By Callum Wyeth '28
Did you know that one of California’s top ten invasive species lives right outside your door? In fact, you can hear its distinctive call in the early evening. It sounds a bit like a humming cow, which may be why it has the word “bull” in its name. It is the American Bullfrog or Lithobates catesbeianus.
The native habitat of the American Bullfrog is eastern North America; however, the American Bullfrog is invasive in the West. Here in California, it was introduced by the gold miners in the late 1800s and early 1900s as a food source, but it quickly spread to aquatic and semi-aquatic biomes throughout the West Coast.
The Bullfrog is a carnivore, or a secondary consumer. It eats mainly primary consumers like small fish, baby birds, small mammals, other amphibians, tadpoles, and lots of insects— basically, whatever it can fit in its very large mouth. It can grow up to 8 inches long and can weigh up to one pound. It has large and powerful legs and, in the winter months, can travel up to a mile per day looking for food.
This massive frog is a dangerous predator here in California and has been decimating our native amphibian populations since it arrived, most specifically the vulnerable red-legged frog and the endangered foothill yellow-legged frog that live in our local watershed. In fact, red-legged frogs were the basis for Mark Twain’s famed short story “The Celebrated Jumping Frog of Calaveras County,” but they are now so scarce that Bullfrogs are used for the competition instead.
There are three main reasons that the American Bullfrog is a threat to our native ecosystems. First, at several stages in its life cycle, this Bullfrog preys upon our native frogs. Although the Bullfrog tadpoles are mostly herbivorous, they will occasionally eat smaller, native frog tadpoles and insect larvae. Further, as adults, American Bullfrogs both compete with and eat native amphibians in such high numbers that one study done by a UC Davis ecologist in San Joaquin County, California, showed a survival rate of just 5 percent of native red-legged frogs in areas with Bullfrogs compared to areas without.
Second, the Bullfrog reproduces at a rate ten times higher than the native frog population and can lay 20,000 eggs per clutch and two clutches per year, compared to the native frogs, which average 2,000 to 3,000 eggs per year. This high reproduction rate and the ability to travel and quickly invade new water systems make the Bullfrog hard to eradicate completely and a dangerous competitor.
Finally, the Bullfrog carries the chytrid fungus Batrachochytrim dendrobatidis, but is not affected by it. The chytrid fungus is a severe skin disease that causes death in susceptible native species. It disrupts the function of the skin in both the tadpole and adult frogs and specifically affects the native Californian mountain yellow-legged frog. This fungus has caused the extinction of one hundred amphibian species worldwide since 1970.
So what can we do about this voracious, opportunistic predator?
Shockingly, California still allows the importation of Bullfrog tadpoles as pets, and adult Bullfrogs are still imported for food. Approximately 2 million are brought to California each year and many of them are released or escape to wreak havoc on native ecosystems. One step we could take in California is to stop this practice. The California Fish and Game Commission could ban the importation or increase the permit fees to make it harder and more expensive to import this invasive species. Also, if you are a resident of California, you can help! You can report Bullfrogs when you see them and not keep them as pets.