Snakes suffered after a frog-killing fungus wiped out their food

Karen Lips knew a wave of frog death was coming. 

The frog-killing Batrachochytrium dendrobatidis, or chytrid, fungus had begun ravaging amphibian populations in Costa Rica in the early 1990s, and by all indications would eventually reach Panama. So in 1997 Lips, a herpetologist now at the University of Maryland in College Park, and her colleagues scrambled to take stock of the biodiversity at El Copé, a tropical forest field site in central Panama, before the wave hit. 

Chytrid did hit El Copé in 2004, eliminating more than 75 percent of the frog population there. But Lips and her colleagues’ foresight allowed them also to assess chytrid’s impact on another part of that ecosystem — snakes.

These elusive frog-eating reptiles can be difficult to detect. Still, the team found that both snake diversity and average body size dipped after chytrid wiped out the frogs, a major food source, researchers report in the Feb. 14 Science. 

“When there’s a collapse [like that in frogs after chytrid], the focus is usually on the group that collapsed,” says Kelly Zamudio, an evolutionary biologist at Cornell University who wasn’t involved in the research. But the new study makes key strides toward documenting the effects of a collapse on other parts of an ecosystem. “It’s an intuitive idea,” she says, but one that has been difficult to demonstrate because biologists need good before-and-after data.

To get such data, Lips and her colleagues looked for amphibians and reptiles along 200- to 400-meter paths around El Copé each year from 1997 to 2012. The team caught whatever they could, noting the species and measuring body size. The final analysis excluded data from 2005-2006, just after chytrid had swept through the region.

“The tropical snake community here is incredibly diverse,” Lips says, “but also really poorly studied.” In part, that’s because the reptiles can be really hard to detect. “Many of these species are rare to begin with. They hide out in hard to reach places, and they’ve evolved to be camouflaged,” Lips says.

So while the effects of chytrid on frogs were obvious — “dead frogs were everywhere,” Lips recalls — it wasn’t clear what the consequences of the fungus were for snakes. (The fungus doesn’t directly harm reptiles.)

The scientists’ surveys show that the number of observed snake species went down after chytrid, from 30 to 21. But since encounter rates are low for many of these species — a dozen of the 36 species ever observed at the site were seen only once in 13 years — simple statistics can’t tell the full story. 

So Elise Zipkin, a quantitative ecologist at Michigan State University in Lansing, devised a different strategy. “Instead of trying to definitively document the absolute number of species that were there before and after, we switched things up and asked what’s the probability that there are less there than there used to be.” 

She and her colleagues used the transect data to run statistical simulations estimating the probability that both observed and unobserved snake species were present in a particular transect before and after chytrid. “We can say with 85 percent probability that there are fewer species present after chytrid,” she says.

Average body size also went down, perhaps due to lack of food, for four of the six species encountered often enough to measure. One of those species was the Argus goo-eater (Sibon argus), which relies heavily on amphibian eggs for food. 

While most snake species were likely harmed, a handful benefited from the disappearance of so many frogs. Five observed species became more common, perhaps because they have more generalist diets. The biggest winner, the eyelash viper (Bothriechis schlegelii), is known to dine on birds, bats and rodents in addition to frogs.

“Overall, chytrid has probably left things worse off for snakes,” Zipkin says. Rare species have disappeared, leaving a smaller, more homogenous community behind. Similar patterns may be occurring elsewhere, she says. “The biodiversity crisis is probably worse than we’re even able to estimate.” 

But just as the collapse of one group can send shockwaves throughout an ecosystem,lifting the health of a species or community could also benefit the whole. “I really don’t think it’s hopeless,” Zipkin says “There’s still so much we can do, like preserving remaining habitats, to preserve biodiversity.”