Tropical Snakes Suffer as a Fungus Kills the Frogs They Prey On

Tropical snakes are masters of disguise, skillfully camouflaged and capable of holding a position for hours without moving a muscle. This made for challenging work for herpetologist Karen Lips, now at the University of Maryland, who spent 13 years counting the snakes of El Copé in central Panama.

Lips had anticipated the arrival of chytrid, a fungus that has been killing off huge numbers of amphibians in Central America since the 1990s. The effects of the disease were well documented—a massive collapse of frog populations was coming. So Lips set up wildlife surveys to track tropical snake populations that prey on amphibians before and after the fungus swept through El Copé. The study, published today in the journal Science, found that it’s most likely that snake species fell as a result of the mass frog die-off.

“It’s hard for us to pinpoint how many species there were before and after, and there’s a wide range of possible numbers,” quantitative ecologist Elise Zipkin tells Ed Yong at the Atlantic. Finding camouflaged snakes makes for touch fieldwork. Instead, she says, “we can talk about the probability of decline. That’s the best we’ll ever be doing, because there’s no scenario where we could just collect more data. We now have probably the strongest evidence that we’ll ever have that there are cascading effects.”

The researchers surveyed animal populations by walking quarter-mile paths around El Copé from 1997 to 2012, catching whatever reptiles and amphibians they could find and recording their species and body size. In 2004, chytrid killed more than three quarters of the region’s frogs, so the final data analysis excluded 2005 and 2006, when the region was adjusting.

The effect on frog populations was made clear by the absences of their nightly songs and the fact that "dead frogs were everywhere," Lips tells Jonathan Lambert at Science News, but chytrid's effect on snakes was harder to measure.

“The tropical snake community here is incredibly diverse, but also really poorly studied,” Lips tells Science News. “Many of these species are rare to begin with. They hide out in hard to reach places, and they’ve evolved to be camouflaged.”

The survey found 30 species of snakes before chytrid hit the region, and 21 species afterward. Some of the snakes the researchers found afterward were skinnier, as if they were starving, according to a statement. But because the snake species are rare and diverse, the data aren’t exactly clear-cut. Some species only appeared in the survey after the frog die-off, but they were probably still in the region beforehand. And vice versa: species that didn’t appear in the after-chytrid surveys may not have disappeared from the region.

“I don’t think I appreciated how difficult it was going to be,” Lips tells the Atlantic. “It took a long time to find someone to help us analyze the data.”

That’s where Zipkin’s data analysis came in. Instead of calculating a direct value for snake species loss, the team calculated the likelihood that snake species are less diverse now than they were before the fungus killed off the region’s frogs. After taking into account the different abundances of local snake species, how many snakes are likely to be along a quarter-mile path and how likely a surveying herpetologist is to spot one, Zipkin’s mathematical model came to a conclusion: “We can say with 85 percent probability that there are fewer species present after chytrid,” she tells Science News.

Snakes that rely heavily on frogs, like the Argus goo-eater that eats frog eggs, fared badly after chytrid. But snakes like the eyelash viper, for which frogs are just one part of a well-rounded diet of bats, birds and rodents, have done well.

“When there’s a collapse [like that in frogs after chytrid], the focus is usually on the group that collapsed,” Cornell University evolutionary biologist Kelly Zamudio, who wasn’t involved in the study, tells Science News. “It’s an intuitive idea,” she tells Science News, to look more broadly at the ecosystem, but one that requires good data from both before and after a collapse.

The likely loss of species is a common story across environments under stress. But the study also points to the ways that conservation and the protection of key members of an ecosystem, in this case frogs, can uplift an environment.

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