Monday, January 20, 2014

Hope for amphibians?

Amphibians are currently affected by a wave of global extinctions. The main reasons for this decline are anthropogenic habitat alteration and fragmentation but mere conservation of amphibian habitats no longer guarantees survival. As a matter of fact, the introduction of infectious diseases has been shown to drive amphibians to extinction even in seemingly pristine habitats. 

The most prominent example is Chytridiomycosis, a fungal disease which is devastating amphibians around the world. It is caused by a skin fungus (Batrachochytrium dendrobatidis) in short called Bd. The fungus infects the skin which for amphibians is a vital part of the respiratory system. If Bd successfully establishes, infections will steadily increase and above a certain threshold, amphibians will start dying. A number of vulnerable species have been lost already, especially in Central America and tropical Australia and the fungus spreads like wildfire. However, prevalence varies significantly at local and regional scales.

Even a single highly susceptible host species, such as the European midwife toad Alytes obstetricans, can exhibit strong variation in the prevalence of infection across small geographic scales. Mortality in this species owing to Chytridiomycosis correlates positively with altitude, which is due at least in part to the effects of environmental temperature. However, this does not explain why sites with equivalent temperature regimes can still exhibit substantial variation in prevalence and mortality associated with infection, or why Bd-positive sites were found to be more similar to each other than would be expected based on chance.

This find intrigued colleagues from Europe enough to start a whole range of experiments, which took over three years to complete, to understand, which differences between different ponds and lakes of the Pyrenees could explain such a pattern. The researchers found several difference between infected lakes and uninfected ones. Their geological characteristics were different and they were surrounded by disparate vegetation. Water samples from the sites showed clear differences in laboratory cultures of the pathogen, as well as in the infection dynamics. A series of additional experiments established that a suite of microscopic aquatic predators, such as protozoans and rotifers, are capable of consuming large quantities of the infectious stage of Bd which in turn reduces the infection pressure for the whole population by reducing the number of infected tadpoles.

I am very careful to not overstate the results of this study but there seems to be a little hope for the amphibians although the main causes for their decline (habitat alteration and destruction) have not changed.

We here show the importance of predation in controlling infections in larvae of two amphibian species and provided direct evidence that zoospore ingestion is the mechanism through which infection is modified. Development of methods that facilitate natural augmentation of predatory microorganisms as a form of Bd biocontrol may hold promise as a field mitigation tool that lacks the downsides associated with introducing nonnative biocontrol agents, such as the use of antifungal chemicals or release of nonnative skin bacteria into the environment, or the reliance of unpredictable environmental temperature to “cure” infections.

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