Taxonomic name: Rana catesbeiana (Shaw, 1802)
Synonyms:
Common names: bullfrog (English), grenouille taureau (French), North American bullfrog, Ochsenfrosch (German), rana toro
Organism type: amphibian Rana catesbeiana (the North American bullfrog), has been widely distributed via aquaculture and the aquarium trade. It is one of the most frequently cultivated edible frogs world-wide. Primary concerns are competition with, and predation upon, native herpetofauna. Description
Large size: adults 10-20cm snout-to-vent length and 60-900g. Dorsal colour is light green to olive to brownish-green, often with a mottled pattern of greens and browns. A fold of skin runs from the eye around conspicuous eardrums. The mostly white ventral surface may be mottled with gray. Similar Species
Rana clamitans
More
Occurs in:
lakes, water courses, wetlands Habitat description
Bullfrogs are highly aquatic and inhabit warm, open, permanent ponds (Bruneau and Magnin, 1980; Bury and Whelan, 1986; Shirose and Brooks, 1995; in Govindarajulu, 2004), with irrigation ponds providing ideal habitat in parts of British Columbia (Govindarajulu, 2004). Bullfrog tadpoles prefer non-vegetated areas and medium depths in the early stages, and deeper water in the later stages (Smith, 1999). General impacts
Competes with native anurans. Larvae can have a significant impact upon benthic algae, and thus perturb aquatic community structure. Adults may be responsible for significant levels of predation on native anurans and other aquatic herpetofauna, such as snakes and turtles. These impacts have been documented for bullfrogs on the west coast of the United States and they are the likely ones for the species globally. Uses
Bullfrogs were intentionally introduced to new habitats as a food item for humans (Moyle, 1973; Jennings and Hayes, 1985; in Kats and Ferrer, 2003). Notes
The population growth rate of bullfrogs is strongly influenced by survival rate of metamorphs and juveniles. One of the factors that may determine survival of metamorphs is the presence of migration corridors and nearness of suitable ponds during the fall migration (Govindarajulu et al. in press). It has been suggested that bullfrog larvae may be able to recognise cues of novel predators, which could contribute to their success as a native species in a region (Pearl et al. 2003). In Oregon, the invasion of bullfrogs appears to have been facilitated by the presence of the non-native sunfish. Native dragonfly nymphs reduce survival of bullfrog tadpoles unless the sunfish is present to reduce dragonfly density (Adams et al. 2003). Geographical range
Native range: The bullfrog is native to central and eastern USA and southeastern Canada.
Known introduced range: It has been introduced to Hawaii, parts of western USA and southwestern Canada, Mexico and the Caribbean, South America, Europe and Asia. Introduction pathways to new locations
Aquaculture: Often initially imported into a country/area for commercial production as food. Sometimes inadvertently introduced along with fishes raised in hatcheries where bullfrog larvae are abundant.
Biological control: In some cases, bullfrogs have been deliberately introduced to control agricultural pests.
Landscape/fauna "improvement": Deliberately introduced as a harvestable game animal, or as aesthetically pleasing wildlife. In British Columbia, Canada they are sold in aquatic garden supply stores for the enhancement of ornamental ponds.
Natural dispersal: Bullfrogs are capable of considerable overland travel, and will eventually disperse throughout entire watersheds, given interspersed patches of suitable aquatic habitat.
Pet/aquarium trade: Released pets- after metamorphosis, the larvae are no longer suitable aquarium subjects, and are released as adults into local waters.
Local dispersal methods
Aquaculture (local): Often initially imported into a country/area for commercial production as food. Sometimes inadvertently introduced along with fishes raised in hatcheries where bullfrog larvae are abundant.
