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The Growling Grass Frog (Litoria raniformis) is listed as endangered because of an estimated 50% population decline over the past 10 years.

In the 1980’s, amphibian researchers conducting species inventories in Australia, began noticing dramatic (up to 50%) declines in the populations of certain frogs. Soon, other researchers on other continents were also noting declining species counts, and even near extinctions. Curiously, the frog species that were most affected–many facing imminent extinction–were those whose habitats were at higher elevations. Despite the more extreme environment, pockets of wet soil and cavernous pools persist in these locales, providing adequate habitat for adapted amphibians.

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At first this decline was attributed to habitat destruction (and habitat “splitting”), and possibly also some effect from global warming. But many of these locales are extreme–high mountain elevations typically–where habitat destruction was minimal. After extensive study and analysis, a fungus found on many of the endangered frogs (or dominant in their local habitat) was identified: Batrachochytrium dendrobatitis (BD), a member of family of fungi referred to generally as chytrids. Some chytrids are uni-cellular and possess a flagellum (a whip-like tail that provides locomotion). Although this fungus is strongly associated with the frog declines, how exactly it contributes to frog mortality is unknown. It is possible that the fungus interferes with a frog’s skin function (inhibiting skin respiration, or, its osmotic regulation). *

Results of a recent study conducted by amphibian experts (Longcore et al, reported in this month’s The Scientist), found chytrid fungi, surprisingly, “dominating”  high-altitude, wet soils. Many species of the fungus were found in locations with little organic matter (something that frogs generally need). However, the BD fungus–believed to be the lead culprit in global frog declines–was not found amongst any of the sampled habitats. This absence of the frog-killing pathogen was of equal surprise to the researchers, perhaps even a disappointment, but nevertheless, the strong presence of a related fungal species offered an important clue to its widespread appearance in other extreme environs.

Mountain tops frequently experience high winds. Many fungi, as part of their life/reproductive cycle, produce spores which can be easily blown aloft and transported over great distances. Such spores, or asexual “resting structures”, are quiescent, protective packets of genetic material that are resistant to heat and dessication (loss of water), and whose growth is triggered only when the spore has found the most favorable environment (such as where frogs inhabit). It is not known currently if the BD fungus possesses this same facility, but if it is shown to, this could help tie together the two biggest known clues: the association of declining frogs with BD, and, frogs that inhabit more extreme environs seem to be the ones experiencing the most dramatic declines.

The study jibes with earlier studies (Lips et al) in Panama and Costa Rica showing that, at higher elevations (above 1000 meters), frog populations were practically decimated by chytridiomycosis (the disease caused by the BD fungus), while below this elevations, many frog species were still unaffected by chytrid fungi.

As always, the biological and ecological interactions between animals and the pathogens that persist in their environments are exceedingly complex. More studies are needed. But for now, this most recent research is providing some of the first clues to understanding these interactions between fungus and frog, and may lead soon to global efforts to mitigate the frog declines and restore their numbers.

* See the comment to this bog for an update on the role of the BD fungus in frog mortality.

photo: CC - By - SA

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