Invasive Bullfrogs Done in by Flash Floods

Most of the time it's great to be an invasive species: You hog all the food, take up all the space and make loads of offspring. Then again, there are those bad times when you have to face the fact that you really did not evolve for this new home and it's the natives who may have the edge.

That's the fatal lesson of invasive bullfrogs who have been making inroads into the small mountain oases of Mexico's Baja California.

Scientists surveying native tree frogs and invasive bullfrogs in the mountain arroyos were surprised to find evidence that the bullfrogs are blindsided by infrequent, powerful tropical cyclones that can hit the region.

The native frogs, on the other hand, are adapted to avoid the violent flooding that ensues after torrential rains.

"The bullfrog is not adapted to these environments," said Victor Luja of the Centro de Investigaciones Biológicas del Noroeste in La Paz, Baja California Sur.

Luja had been monitoring the amphibians of the isolated mountain oases almost every month from 2006 to 2009, when he noticed that after hurricanes passed through, dead bullfrogs could be found dragged through the canyons.

Meanwhile, the native tree frogs were spotted just coming out of their hiding places above the flood zone. He has published his discovery with co-author Ricardo Rodriguez-Estrella in the upcoming October issue of the Journal of Arid Environments.

The storms are essentially a form of natural selection that selects against the invaders, said Luja. Unfortunately, it doesn't appear that there are enough hurricanes to wipe out the invaders.

What keeps native tree frogs from succumbing to hurricanes is timing. The tree frogs of these oases are active during the winter, when wet, cool, but not necessarily torrential storms come in from the Pacific. These frogs stay hidden and dormant during the summer and fall, when hurricanes are likely to strike.

Bullfrogs are just the opposite. They live and breed at the same time the hurricanes are active, which makes them a lot more vulnerable when one crossed the Baja Peninsula, Luja explained.

The discovery meshes with what many biologists have been arguing for other places in western North America, which suffer from invasive fish.

"Fish are the classic example of that here in California," said Peter Moyle of the University of California at Davis. "If you have a dam with regular amounts of water coming out, invasives thrive."

But remove the dam or release water in a way that mimics the extreme high and low flows of many western rivers, and the invasives suffer while the native species gain ground, Moyle said.

"As a general rule, it works," Moyle told Discovery News. And the approach is "sweeping the world of river management," he said.

As for the bullfrogs of Baja California, "It's a nice example of how natural conditions can favor a native species," Moyle said. "These are good kinds of studies to do."

What's more, said Luja, the oases he studies are perfect laboratories for studying such effects, since they are so small and isolated.