Sabin Russell Science Writer

15May/10Off

Extirpation

Yellow Legged Frog Credit:USGS

For people looking for the proverbial "canary in the coal mine," frogs offer plenty of fodder for the dark imagination. A 2004 study by the International Union for the Conservation of Nature and Natural Resources (IUCN) found that 43 percent of 5,743 amphibian species worldwide were in decline; one third of the total met the IUCN's "Red List" criteria of being vulnerable, endangered or critically endangered.    

In California, the loss of the Sierra Nevada frog population has been phenomenal, and the once ubiquitous mountain yellow-legged frogs are today's classic example -- in just a few decades they have disappeared from 93 percent of their range in the high mountain lakes. Two closely related species  are listed as "critically endangered."  

Vance Vredenburg of San Francisco State University and his colleagues have been studying the mountain frogs, and this week published a pair of papers in the Proceedings of the National Academy of Sciences based on 13 years of observations. (http://bit.ly/bg7m1M; http://bit.ly/bXSixn) Vredenburg previously has identified a fungal infection known as chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd), as the primary culprit. This is no ordinary fungus. Chytridiomycosis has been blamed for "the greatest loss of vertebrate biodiversity attributable to disease in recorded history" -- in other words, this is a genocidal bug.     

In his latest work, Vredenburg studied three mountain watersheds in Sequoia and Kings Canyon National Parks. Initially, all three were Bd-free, and home to thriving populations of yellow-legged frogs in 88 different lakes. But his meticulous observations tracked how Bd eventually appeared in each watershed, and chytrid swept like waves through various frog colonies, leading to "extirpation" -- a clinically precise word for "wiped out."  His study of the extirpation process showed how it was a function of  "fungal load," the average amount of spores on the frogskin surfaces. Fungal load would rise and reach a threshold, at which point the frog populations were quickly...extirpated. He even mapped the pace of this grisly extinction -- about 688 meters per year -- similar to the range of individual frogs, indicating the fungus was spreading frog to frog.    

What's unusual and unsettling about this is that, as Vredenburg  points out, "disease is generally not a driving force in extinctions," because typically a pathogen fades away as it knocks down the host population. The dynamics of this fungal infection in the Sierra lakes and ponds are different. Fungal load could soar to the lethal threshold in less than 50 days, causing mass mortality, a die-off so quick and complete that mitigating factors such as an immune response or "density dependence" -- that loss of virulence due to thinning of the host population -- could not kick-in. This is extirpation, disease-driven extinction -- very creepy to contemplate. So intent is this organism on eliminating its host that it lingers over the long winters, when tadpoles of yellow-legged frogs slumber under the ice. When summer arrives and they metamorphose into frogs, the fungus strikes again. Yellow-legged frogs are unusual in that they can remain in the tadpole state for three successive winters. Extirpation, therefore, is not instantaneous. It comes in three genocidal cycles.    

A second Vredenburg paper explains an exception to this dreadful scenario. There are lakes where hobbled populations of frogs seem to live if, and only if, circumstances conspire to lower the fungal load below the lethal threshold -- it's a natural strategy not unlike harm reduction. These are not happy frogs, but a few of them live on, like bunker survivalists.  

What to make of all this? Why fungus? Should we be thinking of patterns, when we also read of the devastating impact of white nose fungus on bat populations, a mammalian species? Some bat populations in Vermont are down 90 percent.     

Fungi may also be playing a role in another genocidal disease that goes under the name Colony Collapse Disorder, which is wreaking havoc in the honey bee population. A suspect -- by no means proven the culprit -- is the parasitic fungus Nosema ceranae.  

There's even a fungus that has clipped the occasional healthy human being. Cryptococcus gattii is a tropical airborne fungus that turned up in British Columbia in 1999, and has drifted south into Oregon.  A paper about this particularly virulent C. gattii strain (http://bit.ly/bTLukr), published in the April 22, 2010 issue of PLoS Pathogens is extremely dense reading and a bit unsettling. An excerpt:     

 The continued expansion of C. gattii in the United States is ongoing, and the diversity of hosts increasing. Cases have been observed in urban and rural areas, and have occurred in a range of mammals. On Vancouver Island and the mainland of British Columbia, cases have been documented in marine and terrestrial mammals including cats, dogs, porpoises, ferrets, and llamas. This trend has continued in the United States, with several cases in agrarian, domestic, and wild terrestrial mammals, as well as marine mammals, adding elk, alpacas, and sheep to the aforementioned list. The co-expansion of the outbreak among mammals and humans is significant for several reasons. Non-migratory mammals serve as sentinels for disease expansion, particularly given that isolation of C. gattii from the environment is difficult, and not yet successful at all in Oregon. Additionally, the threat to agricultural and domestic animals is significant and thus the need for cooperation among health officials is critical. Finally, the widespread spectrum of disease illustrates that the organism is likely to be pervasive in the environment, and that physicians and veterinarians should be well informed of symptoms to facilitate early diagnoses, and successful isolate collection and tracking.   

Since 2004, it has been implicated in 21 human cases in the U.S., causing five deaths -- a 25 percent mortality rate. Not quite extirpation. Infectious disease experts, including lead study author Edmond J. Byrnes, say the yeast-like bug with a penchant for eucalyptus trees is so rare that it is not a significant threat to public health -- more of a scientific curiosity, for now.     

 So... no need to panic about C. gattii.   

But it does cause one to ponder, in our very scientific way... what did we and the frogs and the bats and the bees do to the 'shrooms to make them so pissed off at us?

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Posted by Sabin Russell

Comments (3) Trackbacks (0)
  1. Gosh – what haven’t we humans done in our effort to extirpate our once lovely planet? Those fungi are just launching a little push-back as best they can. Fascinating post!

  2. OOPS. Maybe I won’t saute those Shitaki I got at farmer’s market this morning. Sabin, you just get better and more wide ranging with every post. Loved the Hawking!!!

  3. Maybe it’s time to ask Mescalito about his magical brother.


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