Character displacement in polyphenic tadpoles

Biologists have long known that closely related species are often phenotypi cally different where they occur together, but are indistinguishable where they occur alone. The causes of such character displacement are controversi al, however. We used polyphenic spadefoot toad tadpoles (Spea bombifrons an d S. multiplicata) to test the hypothesis that character displacement evolv es to minimize competition for food. We also sought to evaluate the role of phenotypic plasticity in the mediation of competitive interactions between these species. Depending on their diet, individuals of both species develo p into either a small-headed omnivore morph, which feeds mostly on detritus , or a large-headed carnivore morph, which specializes on shrimp. Laborator y experiments and surveys of natural ponds revealed that the two species we re more dissimilar in their tendency to produce carnivores when they occurr ed together than when they occurred alone. This divergence in carnivore pro duction was expressed as both character displacement (where S. multiplicata 's propensity to produce carnivores was lower in sympatry than in allopatry ) and as phenotypic plasticity (where S. multiplicata facultatively enhance d carnivore production in S. bombifrons, and S. bombifrons facultatively su ppressed carnivore production in S. multiplicata). In separate experiments, we established that S. bombifrons (the species for which carnivore product ion was enhanced) was the superior competitor for shrimp. Conversely, S. mu ltiplicata (the species for which carnivore production was suppressed and o mnivore production enhanced) was the superior competitor for detritus. Thes e results therefore demonstrate that selection to minimize competition for food can cause character displacement. They also suggest that both characte r displacement and phenotypic plasticity may mediate competitive interactio ns between species.