Like many frog populations around the world, Eastern Sierra Mountain yellow-legged frogs are currently on a trajectory toward extinction. The primary culprit here might once have been non-native trout, which prey upon tadpoles and adult frogs, but since recovery efforts in the 1990’s, another threat has brought struggling frog populations back to the brink.
That threat is the amphibian disease chytridiomycosis, or chytrid, which has decimated amphibian populations across the globe. The disease is transmitted via a fungus, Batrachochytrium dendrobatidis, or Bd, that attacks frog skin, depriving it of oxygen. The prevailing theory is that the Bd fungus spread from Africa to other parts of the world because of trade in African clawed frogs.
At Tuesday’s second SNARL (Sierra Nevada Aquatic Research Lab) Spring Lecture, Dr. Thomas Smith of U.C. Santa Barbara’s Department of Ecology, Evolution, and Marine Biology, addressed the potential consequences of the frog’s decline on the Eastern Sierra ecosystem.
As Dr. Smith explained in his lecture, Mountain yellow-legged frogs primarily reside in high alpine lakes at 6,000 to 12,000 feet elevation. The frogs’ historic range once stretched from the Upper Kern River to south Lassen National Park, but since the introduction of non-native trout, populations have declined by over 90 percent.
Then chytrid hit the remaining populations.
“It happens fairly rapidly that frogs all get sick and then die,” said Dr. Smith. “There are now just a handful of really big, healthy frog populations out there.”
In 2007, Dr. Smith began studying what effect if any the disappearance of Mountain yellow-legged frogs might have on insects and algae in high Sierra lakes. He looked at lakes in Kings Canyon National Park with healthy frog populations, lakes with declining populations, and lakes where frogs had gone extinct, surveying each of these 22 lakes three times a year for four years.
Why? Because, as Dr. Smith noted, the extinction of top predators often has what’s called a cascade effect: for example, when wolves were extirpated (eradicated completely) from Yellowstone in 1926, the elk population exploded; elks overgrazed aspen trees, and many riverbanks began to erode.
Mountain yellow-legged frogs are considered a top predator in alpine lakes, but “The story is not that simple in this case,” Dr. Smith said.
What Dr. Smith discovered from his research is that, surprisingly enough, the decline of frog populations had little to no effect on their ecosystem. Mature frogs rely on insects for their diet, while tadpoles rely on algae, yet neither insects nor algae increased in lakes where historic Mountain yellow-legged frog populations had declined or disappeared.
In fact, only Garter snakes may be affected by the disappearance of the frogs. According to research performed by Dr. Roland Knapp, “frogs are major food for Garter snakes,” Dr. Smith said. As frogs have declined, evidence suggests snakes have as well.
“We call this a secondary extinction, or cascading extinction,” Dr. Smith said. The secondary extinction of Garter snakes is, he said, “extremely likely.”
The lesson of Dr. Smith’s four years of research: “Not all extinctions have dramatic consequences,” he said. In the case of Mountain yellow-legged frogs, it turns out algae is more pivotal to the health of alpine lake ecosystems. Algae grows slowly, and because it is not in great abundance in high alpine lakes, all other species; insects, frogs, and snakes, are reliant on it to survive. This means any impact to the ecosystem would come from the bottom (algae) up, rather than from the top (predator) down.
There is a silver lining to the frogs’ decline having no effect on their historic habitat. If healthy frog populations are reintroduced to that habitat, the habitat, having remained healthy while they were gone, will welcome them back.