Introduced bullfrogs (Rana catesbeiana) in British Columbia: Impacts on native Pacific treefrogs (Hyla regilla) and red-legged frogs (Rana aurora)

Introduced species are considered one of the greatest threats to biodiversity, next only to habitat destruction. I studied the ecology, distribution, and impacts of one such introduced species: the American bullfrog (Rana catesbeiana). Bullfrogs were introduced to British Columbia in the 1930's for the farming of frog legs. The frog farms were not economically successful and the bullfrogs were introduced to the wild. Call surveys and information from naturalists and the public was collated to map the current distribution of bullfrogs in British Columbia. Populations of bullfrogs are found in southern British Columbia, on Vancouver Island, on the Lower Mainland, in the Gulf Islands and in the Okanagan.; Natural history data and observations on marked populations in the Greater Victoria area were used to study the ecology of bullfrogs. The population ecology of bullfrogs was very similar to that in their native range in eastern North America and was governed by seasonal rhythms of air and water temperature. Bullfrog populations are expanding their range due to introduction by humans into new habitats and through migration.; Bullfrogs are considered a key species in structuring the anuran community composition in their native habitat. I examined the impact of bullfrogs on native Pacific treefrogs (Hyla regilla) and red-legged frogs (Rana aurora), focusing mainly on larval competitive interactions. Using Capture-Mark-Recapture techniques, I estimated survival of treefrog tadpoles in ponds with and without bullfrogs. I was unable to detect a difference caused by bullfrogs over the natural variation in treefrog tadpole survival rates among ponds. In artificial pond experiments I was able to show that bullfrog tadpoles had a negative effect on the development rate of the two native tadpoles and on the growth rate of red-legged frog tadpoles. Bullfrogs did not affect the survival rate of the native tadpoles in these experiments.; The demography of bullfrog populations was studied using four marked populations in the Greater Victoria area. The life-cycle of the bullfrog consisted of two alternate larval development trajectories. Tadpoles could either metamorphose after a year in the larval stage (fast-track) or spend two years in the larval stage (slow-track). Bullfrogs attained sexual maturity two years following metamorphic transformation. Linear, stage-based matrix models were used to assess the factors controlling bullfrog population growth rate. Population growth rates were most sensitive to the proportion of tadpoles entering the fast-track option of the larval life-cycle and to early post-metamorphic survival rates.; This study adds to previous published studies that have documented negative effects of bullfrogs on native species. I assessed the most effective methods of bullfrog population control. Based on logistics and on perturbation analysis of population matrix models, the most effective stages to cull are the early post metamorphic stages: metamorphs and juveniles. However, all bullfrog control efforts are bound to be expensive and labour intensive. Habitat modification to favour native species and permit coexistence with bullfrogs may be the more effective long-term management option.