From: The Portland Oregonian
March 19, 2003

The eastern bullfrog -- an invader that's pushing out some Northwest native frogs -- has enlisted help from a secret ally: bluegill and other sunfish transplanted long ago from eastern waterways.

Researchers studying Willamette Valley wetlands have shown that the fish clear the way for the invasive frogs by eating the larvae of dragonflies, which would otherwise kill large numbers of bullfrog tadpoles.

"Bullfrogs have greatly benefited from these non-native fish," said Michael Adams, a scientist with the U.S. Geological Survey in Corvallis.

The study, in the April issue of the journal Ecology Letters, is among the first to show how one non-native animal species can increase the invasiveness of another -- a largely unexplored problem.

"We have to start looking more at how these non-native species are interacting with each other to the detriment of native species," said Selina Heppell, an assistant professor at Oregon State University who is tracking the bullfrog's expansion into Oregon.

Bullfrogs are adding to the decline of native amphibians, such as red-legged frogs, which have lost a large percentage of their wetland habitat to farm and city development. Bullfrogs are bigger and more voracious eaters than native frogs, competing for living space and insect prey.

Bullfrogs also sometimes eat other frogs, and they have proved more adaptable to waterways that have been altered by human development. Using radio tags, Heppell has tracked young bullfrogs travelling more than a mile on dry land in search of new ponds.

"We've seen them in abundances that are absolutely phenomenal -- 14,000 in one pond," Heppell said.

To figure out the role of sunfish, Adams and colleagues placed enclosures in a pond and removed all the fish, while leaving a dozen dragonfly larvae and 50 bullfrog tadpoles. In four trials, the dragonfly nymphs killed every last tadpole. Researchers then added fish to the enclosures to prey on nymphs, and a large fraction of tadpoles survived in each enclosure.

The researchers surveyed 85 wetlands across the Willamette Valley for clues to the impact of the fish in real-world conditions. Of all the variables, the presence of sunfish and deeper ponds were the most highly correlated to large numbers of bullfrogs.

Adams said the findings suggest a new strategy for fighting the bullfrog invasion: eliminating bluegill by netting or temporarily draining ponds.

"If we get rid of the fish, that might make wetlands less-suitable for bullfrogs," said Adams. Bluegill have also been shown to prey on native amphibians while ignoring young bullfrogs, which have evolved a chemical defense that makes them unpalatable to the fish.

"Bullfrogs get all the attention, but fish seem to be a bigger problem overall," Adams said. Many of the ponds in the Willamette Valley were fishless before people stocked them with sunfish and other game species.

"Adding fish really changes the system," he said.