Natural dispersal (local): Bullfrogs introduced into ornamental ponds, farm ponds and outdoor aquaculture ponds will disperse into nearby natural bodies of waters, and will use seasonal roadside ditches and streams as corridors in suburban areas (Govindarajulu, 2004). Management information
Physical: Adult R. catesbeiana may be taken by a variety of methods, including shooting, spears/gigs, bow and arrow, clubs, nets, traps, angling and by hand. They tend to be extremely shy but can be caught in the light of a very strong torch (the ones poachers may use) at night or alternatively can be shot (Veenvliet, P., pers.comm., 2003). Collecting egg masses can be an effective adjunct control method. Bull-frogs lay a single mat of eggs that can be removed as they float on top of the surface of the water. A day or so after laying, R. catesbeiana eggs sink to the bottom and then become very hard to find indeed. As the egg masses break easily, a bilge pump may be the most effective way to suck up the entire egg mass (Govindarajulu, 2004). Since this species has a several months long breeding season and each female spawns only once each year, and eggs hatch in less than a week it's very much a matter of either a very strict surveillance or a lot of luck to find any egg masses (Veenvliet, P., pers.comm., 2003). Marking sites where male bullfrogs are heard calling during the night and targeting egg mass searches to these sites may improve probability of detecting egg masses (Govindarajulu, 2004). R. catesbeiana can be controlled using a reptile proof fence: ring the ponds with reptile proof fence to catch the neonates and put traps in the ground so as to catch them as they leave the pond site any eggs will be laid in single layer on top of the water so these can be eradicated with some ease. The tadpoles take two years to metamophosize (tadpoles in most of North America over-winter at least once and some over-winter multiple years, whereas tadpoles in the southern parts of the United States are known to metamorphose in the same year that they were hatched). Care must be taken to make sure that the fence is impregnable and at least two feet off the ground on the vertical piece and buried at least six inches in the ground with a four inch turn at the bottom going inwards to the pond and buried and pushed well down that should contain the neonates. Other ponds and water sources should be looked at as well, as the adults use the river watercourses as roadsystems (Pendlebury, P., pers.comm., 2003). Nutrition
Adult bullfrogs are voracious opportunistic predators of aquatic, terrestrial, and flying invertebrates and vertebrates, including birds (Lopez-Flores et al., 2003) and other amphibians (Chivers et al., 2001). Although diet studies have demonstrated tendencies towards prey selectivity, a complete list of documented prey items betrays a willingness to eat almost any animal it can overpower and swallow whole. In Colusa National Wildlife Refuge, California, USA, crayfish (Procambarus clarkii) make up the majority of the bullfrogs diet (Wylie et al., 2003). Tadpoles can digest the nuisance, bloom forming blue-green algae Anabaena spp., which may explain the competitive advantage they have over native anurans at the larval stage (Pryor, 2003). Bullfrog tadpoles will also prey on the tadpoles of other species (Kiesecker and Blaustein, 1997; in Blaustein and Kiesecker, 2002). Reproduction
Sexual. External fertilisation of unshelled eggs takes place as the eggs are deposited in water. The jelly-coated eggs form a floating raft which may measure as much as a metre across. Eggs hatch in 3-5 days. Bullfrogs breed from early spring through late summer, depending upon local climate. Gravid females commonly contain from 1,000 to 20,000 eggs, with larger females producing larger clutches. Maximum clutch size is over 40,000 eggs. They will produce multiple clutches in a season under favourable conditions. Lifecycle stages
These amphibians undergo a completely aquatic life stage before metamorphosing into semi-aquatic adults. This larval stage is variable in length, from 4 months to over 2 years, taking longer in colder climates. This species has been nominated as among 100 of the "World's Worst" invaders Reviewed by: Major update under progress
Purnima P. Govindarajulu, Ph. D. Post-Doctoral Fellow Department of Biology, University of Victoria British Columbia Canada.
Compiled by: John J. Crayon, U.S.Geological Survey, Western Ecological Research Center, Department of Biology, University of California, Riverside, California, USA & IUCN/SSC Invasive Species Specialist Group (ISSG)
|
|
Last Modified: Tuesday, 3 May 2005
|
