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Of the more than 7,000 known species of amphibians in the world, an estimated one third are now threatened with extinction. Hank breaks down the science behind the decline of amphibians around the world, and what you can do to help.

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I admit it, we've given you some disheartening news lately about the fate of some of the world's coolest animals, like bees suffering from colony collapse disorder, and North American bats dying in epic numbers because of the deadly white nose syndrome.

Well... sorry, but we've got one more modern extinction risk that you should know about to round out our trifecta of doom: the worldwide amphibian decline.

Amphibians include frogs, toads, salamanders, and caecilians. Many of these species live their larval lives in water, and their adult lives on land, and they use their skin as a second respiratory system. Amphibians have been around for an estimated 350 million years, surviving the many harsh changes that smoked so many other animal groups along the way.

But just over the last few decades, they've suffered sudden and serious declines, with nearly 170 species thought to have gone extinct. Today, of the more than 7,000 known species of amphibians in the world, an estimated one-third are now threatened with extinction. A recent study by the U.S. Geological Survey found U.S. amphibians are vanishing from their habitat at an average of 3.7% each year, meaning they'll be gone from half their current habitats in 20 years.

So, what's messing with Kermit, and Mr. Toad, and all their homies? Well, turns out, modern world can be a tough place for these sensitive guys. In addition to problems like pollution from pesticides, heavy metals, and acid rain, as well as mounting habitat loss, studies show that amphibians are steadily declining because of disease. Specifically, the infectious, fatal chytridiomycosis, which is caused by the aquatic fungus known as BD.

We're still not totally sure where the fungus originated, though researchers have detected BD preserved in specimens from African clawed frogs dating back to 1938. This was around the same time that these frogs were being shipped around the world for use, oddly enough, in pregnancy testing. So, one theory believes that they are the source.

Find has recently sequenced the genomes from nearly 30 BD strains from around the world, and, in doing so, realized the evolutionary history of the fungus is more complex than they originally though. So, more research is needed before we have any concrete answers regarding its origin.

But, BD is one bad, bad dude. In just the last 3 decades, it's caused the infection of at least 350 species, many of which are now extinct, in what may be the greatest disease-caused biodiversity-killer in recorded history! 

It works by infecting the amphibian's superficial skin layers that contain the protein keratin; this leads to a thickening, or hyperkeratosis, of the skin, and increased sloughing, or shedding of the skin.

An amphibian's skin is one of its most vital organs. It's thin and permeable, and involved in hydration, thermoregulation, and respiration. Many species breathe, at least partially, through their skin. The skin also absorbs vital electrolytes, like sodium, magnesium and potassium.

But when BD interferes with the absorption, it can cause nutrient deficiencies that ultimately lead to death by cardiac arrest, if the infection hasn't already made the animal so weak that it either starves to death or is eaten.

BD has been found on every continent but Antarctica, which, you know, there are no amphibians there, so... Scientists aren't sure how the fungus moves through an environment on its own, but evidence shows that it gets a lot of inadvertent help from us humans. Mainly, through the international amphibian trade, for use as exotic pets, or human consumption, particularly frogs' legs.

Climate change may also be aiding in the spread of BD. Warming temperatures, extended droughts, and general changes in the timing and amount of precipitation will affect the life cycles of amphibians all over the world. Plus, as temperatures warm, many plant and animal species shift their ranges to higher elevations. Infected amphibians migrating to these new territories bring the fungus with them, and infect new populations.

So, the world's amphibians are in crisis mode, but, like with the bat catastrophe, there are at least some things that you can do to help stop the spread of the disease and slow their decline... like cleaning your boots and gear with a bleach solution after mucking around in ponds and marshes, and not using amphibians as bait, if you're into fishing. Also, definitely not releasing your little sister's pet frog into the wild if she gets bored with it!!

Craig "wheezywaiter" Benzine: Do it for the NEWTS!

 Closing notes

Thanks for watching this episode of SciShow, and thanks to our cameo, from Craig "wheezywaiter" Benzine. If you want to continue getting smarter with us on SciShow, you can go to and subscribe.