If you’ve been following this blog for a while, you may know that for the past six months, we’ve been posting Freaky Frog articles every fortnight. During our journey through the weird and wonderful world of these amazing amphibians, we’ve looked at the delightfully named ‘scrotum frog’, we’ve examined the remarkable defensive mechanism of the ‘wolverine frog’, and we’ve marvelled at the cryogenic wood frog. There have been frogs that brood their young in their vocal sacs. Toads that brood their young inside pockets in their own skin. Frogs with moustaches. Frogs that practise ‘reproductive necrophilia’.
But, as we’ve mentioned multiple times, frogs are also in trouble. Globally, their numbers have been declining dramatically since at least the 1950s, and the populations of many species have crashed. In fact, of all frogs assessed by the IUCN, nearly a third are either extinct or threatened with extinction. And even this is an underestimate because another third or so of the total number of frog species in the world were not included in this tally since there was insufficient evidence with which to assess their status. It seems that even in these extinction-heavy times, amphibians are suffering more than most. They enjoy the dubious distinction of being the world’s most endangered class of animal.
The Frog-Killing Fungus
So what is causing this worrying decline? Well, thanks to their delicate, absorbent skin and eggs, which must be kept moist, frogs are particularly vulnerable to changes in their environment. This means that water pollution, habitat loss and climate change are undoubtedly taking their toll on frog populations. But one of the main problems is disease.
There’s obviously one pandemic that we’ve all been aware of for over a year now (and you can check out our articles on covid-19 here for more information), but there’s another, lesser-known pandemic that’s been silently spreading around the planet for decades. It is caused by a virulent, globe-trotting strain of the chytrid fungus Batrachochytrium dendrobatidis (also known as Bd) and it is decimating frog populations worldwide. It spreads over and degrades the thin, damp skin of amphibians, which they use to breathe and regulate their water levels, often leading to cardiac arrest.
Bd is lethal to many different kinds of amphibian. In some species, it seems to have a 100% mortality rate. Many of the worst affected frogs live in Central and South America, though outbreaks also occur in Europe, North America, Africa and Australia. The only continent where there have been no major declines (apart from Antarctica, of course, which is far too cold for amphibians to live) is Asia, because it is thought that the fungus originated here and has thus co-existed with amphibians for millions of years.
One particular species of frog whose population has been decimated by the skin-eating fungus is the Panamanian golden frog (seen in the image at the top of the article). This tiny frog, whose skin ranges from light yellowish-green to bright luminous gold, is to Panama what the bald eagle is to the United States – a national cultural symbol. In local mythology, it is said that when one of these frogs dies, it turns into gold and brings good luck to those fortunate enough to see it.
Sadly, the chance of seeing a Panamanian golden frog in the wild today is probably zero. For many decades, living specimens were intensively collected and sold for high prices to the pet trade. This, coupled with the steady destruction of its wild habitat and the inexorable spread of Bd, reduced the numbers of this little amphibian to the point that it was in real danger of becoming extinct if nothing drastic was done to help it.
After it was found and filmed in the wild in 2006 for a sequence in the BBC nature documentary Life in Cold Blood (which you can view here), the decision was made to round up the last pocket of surviving toads and take them into captivity. Here, they will be kept and bred until such a time that either a cure has been found for the fungal disease, or it has wiped itself out in this part of the world due to a lack of hosts – if, indeed, any of those two things ever happen. Although the IUCN still lists this frog as ‘Critically Endangered’, in all likelihood it has been extinct in the wild for well over a decade now.
With help, the Panamanian golden frog may yet survive. But many others have not been so fortunate: Bd has already claimed entire species. One of these was Rabbs’ fringe-limbed treefrog. I have chosen this particular species as an example because it was unique – it was the only known frog where the tadpoles derived nutrition by feeding on the skin cells of their father. It became known to science as recently as 2005, living in the forest canopy of central Panama. Its discoverers, knowing only too well about the encroaching threat of the chytrid fungus, collected as many individuals of the then-undescribed frog and sent them to captive breeding facilities.
The last known observation of this frog in the wild was that of a single male heard calling (but not seen) in 2007, only one year after it received taxonomic classification, so the very survival of the species rested on those few captive individuals. Sadly, despite the efforts of several conservation teams, all attempts at captive breeding failed. When the last known female of the species died in 2009, only two males remained, both at the Atlanta Botanical Garden, rendering the frog functionally extinct. In February 2012, one of the two was euthanised to prevent suffering after a decline in health. This left the final male, nicknamed Toughie, as the very last known individual of his kind. When he died in September 2016, so too did his entire species.
