On the study of plant defence and herbivory using comparative approaches: how important are secondary plant compounds

Species comparisons are a cornerstone of biology and there is a long tradition of using the comparative framework to study the ecology and evolution of plant defensive traits. Early comparative studies led to the hypothesis that plant chemistry plays a central role in plant defence, and the evolution of plant secondary chemistry in response to insect herbivory remains a classic example of coevolution. However, recent comparative work has disagreed with this paradigm, reporting little connection between plant secondary chemicals and herbivory across distantly related plant taxa. One conclusion of this new work is that the importance of secondary chemistry in plant defence may have been generally overstated in earlier research. Here, we attempt to reconcile these contradicting viewpoints on the role of plant chemistry in defence by critically evaluating the use and interpretation of species correlations as a means to study defence-herbivory relationships. We conclude that the notion that plant primary metabolites (e.g. leaf nitrogen content) are the principal determinants of herbivory (or the target of natural selection by herbivores) is not likely to be correct. Despite the inference of recent community-wide studies of herbivory, strong evidence remains for a prime role of secondary compounds in plant defence against herbivores.