The Defenses of Legumes against Herbivores

Abstract

The planners of this conference have asked me for a discussion of the interactions between legumes and herbivores. This is a request for an examination of traits of legumes as pertains to herbivores. On the one hand, I am happy to do this because it is a useful sort of focus. On the other hand, I feel it is a bad move. Such an approach fosters the tendency to search for and accept single selective pressures as the evolutionary causes of what appear to be single traits. A single trait, such as the concentration of canavanine in a Dioclea megacarpa cotyledon or the force to crush an Enterolobium cyclocarpum seed is no more the result of a single selective pressure than is the number of times you can open and close your hand in 30 seconds. A single trait, as we describe it in biology, is simultaneously i) under contemporary design constraints, ii) a temporal and spatial anachronism, iii) under contemporary budgetary constraints, iv) multi-functional, and v) optimal (not maximal) in performance rate and pattern. The huge fruit of Cassia grandis is photosynthetic, protective against herbivores with a multitude of defenses, heavy, derived from a minute tissue, expensive per flower, cheap per seed, indehiscent, toxic when unripe, edible when ripe, and probably was eaten by mastodons. We are only just now, again, learning to take a single trait of an organism and simultaneously explore the significance of that trait to that organism's physiology, anatomy, ecology, behaviour, and so forth. And I worry that this review may be taken as a digression from such a holistic approach to understanding form and function. However, while our contemporary research may be becoming holistic, it is clear that most of what we have to work with is not, and that much of the day to day data collection is likewise not. I proceed accordingly.