In March 2019, scientists conducted the first comprehensive study into the damage done by chytrid. The results were very sobering. It was discovered that the killer fungus has driven the declines of at least 501 species of amphibian – and this is a rather conservative number because earlier die-offs from the 1950s and 60s weren’t included for lack of evidence. Worse still, 90 of these chytrid-stricken species have gone extinct or are presumed extinct in the wild, having either been wiped out completely or have at least fallen to population levels so low that scientists can no longer find any trace that they still exist. Another 124 species have declined in number by more than 90% and their odds of recovery seem slim. It would be no exaggeration to say that the chytrid fungus has damaged global biodiversity more than any other recorded disease.
So what can be done to help address this amphibian extinction crisis? In truth, there are no easy solutions. The fungus is already worldwide. No antidote has been discovered that can stop the spread of Bd in wild amphibians, and we can’t reverse the damage that it has already done. Many species have been taken into captivity to start breeding programs so that their populations can be maintained until such a time that their native range becomes safe enough for them to be released into the wild. Not only will captive management and breeding act as genetic lifeboats for many amphibians, potentially preventing the global extinction of hundreds of species, but zoos can also raise public awareness of their plight. The last zoo that I worked at, along with several others that I have visited, do fantastic work in educating the public about the threats frogs face all over the world, and I have personally engaged with many visitors about this very issue.
Of course, captive breeding is essentially useless if there is no safe, suitable habitat to return the frogs to. Unfortunately, once Bd arrives in a new place, it is extremely hard to eradicate. Some people are looking at ways of manipulating the fungus, to make it less lethal; others believe that breeding more tolerant frogs is the way to go.
But, for now, it seems that limiting Bd’s spread remains the best strategy. The global trade in frogs and toads, either for the exotic pet trade, food, research, pest control or even, as we saw in our previous Freaky Frogs post, as biological pregnancy tests, has allowed pathogens such as Bd to spread widely through many different frog populations. A 2018 study revealed that all major strains of Bd, including the one most responsible for the current amphibian crisis, are present in pet-shop animals. If we are to save amphibians, we must, at the very least, greatly curb their global trade and improve screening procedures.
Is there any good news amidst all this doom and gloom? Well, it has been determined that amphibian declines peaked globally back in the 1980s, about 15 years before the chytrid fungus was even identified as the cause. Since then, the pace of worldwide declines has slowed. Even more encouragingly, 12% of the 501 species have shown glimmers of natural recovery. But for the majority of afflicted species, populations are still far below what they once were, with many continuing to fall. And doubtlessly we are losing some species before we even know that they exist.
Frogs are ancient survivors that have been on our planet for 250 million years. They watched the dinosaurs come and go. During all that time, they’ve evolved a raft of amazing adaptations and bizarre breeding behaviours, a few of which we have looked at on this blog. And yet, in just a few decades, one pathogen, unwittingly spread by humans, has decimated their ranks, claiming entire species and triggering serious declines in others. An entire class of animals is being threatened in an unprecedented way. An entire class. If that class were, say, mammals, I think we would sit up and take more notice. But amphibians (and reptiles) have long been underrepresented when it comes to wildlife protection, even though they are some of the most rapidly disappearing species on Earth. Why aren’t we giving more funding and attention to them?
If the current crisis is not averted, it has been predicted that virtually all amphibian species will be considered endangered within a hundred years. If we are to stop our amphibians from becoming extinct, we need rapid and substantial action, not only from conservation organisations, but also from governments around the world. Buying more time for amphibians as we attempt to deal with Bd is crucial, and only by ramping up the protection of their habitats, supporting captive-breeding programs, and curtailing the trade of wild amphibians can we have any hope of achieving this.
For more information about the amphibian crisis and some of the captive management being done to help secure endangered species, visit the Amphibian Ark website.
This was our final Freaky Frogs article, at least for now. Of course, we’ve barely scratched the surface when it comes to these amazing amphibians, so I’m sure The Nature Nook will return to this series in the future. In the meantime, check out our Freaky Frog Archive in case there have been any posts that you’ve missed. And starting from next week, now that lockdown restrictions have eased slightly, we’ll be starting a new series of posts where Alex and myself will be seeing what wildlife we can spot in our local area.