Competitive ability and the coexistence of anuran larvae in freshwater rock-pools

1. I examined the competitive ability of larval common frogs (Rana temporar ia) and common toads (Bufo bufo) in a laboratory experiment by manipulating the number and size of Rana and the number of Bufo. 2. Bufo was the most active tadpole type, whereas small Rana was more activ e than large Rana. 3. Especially at high food availability, the growth rate of large Rana at m etamorphosis was reduced in the presence of small Rana, whereas that of sma ll Rana was not affected by larger conspecifics. 4. Both size classes of Rana were negatively affected by the presence of Bu fo. Size at metamorphosis of Bufo was greater when large Rana was present. These results suggest that activity level is important in determining compe titive ability. 5. To study competition in the field, tadpoles of Bufo were introduced to s mall rock-pools that did not support Bufo naturally, but were with or witho ut Rana. 6. In the experimental pools, there were no interspecific effects on the gr owth rate of either species. However, the growth rate of Bufo was lower in the experimental pools than in pools inhabited naturally. Futher, survival of Bufo was low in small pools. Lower food availability or physical factors associated with small pools, rather than interspecific competition, may be the reason why Bufo avoids breeding in small rock-pools.