Climate change may boost frog disease chytridiomycosis

The BBC reported last month on a very interesting research that considered the effects of a variable temperature, thereby mimicking climate change, on the susceptibility of live Cuban tree frogs for the chytrid fungus.  What makes this interesting is that it looked at the interplay of two potential agents of decline and therefore comes a bit closer to real world conditions than other studies might have done. In my discussion of the article by Beebee & Griffiths from 2005, titled ‘The amphibian decline crisis: A watershed for conservation biology?”, we saw that they formulated several research priorities to counteract the amphibian decline crisis. One of those priorities referred to a larger focus in research on multifactorial approaches: taking into account several agents of decline in the same study to determine whether they have a synergistic effect or not. Although there still remains a lot to be looked at before we can state that indeed there is a synergistic effect between climate change and chytridiomycosis, this research provides a good first step in unraveling that relationship.

Now follows the original article by the BBC

Cuban tree frog. Copyright: SPL

More changeable temperatures, a consequence of global warming, may be helping to abet the threat that a lethal fungal disease poses to frogs. Scientists found that when temperatures vary unpredictably, frogs succumb faster to chytridiomycosis, which is killing amphibians around the world. The animals’ immune systems appear to lose potency during unpredictable temperature shifts. The research is published in Nature Climate Change journal

Chytridiomycosis, caused by the parasitic fungus Batrachochytrium dendrobatidis (Bd), was identified only in 1998. It affects frogs and their amphibian relatives – salamanders, and the worm-like caecilians – and has caused a number of species extinctions.

“I’m not convinced that the effect we’ve discovered could be considered responsible for declines or extinctions in the  way that the spread of Bd can be considered responsible,” said Thomas Raffel, lead scientist on the new research. “It might be, however, that climate change has sped up the decline or extinction after the parasite arrived,” the Oakland University researcher told BBC News.

Over the years, various teams of scientists have conducted a whole raft of experiments to find, for example, whether Bd is more active in warm or cold temperatures. The new research looked at what happens in a more real-life situation – when chytrid fungus is actually on a vulnerable frog. And the key variable the scientists looked at was variability of temperature, rather than temperature itself.

Cuban tree frogs (Osteopilus septentrionalis) infected with Bd were kept under various conditions. In some, the temperature was kept constant at either the bottom or top of their natural range (15C and 25C (59F and 77F)). In others, the temperature was switched predictably between the two values, mimicking the natural day-night cycle; and in a third set, the temperature was switched between 15C and 25C unpredictably.

Bd spores spread from the skin of amphibians. Copyright: SPL

On its own, the fungus fared better in cooler conditions, and when the temperature changes were regular. But when it was already on the frogs, the pattern was reversed; the fungus grew faster under unpredictable temperature change. The explanation is that being a simpler organism, it is able to adapt faster than the frogs’ immune system.

Previous research has found alterations in frogs’ white (immune) cells due to temperature changes. But Dr Raffel suggested it was hard as of now to project what this meant for amphibians and the chytrid threat.  “There’s a lot of observational evidence that climate change is leading to increased variability and unpredictability of temperature and precipitation, and it’s entirely possible that the kind of effects we observed could become more important in the future,” he said. “But I think it’s really difficult to make extrapolations – partly because work needs to be done with additional species, and also because we haven’t done the experiments yet that would allow us to make predictive models in a quantitative way.”

Herpetologist Benjamin Tapley from the Zoological Society of London (ZSL), who was not involved with the study, also suggested it was too soon to draw strong conclusions. “This paper presents some interesting and potentially useful information on climatic shifts and Bd,” he said. “But there are now over 7,000 species of amphibian, and the relationship between each of these potential hosts and Bd will be species-specific; so I would be cautious of drawing broad scale assumptions.”


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