STATESMAN JOURNAL (Salem, Oregon) 13 January 08 Bullfrogs devouring Oregon's native amphibians - One small leap into a pond is one big gulp for a frog (Henry Miller)
When it comes to dining etiquette, the American bullfrog has only two rules:
Dinner has to be alive, and it has to fit in the frog's mouth.
"A bullfrog will eat anything that moves that's smaller than it is," said Chris Rombough, a consulting herpetologist and frog researcher.
The non-native invasive bullfrog's indiscriminate tastes have made the amphibian the scourge of native red-legged and tree frogs and amphibians such as juvenile western pond turtles.
Once they reach adult size, there's not much in the pond that's larger than an adult Rana catesbeiana.
"The creature goes from an almost microscopic larvae to a frog that may weigh 2 pounds or more," Rombough said.
And there's millions of them in Oregon ponds and sloughs that humans have made much more comfortable for bullfrogs than for native frogs and turtles.
The bass-note choruses of the males fill the warm air with a cacophony of "houm, houm" during their midsummer mating rituals.
Because their gullets are so big, scientists use the graphically descriptive phrase "gape-limited" as to what defines bite-sized to a bullfrog.
In other words, anything that fits in their sizable mouths can be swallowed.
"Because they eat anything that moves, I've taken everything from insects to mice from the stomachs of bullfrogs, with a few reptiles, amphibians and fish thrown in there, too," Rombough said. "Other unusual prey items recorded for bullfrogs include blackbirds, crayfish, scorpions ... and even the western rattlesnake."
Rombough, who lives in St. Helens, is a researcher in his company Rombough Biological. As an independent consultant, he has collected data on bullfrogs for state, federal and academic research in the Pacific Northwest for one and a half decades.
Because of their broad dining habits, bullfrogs literally are eating native frogs and turtles out of house and home.
"Bullfrogs and pond turtles -- not a good mix," said Dr. David Shepherdson, Oregon Zoo's conservation program scientist. "Until we started controlling bullfrog numbers, it looked like all of the (turtle) hatchlings would be eaten.
"Not all of them were eaten by bullfrogs, obviously, but a large proportion of them were."
One of the zoo's projects is to raise western pond turtles and release them at a size that's large enough that they won't fit in an adult bullfrog's mouth. (see related story, Page 4A)
Bullfrogs have been in the West for so long that most Oregonians don't realize they aren't natives.
"Most people have no idea," Rombough said. "Heck, most people think bullfrogs are native to Oregon. When I was growing up, I thought that, too."
Bullfrogs came west in the wake of the California gold rush in the mid-1800s.
"The story in California was that bullfrogs were brought out to replace native frogs in restaurants," said Michael Adams, a U.S. Geological Survey researcher at Oregon State University. Adams has studied the interaction of dragonflies, bullfrogs and bluegills.
"There was a commercial frog-hunting market, kind of like market hunting for ducks," Rombough added. "They literally harvested hundreds of thousands of pounds every year, literally millions of pounds across their range.
"And basically they virtually extirpated the California red-legged frog. Not wiped them out, but to the point where they were no longer economical."
Canny speculators had an idea: frog farms using bullfrogs.
They knew a lot about the booming frog-leg market, but squat about biology, Rombough said.
"It doesn't work because not only do they grow slowly, but (bullfrogs) only eat live food, and they're cannibalistic," he said with a chuckle. "So the only ones making any money were the breeders in the East.
"And once they found out it wasn't going to work, they released all their animals."
Rana catesbeiana got to Oregon in one of those classic "it seemed like a good idea at the time" tales similar to the frog farm California bullfrog boondoggle.
The invaders were given the keys to the state and even chauffeured around, according to "The Coming of the Pond Fishes," a highly respected 1946 history of the introduction of non-native fish in Oregon by the late journalist Ben Hur Lampman.
Oregon fishery officials not only brought the adult bullfrogs in from Idaho, but set up a captive breeding program for bullfrogs in the 1920s at the McKenzie River Hatchery.
And from there the offspring were outplanted and spread throughout the state in the 1930s and 1940s.
"One of the misconceptions about bullfrogs is that they're some amazing super amphibian that's way more aggressive and way more adaptable than our natives, and that nothing has a chance to stand up in the face of them," Rombough said. "The truth is that they had a lot of help getting established in Oregon.
"And if you look -- and I actually have stocking records as far back as 1936 -- and they were stocking bullfrogs."
They were classified as a game species for decades, and a fishing license was required through 2000 to pursue bullfrogs in the state, according to that year's edition of the Oregon Sport Fishing Regulations.
"For many years within our regulations, they were classified as a game fish," said Gary Galovich, a warmwater fish biologist in Corvallis with the Oregon Department of Fish and Wildlife. "But, of course, now they're classified as a non-game animal, and there's no restrictions on their harvest."
With that kind of early encouragement, it's no wonder bullfrogs are so widespread in Oregon, Rombough said.
"They were desirable, so they were brought in and had help dispersing," he said. "And they were protected for a number of years."
Ironically, some people still want to move bullfrogs around, Galovicy said.
