude streams while sparing the rest. From his viewpoint, this was a perfect profile for an exotic, water-borne disease that thrived at low temperatures.
Now with a new lead, they arranged a modest amount of funding and set up a PhD student, Lee Berger, with heaps of dead frogs and the mission to unmask their killer. By that time, reports of similar waves of die-offs were coming in from other parts of the world, and Berger added frogs from Panama, Costa Rica, and other parts of Australia to her study.
At first, she felt that a virus was the most likely candidate, and spent about a year at the CSIRO's Animal Health Laboratories in a fruitless search for one. "That was very boring," she remembers, chuckling.
But her luck changed when she began to concentrate on the minute round structures that often appeared in the frogs' skin.
"We saw this thing in the skin, this cyst that was just really superficial," she says. "In a mammal you wouldn't even expect that type of disease to cause death." She experimentally infected some lab frogs with the structures, and the infected frogs died. Then she knew she was on the right track – except she still didn't know what the structures were.
"It [was] just those round things," she says. "We thought it was a protozoan. I sent it to a few protozoologists and everyone was of different opinion. I paged through all these parasitology and pathology books and there was just nothing similar. In the end I got someone to do some PCR work on it."
The PCR, short for polymerase chain reaction, replicates the unknown DNA and creates enough for a sequencing machine. When the results came back as a chytrid fungus, Berger was even more perplexed.
"They don't get a lot of attention," she explains. "Most of them are biodegraders in the environment, and then there are a few that kill insects and plants or algae." Batrochytrium dendrobatidis, as the fungus was named, was the first ever found parasitising a vertebrate.
BEFORE AND AFTER Berger and her colleagues published their surprising findings in 1998, many scientists were sceptical of the idea of disease as the cause, in part because there are very few diseases that have the capacity to drive a species – much less many species – to extinction in the wild.
But when they began looking for chytrid, researchers around the world began to find the fungus in bodies of frogs recovered from their own research sites, and, more intriguing yet, from sites where the presence of chytrid did not prove fatal.
"The [frog] populations that get hit super-hard and die off instantly and crash and do not recover are those that are found [in] optimal chytrid habitat," says Karen Lips, who studies Central American frogs at Southern Illinois University, in Carbondale, USA.
But, she adds, "at least in Central America, it's everywhere you look. It's in the lowlands, it's up in the mountains, it's in the middle elevations, it's in streams, ponds, trees, forests."
Other scientists from South America, Europe, and North America report similar experiences, and it appears now that chytrid has spread to most corners of these continents. Australia and Central America are hotspots, and a few reports have come in from Asia.
But comprehensive surveys for chytrid are time-consuming and arduous, and so far researchers have identified only a few areas, including Hong Kong, where they are fairly certain it is absent.
Understanding where chytrid originated, whether it is still spreading, and where it has yet to reach are key questions that affect how policy-makers allocate limited funds for additional research and plan conservation strategies to protect the species living in its path.
However, as Lips notes, funding is scarce. "The problem is that it doesn't show up very high on the list when you're dealing with global poverty and human diseases like Ebola and tuberculosis and AIDS."
As in any fast-moving field, groups of scientists have very different ideas about the answers to these questions — and therefore about the best projects to fund. The evidence to date does not paint a clear picture, and the debate has been heated.
For many, the explosive waves of extinctions that chytrid caused as it moved through Australia, Central America, and some regions of Africa represent the spread of a novel pathogen. "In the New World," says Lips, "it moves like a wildfire through populations, implying that these frogs have never really experienced this thing before."
Among those who share her viewpoint, the consensus is the fungus originated in one location and has begun to spread around the world recently, perhaps inadvertently introduced by humans as they exported amphibians around the world for all manner of purposes: from their use as a test for confirming human pregnancies to globalising French cuisine. One potential origin of the fungus is southern Africa, where analysis of museum specimens has shown infection as far back as 1938. However, says Lips, "we don't know, honestly, because nobody has really looked terribly hard at too many super-old frogs".
WHATEVER THE ORIGIN, the idea is that when the diseased organisms escaped into the wild in foreign lands, the fungus soon blazed through native ecosystems, resulting in the extinction waves that helped identify the fungus in the first place. If the hypothesis is correct, then quarantines might still keep chytrid out of some areas.
However, other researchers don't think the extinctions represent linked waves. Some, such as JCU associate professor of biology, Ross Alford, haven't been totally convinced that the fungus is moving around – at least not on a global scale.
Most fungi have a resting stage in their lifecycle, he says. Though such a stage has not been found yet for the strain of chytrid affecting frogs, if it does exist the fungus may have simply been lurking in many places in a dormant state elsewhere in the environment before it erupted.
