Photosynthetic Pathways and Selective Herbivory: A Hypothesis

We propose that plants possessing the C4-dicarboxylic acid pathway of photosynthetic carbon fixation are generally inferior food sources for herbivores, and are often avoided by them, relative to plants possessing only the C3-Calvin cycle pathway. As initial support of this hypothesis, we present data from the literature, dealing primarily with insects, that suggest that C4 species are ingested in proportions lower than their availability in a number of natural situations, that they tend to be avoided in laboratory preference tests, and that they result in lower survival and fecundity in herbivores feeding on them. We suggest a number of physiological, anatomical, and nutritional differences between C3 and C4 species that may be involved in generating the observed pattern of herbivore preference. The ecological relevance of the differences between C3 and C4 species has been discussed to date only in relation to hypothetical effects on interspecific competition. If our hypothesis is true, selective herbivory may be another ecologically important consequence of the differences. There is enough evidence pointing toward the validity of our hypothesis that it should be further tested, and the distribution of plants as C3 and C4 species should be taken into account in community and ecosystem studies.