By Dr. Becker
Those of you who are long time subscribers know I love amphibians, so a recent study published in Global Change Biology1 caught my eye. New research suggests that the reason salamanders in the Appalachian Mountains are shrinking in size is climate change. According to scientists, six salamander species of the genus Plethodon are from 2 to 18 percent smaller than they were 50 years ago.
"This is the first evidence of shrinking salamanders," says study co-author Karen Lips of the University of Maryland, in an interview with Nature.com.2 "We don't know if it's a genetic change or a sign that they are able to adjust to new conditions."
Notably, while six species have shrunk in size, salamanders of a related species from the same region appear to have increased in size during the same period. And another eight species seemed not to have changed in size at all.
Research on Declining Salamander Populations Evolves Into Research on Declining Salamander Sizes
Interestingly, Lips and her colleagues didn't start out to investigate salamander sizes or climate change. They were initially trying to determine why salamander populations were in decline.
In 2011 and early 2012, they collected salamanders of several different species from almost 80 sites in the Appalachians. Their goal was to try to determine whether an environmental factor – specifically a fungal disease deadly to amphibians – was to blame for the diminishing numbers. They discovered that less than one percent of the animals they collected were infected.
That's when it occurred to the researchers that perhaps climate change was a factor, since scientists have speculated that wildlife might change in size as a way to adapt to warmer, sometimes drier conditions.
Lips and her colleagues decided to look at the body lengths of the salamander specimens they had collected and compare them to specimens collected from the 1950s through 2000s by a now-retired University of Maryland ecologist. In total, the researchers examined over 9,000 specimens across more than a dozen salamander species.
Over a 50-Year Period, Metabolic Activity of Two Species of Salamanders Increased 7 to 8 Percent
To test how climate conditions might affect the survival of salamanders, the researchers created a biophysical model to simulate daily activity of the animals. They also applied temperature and moisture data from the National Weather Service.
What they discovered was that the metabolic activity of two species of salamander – one tiny and one much larger – increased between 7.14 and 7.86 percent over the five decades between 1957 and 2007. Salamanders collected after 1980 were eight percent smaller than those collected up to that point, and each generation of salamander shrunk by one percent. According to Lips, unless salamanders eat more to offset a faster metabolism, they will shrink in size.
The reduction in size was highest in animals from the southernmost collection sites, in which temperatures increased the most and rainfall decreased the most from the 1950s to the 2000s.
The researchers don't yet know what biological processes resulted in smaller salamanders. They speculate that perhaps bigger salamanders either died off or didn't reproduce at the same rate as smaller species. Or maybe the animals have the ability to adjust biological features in response to environmental changes. They also wonder if an increasingly warmer or drier climate could alter gene activity or other biological processes, resulting in stunted growth.
Based on their computer models, Lips and her colleagues suspect that today's salamanders may have to burn more energy in warmer, drier conditions to stay as active as their ancestors did. If so, that means modern salamanders may spend more time searching for food and hiding from the heat, which makes them more vulnerable to predators and gives them less time for mating.
Future Studies Will Determine if Declining Salamander Populations and Body Size Are Related, or Separate Issues
While some scientists view the findings of this study as proof that certain species are especially vulnerable to climate change, as you might guess, the study also has its skeptics.
An ecologist at Loyola University points out that the salamander specimens collected between 1950 and 2007 weren't intended for a study on climate change. He argues that the specimens weren't collected at night, which could produce a different size distribution of animals from the same populations.
And another ecologist, Ken Dodd of the University of Florida, who in the 1970s worked alongside the man who collected the original specimens, believes Lips' study overstates the role of climate change. He contends that other potential factors, including long-term air pollution, habitat changes and forest-structure changes, weren't considered.
For their part, Lips and her team are planning field tests and laboratory experiments to answer open questions from the study. They hope to simulate past, present, and future climactic conditions for salamanders as part of future studies.
"We also need to figure out the relationship between declining populations and body size," says Lips, "to see if they're related or two separate issues."