Image by N. McMinn
A study done in a California river by Sarah Kupferberg showed that
Bullfrog tadpoles reduced the survival of Yellow-legged Frog tadpoles and the growth of both Yellow-Legged and Pacific Chorus Frog tadpoles.
In a series of studies, Joe Kiesecker and Andy Blaustein, at the University of Oregon, showed that
Bullfrogs have a variety of negative impacts on Red-legged Frogs and that these impacts were exacerbated by the introduction of non-native
fish and habitat change by humans. Recent studies by Mike Adam, Christopher Pearl and others at the USGS research labs in Oregon, support the
role of non-native fish and habitat change in the negative impacts previously attributed solely to Bullfrogs.
Bullfrogs have co-evolved with the non-native fish such as sunfish and bass, and are not readily vulnerable
to predation by these fish. These fish also eliminate the aquatic insect predators of bullfrog tadpoles. Native tadpoles are extremely vulnerable
to predation by these non-native fish. In addition, habitat change such as the removal of shoreline vegetation and upland forested habitats
reduces the available ecological space for native frogs forcing them into close contact with predatory Bullfrogs. These same changes increase
water temperatures, which benefit Bullfrog tadpoles over Red-legged Frog tadpoles. These studies highlight the complexity of ecological
interactions and threats to native amphibians.
Photograph by C. Bingham
Photograph by S. Price
The pond on the left with the intact shoreline vegetation and surrounding forested habitats shows a pond where
native frogs persist in the face of competition from Bullfrogs. The highly simplified habitat of a backyard pond on the right shows landscapes
where Bullfrogs outcompete and extirpate native frogs.
Bullfrogs also appear to be a vector for an amphibian disease called Batrachochytrium dendrobatidis (Bd).
It is unclear how Bd kills amphibians, but it has been known to cause mass mortalities around the world. Once a Bullfrog enters a wetland it
can introduce the disease and even if all Bullfrogs are eradicated the disease may remain in the water body and infect other amphibians.