Phenotypic plasticity has been studied intensively in experimental settings but infrequently in nature, and therefore the relevance of experimental findings is poorly known. This is especially true for morphological plasticity in amphibian larvae induced by predators and competitors. This paper describes a seven-year survey of head and tail shape in eight
species of anuran and newt larvae in northern Switzerland, involving 6824 individual larvae
and 59 ponds. I tested relationships between geometric measures of size and shape and five
habitat gradients: pond permanence, cover by forest canopy and aquatic vegetation, and the densities of predators and competitors. Responses to competitors and predators were often similar to those reported in experiments. High competitor density was associated with small size and a large head in newt larvae, a long or deep head/body in anuran larvae, and a short or shallow tail in newts and some tadpoles. High predator density was correlated with a deep tail
fin and tail muscle in many species. In anurans, the change in shape between low- and highpredator
ponds in nature closely paralleled the plastic response to nonlethal predators in mesocosm experiments. The survey revealed many previously undescribed relationships between morphology and the other habitat features. Several species had relatively large tails in ponds that were shaded or thickly vegetated. Associations between year-to-year changes in shape and habitat within ponds implicated phenotypic plasticity rather than genetic population divergence, at least in anurans. These results inspire confidence in the relevance of experiments and highlight many new patterns that will merit further study.