Trade-Offs and Constraints in Plant-Herbivore Defense Theory: A Life-History Perspective

Abstract

Trade-offs and constraints are key ideas in plant defense theory. Here I argue that trade-offs, such as those between defense and growth, may be better understood by incorporating insights from life-history studies. Specifically, I set out criteria for characterizing resource based trade-offs, because these lie at the heart of current defense theory. I show that defense theory is often supported by inappropriate evidence from negative trait associations seen in the field, or from genetic studies. Such evidence is inappropriate because trade-offs grounded in genetic constraints can differ fundamentally from those conceived in terms of limiting resources. In work on resource based trade-offs there is often a failure to consider the critical issues of whether the resources involved are limiting, and whether this limitation is imposed by the assimilatory capacity of the organism or by a shortage in its environment. Another important consideration is whether a trade-off is between two that jointly consume all resources. Where this is not the case, unforeseen trade-offs with third party traits may confound analyses. I discuss the practical issues of resolving these problems with plants, and present a program for future research on resource based trade-offs involving plant defense.