Plant morphology, and recruitment of the third trophic level: Subtle and little-recognized defenses?

We present the new hypothesis that plant morphological traits that make plants more accessible to predators and parasitoids (in contrast to attracting them) may be shaped by the third trophic level. We review data that demonstrate selective impacts of the third trophic level, other than ants and mites, on plant fitness. This third trophic level impact comes about because predators and parasites reduce damage caused by herbivores of those plants. Because of this impact, we suggest that there may be selection for changes in plant morphological traits that make herbivores more accessible to the third trophic level, Both invertebrate and vertebrate members of the third trophic level may be involved, potentially influencing such traits as pubescence type and density, leaf morphology, canopy density, perch and stem shape, branching angles, and distance between leaves and perches, Such influences may have been previously overlooked because: 1) there has been little investigation of third trophic level impacts on plant fitness other than by predatory ants and mites; and 2) changes in plant morphology that make plants accessible to the third trophic level are likely to be subtle.