Frog Decline Strikes Snakes as Well

In recent decades, the populations of more than 500 amphibian species have declined, and 90 have gone extinct because of a deadly fungal pathogen, but a new study by University of Maryland and Michigan State University researchers shows the ripple effects hurt more than just amphibians.

The results, published today in the journal Science, reveal that after Batrachochytrium, commonly known as chytrid, swept through a remote forest in Panama, decimating frog populations, the number of snake species scientists could find there also declined dramatically.

“This study highlights the invisibility of other changes that are occurring as a result of losing amphibians,” said Karen Lips, a professor of biology at UMD and a co-author of the study.

Frogs and frog eggs are part of many snakes’ diets, so the researchers expected a decline in frogs to reduce snake populations. But because they are notoriously cryptic and difficult to study in the wild, how snakes fare following chytrid epidemics was mostly a matter of conjecture until now. 

Lips and her colleagues compared seven years of survey data collected in a national park near El Copé, Panama, before the 2004 chytrid outbreak caused mass amphibian die-off, with six years of survey data collected after the die-off. 

“Comparing the after with the before, there was a huge shift in the snake community,” Lips said. “The community became more homogeneous. The number of species declined, with many species going down in their occurrence rates, while a few species increased. Body condition of many snakes was also worse right after the frog decline. Many were thinner, and it looked like they were starving.” 

In addition to Lips, authors include UMD biological sciences alumnae Elise Zipkin Ph.D. ’12, now at Michigan State University, and Grace DiRenzo Ph.D. ’16, now at the U.S. Geological Survey.

The researchers can’t say with certainty how many snake species declined, because snake sightings are so rare that some species had only been seen once in the pre-chytrid surveys. The researchers could not confirm that a species had disappeared just because it was absent in the post-chytrid surveys. However, over half of the most common snakes (those observed more than five times throughout the total study) had declined in occurrence rates after the frog die-off. Further statistical analysis of the data confirmed a considerable drop in species diversity.

The remoteness of the El Copé research site and the fact that Lips had been conducting annual surveys in the years prior to the chytrid epidemic combined to provide a rare window into the rapid changes in an ecosystem following the catastrophic loss of amphibians.

“This work emphasizes the importance of long-term studies to our understanding of the invisible, cascading effects of species extinctions,” Lips said. “Everything we watched changed after the frogs declined. We have to know what we are losing, or we run the risk of undermining effective conservation.”