The fungus's normal state "could be like many other chytrids: a parasite of plants or as a decomposer living in leaf litter or shed bits of reptile skin or who knows what," he explains. A recent change in the environment could have weakened frogs' immune systems or destroyed other microbes living on them that would ordinarily out-compete the fungus, allowing the chytrid to begin infecting frogs.
Jodi Rowley sees an 'invisible' dormant stage as a real possibility. "We're not really good at environmental sampling yet," she told me from her new office at Conservation International in Cambodia. "In other chytrid species, you can sample and find nothing for a very long time, and then all of a sudden: bang – you get hundreds and hundreds."
As for what environmental change might have prompted the chytrid explosion, there is no shortage of ideas being discussed. "This could be caused by global climate change, it could be caused by the trace quantities of various sorts of pesticides and antibacterial agents and all sorts of things that are pretty much present absolutely everywhere anybody looks," says Alford.
The climate change idea in particular attracts bountiful controversy. A 2006 Nature paper linked disappearances of more than 30 frog species in Costa Rican forests with patterns of abnormally warm temperatures. Chytrid seems to have been the immediate cause of their deaths, so the correlation was odd since the fungus grows best at lower temperatures.
To explain the apparent contradiction, the authors suggested the warmer temperatures could have increased evaporation and thus cloud cover over the forests, leading in turn to lower daytime temperatures and more optimal conditions for the chytrid.
"Disease is the bullet killing frogs, but climate change is pulling the trigger," insisted lead author J. Alan Pounds, a researcher at Costa Rica's Monteverde Cloud Forest Preserve at the time.
Some, including Andrew Blaustein, think the idea is entirely plausible. In a commentary Blaustein co-authored which accompanied the original paper, he described the work as "a breakthrough, as it resolves the paradox and offers a theory to explain the widespread 'enigmatic' declines".
But other researchers have not hidden their concerns with the idea. "It just doesn't make sense," says Berger – original discoverer of the fungus. Lee Skerratt, Berger's husband and an expert on chytrid epidemiology in his own right, implies that Pounds' naïve hypothesis was drawn solely from his expertise in climatology, without knowledge of either frogs or chytrid.
Lips agrees. "Alan claims that these shifts in climate are making things optimal for the fungus, but the problem is [that] where he works, in Monteverde, and [at] all my sites in Central America, they were already optimal to begin with!"
HEALTHY DISAGREEMENTS NOTWITHSTANDING, Lips, Blaustein, Alford, and many others recognise the need to act immediately to protect the world's frogs while they search for a way to deal permanently with chytrid and all the other threats facing these creatures.
To that end, 50 concerned researchers co-authored a policy letter in the U.S. journal Science calling for "an unprecedented conservation response" and the need for, among other initiatives, captive breeding programs.
One of the outcomes of that stance is the Amphibian Ark (AArk), spearheaded by Joseph Mendelson, curator of herpetology at Zoo Atlanta in Georgia, USA. The project aims to organise and support directed breeding programs at zoos worldwide.
Most acknowledge, though, that captive breeding programs are an interim measure at best. Long-term solutions are being studied, though none have shown encouraging results yet. Neutralised versions of the fungus are being investigated, as are vaccines.
One idea for species that have already been affected is to breed up the eggs of survivors in captivity.
The idea, being tested at the Amphibian Research Centre in Melbourne, Australia, is that the few individuals left after the fungus has established itself are likely to be somewhat resistant. So, making sure that all of their eggs survive to adulthood might help the population recover.
More broadly, says Alford, "all multicellular organisms [are] patches of habitat that support really complex communities of microbes". If researchers can find and isolate another, more benign frog symbiont that out-competes the chytrid fungus, he says, then after thorough testing they might be able to introduce it to threatened areas.
With the number of species it encompasses and the percentage of the Earth they inhabit, the crisis facing amphibians today may dwarf any other conservation problem in recorded history.
While the prognosis looks bleak, researchers point out that less than 10 years ago, the fungus hadn't even been described, and yet today its genome has been sequenced and research to find a solution is underway on many fronts. It's an overwhelming problem, says Lips, but "we've made a lot of progress in eight years".
For now, though, research budgets remain frustratingly small and chytrid shows no signs of ending its rampage. It seems inevitable that, until the research of amphibian advocates bears fruit, more and more of the world's rainforest preserves will lose forever the tiny amphibians they proved unable to protect.
Benjamin Lester is a freelance science writer based in Washington, DC, USA. While on exchange for a semester in 2005 at James Cook University in Townsville, Australia, he studied ecology and marine biology.
