Rapid geographic divergence in competitive ability of cane toad larvae (Rhinella marina) during a biological invasion.

Low population densities at the expanding edge of a biological invasion are predicted to reduce intraspecific competition such that invading organisms evolve to prioritize adaptations for rapid rates of population growth and dispersal over those that facilitate success in competitive interactions. The larvae (tadpoles) of anuran species that breed in isolated waterbodies provide excellent models with which to test this prediction, because they compete for limited food resources and are unable to disperse until after metamorphosis. Consistent with our predictions, experiments with captive-raised larvae of cane toads (Rhinella marina) show that tadpoles from range core populations were more effective competitors (based on rates of growth and development), especially against tadpoles from invasion front populations, and especially when food was limited (intensifying competition for resources). By contrast, response to predators-a selection pressure presumed constant regardless of invasion history-was largely unchanged between core and edge populations. Limiting food availability reduced survival rate and size at metamorphosis by similar amounts in both core and edge populations. The spread of cane toads across tropical Australia thus has been accompanied by a reduction in the competitive ability of tadpoles, as predicted under the hypothesis that competitive ability will decrease at an expanding range edge. Other traits, associated with response to predation and food limitation, appeared unchanged.