Around 250 million year ago the first true amphibians started a new evolutionary path from their fish ancestors. Out living dinosaurs and surviving several mass extinction events, amphibians have been key figures in Earths living history for millennia. Today there are approximately 7000 species spread across every continent expect Antarctica. Unfortunately amphibian species across the world are finding it hard to survive the current mass extinction event, the Anthropocene. Can these ancient creatures prosper in our human dominated world, or will we lose them all together?
Amphibians are ectothermic, tetrapod vertebrates who are intermediate in many characters between fish and reptiles. All species have gilled aquatic larvae who mature to be air-breathing adults. There are three modern orders of amphibians; Anura (frogs and toads), Urodela (salamanders and newts), and Apoda (limbless caecilians). We have all grown up with amphibians in our lives, whether with pond dipping in school, watching the life cycle from tadpole to adult in the garden pond or hearing the nightly chorus of frogs in the tropical jungles. However, for future generations this may not be the case.
Half of the world’s amphibian species are now globally threatened, many of which could be considered as functionally extinct. Habitat loss and degradation and over consumption have played a major role in many of the reported declines. But these factors cannot explain the decline of amphibians living in or near streams in the protected rainforests of eastern Australia, and central and South America. Scientists across the globe are now in agreement that amphibians are facing their biggest challenge yet, a deadly fungus called chytridiomycosis (chytrid).
The impact of chytrid has since been described as “the most spectacular loss of vertebrate biodiversity due to disease in recorded history”. Only identified in 1998, the fungus has already caused 200 recorded extinctions, and many more are expected. Caused by the parasitic fungus Batrachochytrium dendrobatidis, this deadly fungus has been implicated in enigmatic declines in several continents around the world. But how has a fungus been able to cause so much destruction to all amphibians?
The answer can be explained through one important biological function of amphibians. Most amphibians “drink” water and absorb important nutrients such as electrolytes through the skin and not through the mouth. Other amphibians like the lungless salamanders use their skin to breathe. When chytrid fungus attacks the outer skin layers of the animal the skin stops functioning normally. Abnormal electrolyte levels as the result of chytrid damaged skin cause the heart to stop beating and the death of the infected animal. In the case of the salamanders, the fungus stops skin function and causes the animal to suffocate. Although extremely gruesome on an individual scale, the fact that the fungus has become so globalised and able to devastate entire populations is the most pressing concern.
Following on, scientists have been doing their best to find the reason why the fungus has been able to spread so quickly and only recently become so devastating. Many believe that the fungus has been able to flourish under the current climatic changes we are undergoing through human induced climate change. Research in Central America (Pounds 2006) found that most of the missing harlequin frog species vanished in warmer than average years. It was also found that when temperatures vary unpredictably, frogs succumb faster to the fungus. It appears that warmer conditions are optimum for the pathogen and that amphibian’s immune systems lose potency during unpredictable temperature shifts. Further research found that it is not as simple as warming conditions, the pathogen can flourish as the daily average minimum and maximum temperatures of warm months and cool months have converged. Keeping the climate more stable and allowing the pathogen to make a strong hold.
Although climate change can explain why in some areas the pathogen has been able to flourish, scientists are not fully convinced this is the reason why the fungus has been able to spread so far. Optimum conditions don’t necessarily mean the fungus will be there. The actual spread of the fungus appears to have occurred thanks to the international amphibian trade. Australian researchers calculated that more than a billion frogs were raised each year for human consumption. Demand for amphibians through local and international trade is high and fuelled by use of frogs as pets, food, bait and as a source of traditional ‘medicine’. The fungus expanded greatly between 50 and 120 years ago coinciding with the rapid global expansion of intercontinental trade. Thus, the movement of amphibians by humans – such as through the pet trade- has directly contributed to spreading the pathogen around the world as climate change gives the pathogen perfect conditions to reproduce and flourish in.
Moving on, amphibian species across the world are in trouble and we know the reasons why, but what can we do to save them? Humans need to step in and help these species recover. Critical management actions need to be taken. Action such as broad-scale surveys, precise risk assessment, and development of husbandry techniques for assurance colonies and further research into re-introductions and translocations of amphibians must be given top priority. Bringing wild animals into captivity is a last resort but when it comes to many amphibian species, this is what it has come too. For example, in Central America scientists have bought some frog species into captivity to spare them, today that’s the only place they exist. An international effort called Amphibian Ark is underway to keep captive populations of some 500 amphibian species that would otherwise become extinct. By keeping these insurance populations we can stop these species vanishing, but we must protect or create suitable habitat in the wild to eventually put them back where they belong.
Furthermore, some scientists are taking this captive population idea further and trying to introduce individual amphibians back into the wild that have been made immune to the fungus. For example the endangered mountain yellow-legged frog (Rana muscosa), of which the wild populations have seen a 90% reduction, has been put on a medical trail trying to immunise the species against the chytrid fungus. If the immunised frogs survive the worst of the fungus outbreak, the frogs may eventually be able to become resistant to it on their own, generation after generation, without human help.
Although it is easy to become dejected with conservation stories like this one, there is hope. A number of specie that were reported to have declined have since been rediscovered in remnant populations. The ability of these populations to persist is of considerable interest, have they found areas where the fungus cannot persist, or has the population naturally developed immunity from the fungus? For example in Panama in 2004 the fungus wiped out thousands of frog species, with bodies littering forest floors. Recently in these same areas researchers have documented the recovery of nine frogs showing no ill effect from the fungus. Nature is resilient and can survive many potentially threatening risks, although humans pose a very serious threat with multiple offences.
Amphibians have been on this earth for millions of years before us, and yet we are pushing them towards extinction. Humans are not the cause of the chytrid fungus, however we put enormous pressure on all of the planets species through habitat loss and degradation, over consumption and human induced climate change. Amphibians are finding it hard to cope against such an onslaught of issues, leaving them vulnerable to the chytrid fungus. It is our job to stop amphibians and all other species from disappearing altogether. Conservationists work hard around the world to understand and protect our wildlife, but the rest of the planets people must seriously change their lifestyles in order for our biodiverse world to stay that way.