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What’s driving amphibian decline? The world’s worst disease for biodiversity

A pair of fungi are implicated in 501 species declines, exceeding the havoc caused by rats — and house cats.

chytrid fungi
The chytrid fungi known as Bd and Bsal have done far more harm than the headline-grabbing white-nose syndrome.

A pair of skin-eating fungi responsible for the so-called amphibian apocalypse are now credited with driving toward extinction some 501 species — more than double the previous best estimate. This means they have likely caused more biodiversity loss than any other disease in world history.

For example, according to a new paper in the journal Science, they are responsible for more species declines and extinctions than such well-known predators as rats and other rodents (threatening some 420 species) or Felis catus, better known as the house cat (in the lead until now, apparently, with 430).

Against some familiar wildlife-killing pathogens, the chytrid fungi known as Bd and Bsal have done far more harm than the headline-grabbing white-nose syndrome, a fungal disease that has afflicted just six bat species, or the West Nile virus, which burdens 23 types of birds.

The sudden die-offs of frogs, toads and salamanders has been a major ecological mystery over the last 50 years or so, initially because they so often occurred in woodland environments that had generally escaped deforestation and industrial toxins.

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For a time, too, the potential role of Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans was overlooked because they belong to a class of fungi that are ubiquitous and, typically, benign. Even these two are harmless to frogs across a large swath of Asia, where they apparently originated.

Now the pair are recognized as especially effective killers. It’s not just the aggressive way they attack skin proteins, but also their willingness to feed on a wide range of host species (nearly 700). They spread by contact or through water; some accounts suggest they can “swim.” They have an ability to infect some hosts without harm, converting them into reservoirs, and when they do kill, they do it slowly — reproducing all the while.

Still, establishing the scope of their impact from scores of population declines observed across the woodlands of South and Central America, Africa, Australia and Europe was a challenge — a problem of visualizing the forest, you might say, instead of the trees.

By 2007, scientists using the tools of meta-analysis drew on collected studies to credit the fungi for declines in 200 threatened amphibian species. That inspired Benjamin Scheele, a research ecologist at the Australian National University, to assemble a team and cast a wider net — gathering more studies, interviewing researchers about unpublished findings, visiting museums to test amphibian specimens for fungal DNA. His list of co-authors grew to 40.

Their ultimate finding: 500 afflicted frog and toad species threatened by Bd, plus a salamander species afflicted by Bsal.

Of course that’s a conservative measure, because not all species declines have been traced to a cause; habitat loss, pollution and climate shifts are serious problems, too. Many declines have probably gone undetected.

Still, 501 amounts to 6.5 percent of the amphibian species known to science, and if that strikes you as a small portion then I offer you another fine example of “social math” from The Atlantic’s Ed Yong:

The scale of these losses can be hard to appreciate, especially if you think that a frog is a frog is a frog. But amphibians are ancient survivors that have been diversifying for 370 million years, and in just five decades, one disease has nearly decimated their ranks. Imagine if a new disease started wiping out 6.5 percent of all mammal species—that would be roughly everything with hooves and everything with flippers. The world would freak out.

90 extinctions so far

Have the 501 species been afflicted equally? Not at all. Some 90 are known to have gone extinct; among the survivors, 39 percent are in a continuing decline and only 12 percent show signs of recovery.

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Among those showing initial signs of rebound, Scheele notes, “recoveries generally represent small increases in abundance of individual populations, not complete recovery at the species level.” Scores of species have a current status that’s simply unknown, a function of the difficulty in counting creatures that are small and elusive, in habitats that are rugged and remote—the challenge of focusing on trees instead of forests.

Time is a factor as well; after tailing off somewhat after the 1980s, the pace of new infections has risen again since the early 2000s, for reasons that remain unclear.

Because I hope we’re all past the point where creatures have to justify their ecological niches, I won’t go on and on about the services rendered by frogs and toads except to note that they are important controls on insects and algae.

Let’s look instead at the human contribution to the amphibian collapse, which Scheele and a great many other scientists rank as an important component of the planet’s ongoing Sixth Extinction.

Had Bd remained on the Korean peninsula, believed to be its place of origin, and among amphibians adapted to its presence, the decline of 500-some species worldwide would be the stuff of dystopian fiction.

But, you see, we have this global pet-shop trade that ships amphibians from places they naturally belong to places they don’t, with an especially capable pair of pathogens riding along.

This has moved destination countries like the United States, somewhat belatedly, to place restrictions on the import of, say, a selection of  salamander species likely to be carrying disease. Which prevents the introduction of new problems, at least for a while; but nothing really can be done retroactively to limit the impact of previous arrivals. As Scheele put it in his university’s announcement of the findings:

It’s really hard to remove chytrid fungus from an ecosystem — if it is in an ecosystem, it’s pretty much there to stay unfortunately. This is partly because some species aren’t killed by the disease. On the one hand, it’s lucky that some species are resistant to chytrid fungus; but on the other hand, it means that these species carry the fungus and act as a reservoir for it so there’s a constant source of the fungus in the environment.

No barriers to global spread

In the paper itself, he put it more sharply, arguing that human action has returned the world to the state of unobstructed migration available to life forms when there was but a single land mass and a single ocean:

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Global trade has recreated a functional Pangaea for infectious diseases in wildlife, with far-reaching impacts on biodiversity, livestock, and human health. Effective biosecurity and an immediate reduction in wildlife trade are urgently needed to reduce the risk of pathogen spread.

An accompanying commentary in the same issue of Science expanded on this point. Here’s what a pair of Canadian biologists, Dan A. Greenberg and Wendy J. Palen of Simon Fraser University, had to say:

Unless the winds of wildlife trade shift considerably in the near future, the expanding rate of biotic exchange suggests that there will be greater opportunities for once­ isolated pathogens to proliferate across the globe.

It is troubling that Bd had apparently spread globally long before it was identified by scientists or before any action could be taken. Although technology and knowledge have improved, biosurveillance may ultimately be ineffective, given the potentially vast diversity of currently unknown pathogens. Though Bd is likely exceptional, owing to its aforementioned traits, it may still be a harbinger of other disease outbreaks to come as humans continue to inadvertently spread pathogens around the world….

Just as trade-driven diseases have left their mark on our own history, so too has the human-mediated spread of wildlife pathogens already begun to reshape the evolutionary history of life on Earth.

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The Scheele paper, “Amphibian fungal panzootic causes catastrophic and ongoing loss of biodiversity,” along with the commentary, “A deadly amphibian disease goes global,” can be found here but access is not free.