Biodiversity: Natural micropredators key to controlling frog-killing chytrid pathogen

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Colorado’s boreal toads have been devastated by the chytrid pathogen, as well as habitat loss and water quality degradation. bberwyn photo.

Findings may help protect amphibians from deadly fungus

By Summit Voice

FRISCO — Biologists continue to unravel the mysteries of the amphibian-killing chytrid pathogen, raising hopes that there may be a way to slow or stop the disease, which has wiped out populations of frogs worldwide.

Researchers have long wondered why the pathogen sometimes kills all the amphibians in one habitat, while apparently leaving other nearby populations unharmed. After studying mountain ponds in the Pyrenees, the scientists concluded that, in some regions, naturally occurring predators may control the pathogen.

This natural behavior will reduce the infection pressure on potential amphibian hosts and a goes a long way towards explaining the distribution of chytridiomycosis, at least in temporal climatic regions.

“The current amphibian decline is a disaster for ecosystems around the world,” said Dr. Dirk S. Schmeller, with the Helmholtz-Center for Environmental Research. “Amphibians have key roles in freshwater ecosystems, and when they are gone, far going changes are unavoidable,” Schmeller added.

Anthropogenic habitat alteration and fragmentation are the most important causes of amphibian biodiversity loss. But merely conserving amphibian habitats no longer guarantees survival. Indeed, the introduction of infectious diseases has been shown to drive amphibians to extinction even in seemingly pristine habitats.

“Bd needs to establish in a new environment and has usually a tight time window to infect a suitable host, either an adult amphibian or tadpoles and larvae of this species group,” said veterinarian Prof. Dr. Frank Pasmans, with the University of Ghent.

If Bd successfully establishes itself infections steadily increase. Above a certain threshold, amphibians start dying. In vulnerable species, local extinction can occur. Many species have already been lost, especially in Central America and tropical Australia.

After seeing a patchwork pattern of chytrid infection in the Pyrenees, the biologists embarked on a three-year study to analyze the differences between various ponds.

“The infected lakes and ponds did not look like the uninfected ones, neither in regard to the vegetation nor in regard to the geological characteristics,” Schmeller said. “When we brought in water from infected and uninfected sites, in some cases with help from donkeys, we saw clear differences in laboratory cultures of the pathogen, as well as in the infection dynamics.”

A series of additional experiments than clearly established that some microscopic aquatic predators, such as protozoans and rotifers, are capable of consuming large quantities of the infectious stage of Bd.

“The consumption of zoospores reduces the infection pressure for the whole population by reducing the number of infected tadpoles”, said Mark Blooi, with the University of Ghent.

Water bodies that do not support a diverse and abundant micropredator community, such as those that suffer from anthropogenic and environmental pressures, could lead to higher infection rates that lead to outbreaks of disease and amphibian population crashes.

“The big question to rapidly answer is, if by steering micropredator abundance and community composition, can we alleviate the impact of chytridiomycosis in natural amphibian populations? And if so, does this offer a realistic method for preservation of amphibians in Bd infected areas around the world?” said Dr. Adeline Loyau, also with the Helmholtz-Center for Environmental Research.

The work, conducted by an international research team financed by the Biodiversa-Project RACE, raises the hope for an effective biocontrol against the Chytrid fungus, one without 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.

The study also contributes to a better understanding on how ecosystem health is linked to the establishment of pathogens in new environments, as only in healthy ecosystems the community of microorganisms might be able to consume zoospores effectively. The results have been published in the January issue of Current Biology.

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