Phenotypic Plasticity Across Gradients of Predation and Competition in Larval Amphibians

ABSTRACT Phenotypic plasticity allows individuals to produce different traits when they experience different environments. These trait changes are thought to evolve as a result of experiencing environmental variation and fitness tradeoffs among alternative phenotypes. For example, predators and competitors often induce opposite traits, yet a large proportion of plasticity studies have examined 2 × 2 factorial combinations of predator and competitor environments, thereby hindering our understanding of the full shape of reaction norms under different environmental combinations. We examined plastic responses of gray treefrog tadpoles ( Hyla versicolor ) when raised under 25 combinations of predation risk and competition intensity. We discovered that tadpole mass was affected by competition and a competition‐by‐predator interaction. Both factors affected tadpole behavior (number seen and activity) in a continuous fashion, but only the number seen exhibited interactive effects, with responses to predators weakening under high competition. Changes in relative (i.e., mass‐adjusted) morphology were also continuous; predator cues induced relatively deeper tailfins and deeper bodies, while competition induced relatively shallower tailfins and tail muscles, longer and shallower bodies, and wider mouths. Predation and competition caused mostly additive effects on morphology; the exception was relative tail depth, which responded more strongly to increased predation risk under low competition than under high competition. While our results suggest that predators and competitors induce traits in opposing directions and in a continuous manner, only a subset exhibits interactive effects.