Keisuke Atsumi, Samuel Ross, Osamu Kishida

1. Through its influence on trait expression, phenotypic plasticity can shape variation in strengths of interspecific interactions across environmental gradients. If species exhibit interpopulation differences in phenotypic plasticity, their genotypes and environmental conditions may jointly determine the strength of interspecific interactions.
2. To examine this prediction, we experimentally investigated the trophic interactions between different populations of predators and prey, using amphibians that vary in the plasticity of offensive and defensive morphological traits, respectively. Cannibalism-induced gigantism of Hynobius retardatus salamander larvae produces salamanders with wide enough gapes to consume Rana pirica frog tadpoles, an otherwise inaccessible large prey species that, in turn, possesses an inducible morphological defense.
3. By manipulating combinations of two populations of salamanders and tadpoles and the size distribution—hence, prevalence of cannibalism—of salamander hatchlings, we found an interactive effect of salamander population identity and size distribution on tadpole consumption.
4. Early life salamander size heterogeneity resulted in interpopulation differences in salamander gigantism. Salamanders from the population with weaker cannibalism-induced gigantism were largely unable to consume tadpoles. However, salamanders from the population exhibiting striking offensive gigantism consumed tadpoles from both populations, though tadpole inducible defenses were stronger in the population with higher prevalence of salamander gigantism.
5. Our results suggest that the likelihood of emergence of a trophic interaction between salamanders and tadpoles differs among salamander populations based on an environmental determinant of distinctive inducible offense. Ultimately, this implies that geographic variation in the trait plasticity of a species can determine geographic variation in interspecific interaction strengths.