Bullfrogs: Garbage disposals of the wetlands
By Kevin Krutzsch
“On three separate occasions, we found a bullfrog inside a bullfrog, then another bullfrog inside that bullfrog,” said Cecil Schwalbe, a herpetologist at with the U.S. Geological Survey stationed at the University of Arizona.
Sitting underneath a tree in the courtyard of the UA School of Natural Resources and the Environment, he explained that a bullfrog is capable of eating anything it can shove into its gullet. This has created problems for other species.
Bullfrogs have already decimated some populations of species indigenous to the United States, such as the Western spadefoot toad and Mexican garter snake. In the Southwest, populations of Chiricahua leopard frog have dwindled, in large part because of competition between bullfrogs, as well as a fungal disease carried by bullfrogs. The threatened Chiricahua leopard frog now only resides in about 25 percent of its traditional homeland.Researchers and environmental groups are involved in efforts to eradicate bullfrogs because of their potential to harm native species such as the Chiricahua leopard frog.
Overpowering the competition
“They have been known to snatch bats right out of the air,” Schwalbe said, noting that their ability to feed on almost any type of prey is one of the key elements to the bullfrog’s devastation on indigenous species. In addition to garter snakes and other frogs, mice, fish, crawfish, and even birds can find themselves ending up as a food source to bullfrogs.
Bullfrogs are the largest frogs in North America. They have been known to reach sizes of up to 8 inches, with the legs adding another 7 or so inches onto their length.
Besides having the advantage of eating almost anything they can fit into their mouth, bullfrogs can use sheer numbers to overwhelm their competition. Female bullfrogs have clutches containing 15,000 to 20,000 eggs, Schwalbe said. That’s about ten times more than a typical frog, including the Southwest’s threatened Chiricahua leopard frog.
Partly because of their ability to eat a wide variety of prey, bullfrogs can survive anywhere with a permanent source of freshwater. They recently have been found to travel longer distances than previously believed. Previously, bullfrogs were thought to have a range of only two miles from a permanent body of water. Recent studies by U.S. Geological Survey and University of Arizona researchers have found they are able to travel six miles or more from a body of water.
“We actually saw bullfrogs over 10 miles from a source,” said Phil Rosen, a research scientist with the UA School of Natural Resources and the Environment who holds a doctorate in ecology and evolutionary biology.
“Many leopard frogs have been eliminated from their native habitats ” said Schwalbe. The leopard frog population in the Southwest now is restricted to small bodies of water where the bullfrog doesn't thrive.
Other animals affected by these bullfrogs are Mexican garter snakes (Thamnophis elegans errans). The bullfrogs are able to eat juvenile garter snakes, and many larger snakes have visible scars on their tails from bullfrog attacks.
“The bullfrogs can even eat full grown males,” said Schwalbe. They can threaten the population because adolescent snakes are eaten by the bullfrog before they can reach maturity. “The only ones left are the older females and one of them died because of infection from the bites on her tail,” said Schwalbe.
Carriers of disease
The bullfrogs also have another way of hurting local amphibians – through the spread of a deadly fungus called the chytrid fungus. Bullfrogs are carriers of this fungus, as it resides on their damp skin, waiting to spread to another organism.
Recently a team of international researchers took skin samples from bullfrogs in North America, Brazil, Uruguay, the United Kingdom, France, Italy and Japan, according to National Geographic Society studies reported by the BBC News. All of the skin samples from bullfrogs that were taken tested positive for the chytrid fungus except for the ones from Japan.
This information worries the scientists who are trying to eradicate the bullfrogs from infected areas, because the spreading of this fungus would prove deadly to many indigenous frog species. Recently one third of all amphibians were put into the high risk of extinction category, with a main concern being this destructive fungus, the BBC reported. The fungus attaches itself to amphibians’ skin and once the organism is affected it is called chytridiomycosis.
Efforts to eradicate
Because of bullfrogs’ effect on indigenous species such as the leopard frog and the garter snake, scientists have been testing and attempting to perfect numerous removal techniques to combat them.
Schwalbe, along with numerous other scientists, is researching new removal techniques to salvage the indigenous species from being extirpated from an area or driven to extinction by the bullfrog introduction. There are many plans of attack on the bullfrogs. Rosen said one is to completely drain the water source, in some cases after removing desired species for reintroduction later. The technique is effective but time-consuming and may pose conflicts with ranching operations since cattle also use these ponds.
Another method is “gigging,” or exterminating all bullfrogs by spearing them with a metal rod. This method is effective but if any bullfrogs are missed it will not work. Also, many bullfrogs are cautious and jump into the water as soon as a human is within six feet. The bullfrogs that are killed by this method are used to gain a greater knowledge of the bullfrog diet, with the content of their stomachs revealing their recent prey.
The newest method, still in development, is creating steep topography between bodies of water. “Never in any of our data has a bullfrog dispersed across a topography greater than 11 percent,” Schwalbe said, which amounts to roughly a 5 degree slope.
Schwalbe thinks that an increase in the slope around ponds might be the key to stopping bullfrogs from invading or coming back. This factor in bullfrog dispersal was discovered because of the evidence of untouched leopard frog populations where the incline was above 11 percent, or a steep topography separated two bodies of water.
The removal techniques being tested and used now are still far from perfect and are not without consequence to other native species.
“The biodiversity we have on this planet is a storehouse for information. When invasive species threaten this we lose that information for the future,” said Rosen. “If something is not done we are homogenizing the earth and losing many beautiful things.”
Kevin Krutzsch is a junior at the University of Arizona majoring in wildlife conservation. He was born and raised in Tucson and always had a passion for herpetology.