Introduced Bullfrogs (Rana catesbeiana) in Western Canada: Has Their Ecology Diverged?

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Organisms can diverge in life history when introduced outside their native range due to release from predators, competitors, and parasites, and also due to genetic drift and local adaptation. We studied the ecology of the American Bullfrog (Rana catesbeiana) in its introduced range in British Columbia (BC). To assess differences between introduced and native populations, we compared the population ecology of BC bullfrogs to published life-history parameters from the bullfrog's extensive native range in eastern North America. In BC, bullfrogs emerged from hibernation in late April and early May. Breeding choruses developed fully when air temperatures exceeded 20°C. Eggs were laid from mid-June to mid-July when the mean water temperature was 20°C. Mean egg mass size was 13,014 ± 7,296 eggs (mean ± standard deviation). Tadpoles hatched in 3 to 5 days and overwintered the first year as tadpoles. Approximately 68% of the tadpoles metamorphosed at the end of the following summer, but the remaining spent a second winter in the pond. Bullfrogs were large enough to attain sexual maturity 2 yr after metamorphosis. Bullfrog population density among ponds varied from 4.1 to 530 frogs/ha. Terrestrial insects were the primary prey item of bullfrogs < 150 g, whereas frogs were the primary prey item of larger bullfrogs. The life-history parameter values estimated for BC bullfrogs were within the range observed for bullfrogs in their native habitats. Due to milder weather conditions in southwestern BC, the seasonal pattern and growth rate of bullfrogs were similar to lower latitude populations in Kentucky and Missouri. We found no evidence to support the hypothesis that when released from native predators and parasites bullfrogs build up to unusually high population densities or attain significantly larger sizes in their introduced range.