Are Frogs Becoming Resistant to the Amphibian Chytrid Fungus?

The arrival of a new disease into a naive animal or human population usually results in strong impacts initially, but the severity of these effects often subsequently declines as the host animals develop resistance to the disease. Recent observations of amphibians persisting following die-offs caused by the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") has raised speculation that these post-crash amphibian populations, including those of the mountain yellow-legged frog, are resistant to Bd. Development of resistant populations would dramatically increase the likelihood of population persistence despite the presence of Bd, and would provide ideal source populations for reintroduction efforts.

However, are amphibians really developing resistance to Bd? Unfortunately, as of yet there is no evidence of this in mountain yellow-legged frogs or in other species. Mountain yellow-legged frogs taken from populations that are persisting with Bd and introduced into nearby lakes often experience Bd-caused die-offs within a couple years. Furthermore, frogs from persistent populations succumb to Bd when held in the laboratory under a wide range of environmental conditions. Both results indicate that frogs from persistent populations are clearly not resistant to Bd. So, if frogs aren't developing resistance, how do we explain the persistence of some populations following Bd outbreaks?

There are at least three explanations in addition to frog resistance to Bd. First, it is possible that Bd strains differ in their virulence and that infection by a less virulent strain allows persistence while more virulent strains cause extinction. Second, habitat conditions (e.g., water temperature) may influence the amphibian-Bd interaction, either by changing Bd virulence or by changing the amphibian immune response. For example, in warm climates where air and water temperatures stay in a range that is detrimental to Bd (above 30 degrees C), the effect of Bd may be much less severe compared to in cooler climates that are more favorable to Bd. Third, persistence of amphibian populations in the presence of Bd may simply be a consequence of density-dependent disease dynamics. If higher frog densities allow higher Bd densities ("positive density dependence"), then after a frog population crash Bd densities should decline. Under these conditions, it is possible that frogs would experience Bd infections of lower intensity, allowing frog persistence. This persistence may be short lived, however, because if frog density subsequently increase so too will Bd, likely resulting in another frog population crash.

All four of these explanations for frog persistence are plausible, but all remain speculative. We're working to understand the role that each of these might play in allowing frog persistence but with numerous field and laboratory experiments either underway or planned in the near future, we are likely several years away from definitive answers. With Bd having already caused massive declines of mountain yellow-legged frogs across their entire range, time is definitely not on our side.

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