The American Bullfrog is an eastern species of frog that has been introduced to British Columbia. It is the largest frog species in North America, and is found in permanent vegetated ponds, lakes, and ditches.
The bullfrog may reach lengths of more than 200 mm, excluding the legs, with females larger than males (Matsuda et al. 2006). In BC, Bullfrogs area readily identified by their deep booming calls and by their large size. Adults are white with mottling below and may be green, brownish-green, or olive-coloured on the back, with tiny black specks or mottled irregular patches (Matsuda et al. 2006). The underside of the chin is often white (sometimes yellowish), but it can be a very noticeable bright yellow in males breeding season (Matsuda et al. 2006, and others, personal observation). The tympanum (hearing organ) in Bullfrogs is conspicuous In females, the tympanum is equal to the eye in size, but it is larger than the eye in males. Both sexes have a prominent fold of skin that curves around the typmanum. This fold of skin around the tympanum is lacking in the very similar, though smaller, Green Frog (Lithobates clamitans).
The Bullfrog has a prolonged breeding season that begins in late spring and continues throughout the summer (Matsuda et al. 2006, Emlen 1976). During this time males can be observed congregating in groups or 'choruses' in a pond or wetland (Matsuda et al. 2006, personal observation). These breeding groups or choruses are temporary and shift locations throughout the breeding period (Emlen 1976). Breeding activity is easily noticed in ponds and lakes because of the loud splashing and obvious activity by the frogs (personal observation). Males will float on the water surface, inflated, while calling and showing off their bright yellow throats (see photo).
Recent research has shown that Bullfrogs readily move between permanent and seasonal ponds: 'Until pools completely dried, density of bullfrogs was greater in seasonal pools than in the permanent breeding ponds, underscoring the significance of seasonal pools as non-breeding habitat' (Gahl et al. 2009).
Although insects form the major part of the diet for young Bullfrogs, but as they grow they will eat a variety of things, including young birds, mice, fish, and smaller Bullfrogs. According to Wikipedia (2009): 'Bullfrog stomachs have been found to contain rodents, small turtles, snakes, frogs including bullfrogs, birds, even a bat, as well as the many invertebrates which are the usual food of ranid frogs. These studies furthermore reveal the bullfrogs diet to be unique among North American Rana in the inclusion of large percentages of aquatic animals, e.g., fish, tadpoles, Planorbid snails, Dytiscid beetles'.
Distribution in BC
The bullfrog was introduced to British Columbia following the second world war as a food source, and when this was unsuccessful, Bullfrogs were released into the wild. Bullfrogs are now found in the Lower Mainland, Vancouver Island north to Campbell River, and on Lasqueti Island. They are also reported from the South Okanagan (BC Ministry of Environment 1998).
Bullfrogs are widely believed to displace native frog species in areas where they are introduced, resulting in native species loss. Although amphibian decline is a complex issue, and other factors have also played a role, research has shown that Bullfrogs do play a role. In California, for example, Bullfrogs have been suspected of displacing the native California Red-legged Frog (Lawlor et al. 1999): research has shown that the survival of California Red-legged Frogs in study areas in the presence of bullfrog tadpoles was less than 5%, and that Bullfrogs strongly affected California Red-legged Frog populations (Lawlor et al. 1999). In BC, there is research-based evidence (range changes) that indicates that Bullfrogs have displaced two native species on Vancouver Island: the Red-legged Frog (Rana aurora) and the Pacific Chorus Frog (Pseudacris regilla) (Govindarajulu 2005). Research is now ongoing (Govindarajulu 2009) to explore how to halt this. Removal of tadpoles and adults has been used for control. However, some research is showing that removal of tadpoles and breeding adults may have the opposite effect, and timing of removal is important.