"With bullfrogs, I still get requests occasionally from folks who realize they're good eating, and maybe they're not familiar with the impacts on native wildlife," he said. "And they say, 'Hey, maybe it's a good idea if maybe I bring some into my pond.'
"Actually, they are considered a controlled amphibian in the state. That means you can't import, purchase, sell, barter or exchange bullfrogs."
"In some situations bullfrogs take the heat for the introductions of a lot of other stuff," Rombough said about other predatory and opportunistic non-native invasives. "Things caused by bass ... bass are extremely efficient predators. But nobody's going to pick on bass because they're a favored game fish."
Native yellow-legged frogs and Umpqua chub have been almost wiped out by bass on the Umpqua River, according to his surveys that he did for one study.
But there are places where bullfrogs are definitely a factor in the decline of native amphibians, he said.
"I've seen cases of, for example, breeding ponds of Pacific tree frogs where bullfrogs, once they become active in the spring, will probably clean out the entire breeding population," Rombough said.
"I've opened up bullfrogs that are just packed with Pacific tree frogs."
But a lot of the bullfrog's success in Oregon, and the attendant problems that has caused, have to do with the way that humans have changed the environment that help the non-natives, he added.
Bullfrogs are the antithesis of the canary in the coal mine.
A chirping bird indicates the air is safe to breathe.
The bullfrog's booming call indicates environmental changes that are detrimental to native species such as frogs and turtles.
Bullfrogs in their native range on the East Coast and from east of the Rockies south into northern Mexico, thrive in warm, year-round, dead-water ponds and lakes with lots of water plants.
In Oregon that perfectly describes a farm pond, ornamental backyard water feature, gravel borrow or backwater slough, particularly if they are choked with non-native invasive weeds.
None of those situations is the historic natural Oregon environment west of the Cascade Range. Native frogs and turtles thrive in the streams, rivers and seasonal pools and ponds with streamside trees and bushes.
"In a lot of places where you see bullfrogs, it's not because they have successfully outcompeted the natives or wiped out an area, but it's because the habitat's been degraded, and the bullfrogs are better able to tolerate it," Rombough said.
Requirements to replace wetland lost during a construction project are a classic example of the kinds of changes that favor the invaders ahead of the natives, he added.
"They make a mitigation wetland, and it basically just ends up being a duck pond," he said. "It's just a permanent pond sitting out there in the middle of a field, and it's quickly colonized by bullfrogs, which can tolerate the lack of cover because they're very aquatic.
"Red-legged frogs need a forested marsh that they can escape into. They don't spend a lot of time in the pond itself."
Along with habitat changes as well as lack of predators, diseases and parasites that are found in their native range, bullfrogs have several other natural advantages.
Fortune favors the fecund.
And no native amphibian produces more eggs than a female bullfrog.
"For an example, our most fecund native frog would be either the Oregon spotted frog or the northern red-legged frog," Rombough said. "And they'll produce a huge egg mass, maybe 5,000 eggs.
"But a small bullfrog just starting out will lay 6,000, 7,000 eggs. And a big bullfrog will lay up over 20,000. And sometimes they'll double-clutch (lay twice in the same year)."
The tiny, pinpoint eggs that are laid in a glossy layer on the surface of the water hatch rapidly, most in less than a week, Rombough said.
And because they are on the water surface and exchange oxygen with the atmosphere, the eggs can survive in ponds in which dissolved oxygen levels are almost non-existent, typical of a water body choked with non-native invasive weeds.
The bullfrog's astronomical egg numbers ensures its survival because under normal conditions in Oregon it takes two years to metamorphose from larvae through large, thumb-sized tadpoles to frogs.
And for most of that time they are a food item for everything from native herons, mink and otters, to non-native species such as bass and bigger bullfrogs.
"Just about everything eats bullfrogs," Rombough said. "So it's just a numbers game.
"You put enough out into the environment, some of them are going to make it. You figure that maybe 4 (percent) to 5 percent of what's getting laid as eggs is actually making it to metamorphosis. Everything else is getting hammered."
But back to that gape-limited diet. Once they metamorphose from tadpoles to frogs, there's no native frog that can eat a bullfrog, and the tables are turned.
And that's where another advantage enjoyed by bullfrogs kicks in.
If they make it to adult size, they're around for a long time, as much as seven years to nine years in the wild.
Another biological advantage for bullfrogs, and one that could be far worse for native amphibians in the long run than environmental changes or their rapacious appetite, is the arrival of a deadly chytrid fungus, chytridiomycosis.
It's been blamed for decimating, and in some cases wiping out, populations of frogs worldwide.
"What we've realized is a big factor in some of the amphibian declines around the world is this disease," Adams said. "And what seems to be emerging is that bullfrogs carry the disease. They carry the pathogen, but they are not, at least in some cases, as sensitive to it as the natives."
Adams said that a preliminary study that he did was inconclusive on the bullfrog-as-carrier hypothesis.
"But it's something that needs to be looked at," he said. "It's something we're concerned about."
That research is continuing, he added.
Bullfrogs devouring Oregon's native amphibians