Fans of Sesame Street's ambassadorial anuran, Kermit, may have perked up when a global conservation coalition designated 2008 the Year of the Frog, but it was actually a sad day for anyone concerned with the fate of the planet's wildlife.
Year of the Frog was essentially a PR response to a growing biological crisis in which up to half the Earth's amphibian species were predicted to disappear in the immediate future. It signalled a consensus global conservation issue of historical proportions, in which thousands worldwide would set to working feverishly to mitigate the disaster. Yet almost a decade on, little has changed.
Though amphibian declines reached critical proportions in the new millennium, the crisis wasn't exactly new. Biologists had known frogs to be in worldwide decline for decades. The difference was that they were now vanishing at alarming rates — even from pristine wildernesses. The Golden Toad (Bufo periglenes) of Costa Rica's Monteverde cloud forest disappeared completely between 1989 and 1991, and genus Rheobatrachus, Australia's startling gastric-brooding frogs (the young developed in the mother's stomach), was lost literally overnight from the rainforests of northern Queensland.
Humans have cultivated a longstanding cultural dichotomy of fear and fascination with amphibians. Art, mythology, magic, witchcraft and commercial logos have all employed these animals to symbolize everything from intelligence to fertility to resurrection to evil. Frogs, which occur on every continent save Antarctica, are familiar to everyone. But having survived several extinction events since appearing 200 million years ago, they are now undergoing the worst global extinction event of any vertebrate group since the dinosaurs bowed out 60,000,000 years ago. Long humanity's coalmine canaries of ecosystem health, frogs are quickly blinking out of existence. When the canaries are gone, whither the coalmine?
In the past, amphibian declines were tied to anthropogenic habitat destruction, loss of wetlands, or chemical pollution, but there was another factor contributing to the current crisis: global spread of a deadly fungus, Batrachochytrium dendrobatidis, or Bd. Vulnerability to Bd was increased by the previously listed factors as well as large-scale climate change. Warming affects interaction with — and resistance to — the fungus: hot/dry conditions mean frogs can't re-hydrate properly, and water stress causes them to huddle together where chytrid spores are more easily passed; changing climate can also impact the timing of breeding and spatial distribution of a species; higher water temperatures lead to lowered oxygen levels and increased parasitism. The result? Chytrid became a fungal meteorite plowing through the world's frogs. A bona fide F-bomb.
Of approximately 6,000 recognized amphibian species, 122 have fully disappeared since 1980, and some 30 to 50 per cent are nearing extinction. In the late 1990s, extinctions in Central America comprised a wave of disaster moving southeast around 28 km/year. A multi-country conservation initiative concerning genus Atelopus found that of 32 species, the status of one was unknown, two were endangered, and 29 were extinct or critically endangered. Chytrid may have been the smoking gun, but where did it come from?
Forensic geneticists from the U.S. Centers for Disease Control traced Bd's origin to Africa and linked its unseen spread to international trade in the African clawed frog, Xenopus laevis, an amphibian "lab rat" exported around the globe for use in pregnancy tests in the mid-20th century. Xenopus carry chytrid but aren't affected by the fungus. Thus, beginning in the 1930s, Xenopus cross-contaminated native species in university and hospital laboratories, via the pet trade, and through escapees that established wild populations. Accelerated globalization further facilitated movement of this emergent pathogen, a scenario frighteningly familiar to human epidemiologists.
Chytrid isn't just a tropical problem — it's found on Vancouver Island and in northern British Columbia, as well as Alaska, Japan, Russia and Europe. Which, if any, amphibian species are threatened in these regions is unknown, but those already on the brink for one reason or another are most at risk. In 2010, an even deadlier chytrid surfaced, wiping out populations of the already endangered European Fire Salamander. Bsal (B. salamandrovensis) originated from Asia through the pet trade, causing convulsions among biologists, who lobbied governments for a total ban on salamander imports. Though but a finger in the epidemiological dike, temporary bans were enacted in the U.S. (2016) and Canada (2017).
Though the prognosis for global ecosystems of this explosive loss of amphibians remains to be seen, one thing is clear: it won't be positive.