Climate Change and Amphibian Declines: putting the puzzle pieces together

By Jason Rohr, Ph.D. and Thomas Raffel, Ph.D.

Tree frogWe had similar interests as kids: a love for amphibians and exploring wetlands, and a passion for puzzles and mystery and detective movies. So, it should be no surprise that we both turned out to be scientists studying the mystery of global amphibian declines. Working to figure out how all the “puzzle pieces” of climate change, environment, and other factors contribute to the mysterious global decline of amphibians is like living in our own mystery movie: a pair of scientist  Sherlock Holmes-like detectives serving as interpreters for the frogs who cannot reveal clues or communicate directly.

Upon examining the literature, we noticed that declines of Latin American frogs were occurring under three scenarios: declines were occurring in (1) warm years, in (2) cool seasons, and at (3) high elevations.  What caused this peculiar pattern? The key was to find the common link among all three. 

What we discovered, with research supported by EPA funds, was that the common link was temperature variability.

Frogs exposed to (1) warm years, (2) cool seasons, or (3) high elevations all experienced more variability in temperature—at  both daily and monthly time scales—than those not (that is, frogs exposed to cool years, warm seasons, and  low elevations). 

Another clue: chytrid fungus. We found that both daily and monthly temperature variability served as positive predictors of amphibian declines thought to be caused by the fungus.  Hence, the missing link seemed to be temperature variability.

We suspected that we were on to something.  We hypothesized that pathogens such as chytrid fungus might benefit from temperature shifts and extremes because they are smaller and have faster metabolisms than their ectothermic (also known as “cold blooded”) hosts, and thus might acclimate faster than their hosts after an abrupt shift to a new temperature. 

We conducted a series of experiments to test our hypothesis and to see if it offered a causal (“cause-and-effect”) explanation for the patterns we saw for frogs living in the field. 

We discovered that frogs exposed to temperature shifts at daily and monthly time scales not only had more chytrid fungus, but were also more likely to die from these infections than frogs living in areas with more constant temperatures.  Given that temperature variability and extremes are increasing in many regions, this work suggests that climate change might be a culprit in amphibian declines.

We now are trying to serve as detectives and interpreters for other declining animals by testing whether temperature shifts spark increases in their disease risk, and whether such temperature shifts benefit pathogens in general.  That is, we are trying to determine if the story of the frogs is just one piece of a larger puzzle.

About the authors:

Jason Rohr, Ph.D., is an EPA-funded Associate Professor in the Integrative Biology Department of the University of South Florida.  He studies interactions among climate change, pollution, and disease.

Thomas Raffel, Ph.D., is an EPA-funded Assistant Professor in the Department of Biological Sciences at Oakland University.  He uses a combination of field studies, experiments, and modeling to study the ecology of parasitism in aquatic systems.

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