In a Paradox, Good News for City Frogs

Urban living may mean less exposure to deadly fungus

Spring peeper (Pseudacris crucifer) calling. Photo by Taylor Hall.
Spring peeper (Pseudacris crucifer) calling. Photo by Taylor Hall.

In a world rapidly losing its species diversity, amphibians have the highest rate of extinction among vertebrates. Although the usual culprits of habitat loss and human incursion play a major role, a fungus called Batrachochytrium dendrobatidis (Bd) that causes an often fatal skin disease in amphibians has played a major role in the decline or extinction of about 200 frog species in less than a decade.  

A new study by a U.S. Forest Service scientist and collaborators shows that degraded urban areas may actually serve as refuges for some species of frogs from the deadly fungus.

Dan Saenz, research wildlife biologist with the Forest Service Southern Research Station (SRS), compared occurrence rates of Bd in spring peepers collected from forested and urban breeding sites near Nacogdoches in eastern Texas. While other studies have compared Bd rates in frogs collected from forested and deforested areas, this is the first to make the comparison using urban sites such as drainage ditches and flooded soccer fields.

The results of the study are dramatic. The frogs collected from forested habitats showed almost three times the incidence of Bd than those from urban sites.

“We chose spring peepers as our subjects because they’re known to occur in both urban and forested wetlands and they actively breed in the cooler months when the prevalence of Bd should be highest,” says Saenz. “They’re also known to have a relatively high incidence of Bd in our study area.”  

One of the urban sites where researchers collected frogs for the study. Photo by Dan Saenz.
One of the urban sites where researchers collected frogs for the study. Photo by Dan Saenz.

The researchers collected individual peepers in one visit to each of 14 different sites by listening for their distinctive calls and capturing individuals by hand. They swabbed each individual using sterile techniques, returning each frog to the capture site immediately.

“We think we captured over 95 percent of the frogs we heard calling in both habitat types,” says Saenz. “We collected 41 spring peepers from the six urban wetlands and 89 individuals from eight sites in forested habitats on two different national forests. These numbers suggest higher densities of spring peepers in forested habitats than in urban, which may be a factor in higher rates of Bd.”

Of the individuals collected in the city, 19.5 percent tested positive for Bd, while 62.9 percent of those collected in the forest tested positive. The results are similar to those of studies comparing forested and deforested sites but with more dramatic differences.

“The greater than three-fold difference we found between urban and forested sites suggests to us that something in the urban environment may augment the previously established effects of deforestation on Bd infectiousness,” says Saenz.

Previous studies concluded that cooler temperatures and wetter areas promote Bd infectiousness; lack of vegetation cover in deforested and urban areas results in higher temperatures that restrict Bd. It’s also been suggested that certain pollutants might actually have negative effects on the Bd fungus, but more research is needed in this area before any conclusions can be made.

Still, the results reveal a paradox, life protected by habitat degradation.

“Urbanization produces some of the greatest extinction rates and has already eliminated large numbers of native species,” says Saenz. “Our study adds to the paradox that disturbed habitats, including urban areas, may be acting as refuges from diseases such as Bd for species able to tolerate them.”

Read the full text of the article.

For more information, email Dan Saenz at dsaenz@fs.fed.us.

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