Life history and competition with Northern red-legged frog: a proposed management strategy
Humboldt State University
Native to eastern North America, the bullfrog (Lithobates catesbeiana) was introduced into California in the late 1800’s as a food source for the gold miners. The bullfrog’s large body size and high reproductive rate gave it a competitive advantage over native amphibians, allowing it to invade a wide variety of aquatic habitats throughout the state.
Bullfrogs are voracious predators with a broad diet that includes the eggs, juveniles, and adults of aquatic invertebrates, fish, amphibians, reptiles, birds, and small mammals. Their prey includes the threatened California red-legged frog (Rana draytonii) and endangered California tiger salamander (Ambystoma californiense). They are also carriers of the chytrid fungus (Batrachochytrium dendrobatidis), which has decimated amphibian populations worldwide. Bullfrogs are currently listed on the IUCN’s list of 100 worst invasive species, and are considered among the top reasons for native amphibian decline in California.
The eradication of bullfrogs from critical habitat is essential for the management of California’s rare amphibian species. However, management agencies currently lack an effective method of bullfrog control. Typically, adults are removed by netting, gigging, or hand capture, which is labor intensive, expensive, and often fails to reduce bullfrog numbers to the desired level. Tadpoles can be removed by draining ponds, but this method has potentially adverse effects on non-target species.
With support from a conservation grant from Sequoia Park Zoo, Tim Girod is conducting a field study to test the effectiveness of traps for removing bullfrogs from local ponds. Trapping may be an efficient method of bullfrog removal, because relatively little effort is required to set and check the traps. The traps are transparent containers with one-way gates that allow frogs in but not out. Similar traps have been used successfully by FrogWatch to capture cane toads in Australia. Tim will also be testing the effectiveness of two types of bullfrog attractants: recordings of male bullfrog calls and feeder cricket baits. If these tests are successful, the traps will offer managers a method that saves time and reduces costs associated with bullfrog control in aquatic habitats.