An Examination of the Frog Extinction Epidemic

Although many factors are involved in this epidemic, one of the worst is the Chytrid fungus epidemic. It is being spread by Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis. This fungal disease is devastating frog populations all over the world, but particularly in Australia, and North, Central and South America.

The devastation in Central America has been particularly acute, with many species simply vanishing from the face of the Earth. Bd is just now spreading here in the US, with serious devastation of Sierra Mountain Yellow-Legged Frog populations in the Sierra Nevada. However, some populations are apparently surviving the epidemic with some some survivors intact and thereupon rebuilding their populations.

A paper in Nature (Pounds 2006) made the case that the chytrid epidemic was being driven by global warming. They suggested that Bd had always been there but had only become pathogenic in the face of global warming.

A new paper (Lips 2008) in the journal PLoS Biology challenged that theory with some interesting data. I did not read the Pounds paper, but the Lips paper was quite convincing.

Their argument is rather simple. If Bd had always been there, it would not show a spread rate typical of a spreading disease epidemic. Instead, it would tend to erupt in all places at once.

Lips' team showed first of all that Bd had not always been in the environment, that is, it was not an endemic. It appears to have escaped from an Australian lab around 1970 and from there spread through Australia. From Australia, it made its way to the Americas.

We can see several places where it seems to have been introduced, and we can plot the years of introduction on a map. So Bd is acting like an invasive alien species.

Bd appears in Costa Rica in 1987 and then heads south to Panama. It seems to be following mountain ranges there too. The number of species lost in Costa Rica is very large.

Bd spread in South America following two introductions, one in 1977 and one in 1980. The 1980 Ecuadorian introduction heads both north and south along the Andes. The 1977 Venezuelan introduction heads south along the Andes. For some reason, Bd in South America is sticking to the Andes.

This is precisely how we would expect an epidemic following an introduction by an alien species to operate - a geographical spread from a point of introduction with a rate of spread in miles per year. Furthermore, the testing of many specimens in museums failed to find Bd in any of them prior to 1977. This suggests strongly that Bd is an invasive alien fungus that was not present in the environment before.

An alternative hypothesis was not tested but did occur to me: That even though Bd was an alien exotic invasive fungus spreading after accidental introduction, global warming had somehow made Bd much more lethal to frogs. I can't figure out a way to test that hypothesis, and I guess none of the researchers are considering it. The Pounds team is sticking to their guns on this one, but I think that they are wrong.

It's a good mind exercise to read academic science journal articles that test scientific hypotheses against competing hypotheses. It's hard to read that stuff, but if you can get through it somehow, personally I find these brain puzzles to be a lot of fun. If you see learning as virtually a sensual activity as I do, this kind of stuff is almost as fun as a vacation, sports, sex or any other other purely sensual activity.

Learning and thinking is actually a blast, to me anyway. Try it sometime!

References

Lips, Karen R., Diffendorfer, Jay, Mendelson III, Joseph R., Sears, Michael W. 2008. Riding the Wave: Reconciling the Roles of Disease and Climate Change in Amphibian Declines. PLoS Biology 6:3.

Pounds JA, Bustamante MR, Coloma LA, Consuegra JA, Fogden MPL, et al. 2006. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 39: 161–167.

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