Do plant toxins impose constraints on herbivores? An investigation using compartmental analysis

Abstract

Plant secondary metabolites are often acutely toxic to animals, but the influence of this property on interactions with herbivores has been debated vigorously. In this paper we apply a compartmental model, developed to investigate the movement of insecticides through the bodies of animals, to quantify the effect of natural plant toxins. A simulation based on the model is also used to examine the degree of tolerance necessary for a herbivore to avoid illness. Comparison with studies from insecticide science suggests that the required level of tolerance is high enough to impose major ecological limitations on herbivory at naturally occurring concentrations. Various adaptive mechanisms could be employed to overcome this inhibitory effect, and the model allows a precise analysis of their relative importance in a given species. More generally, elasticity analysis shows that these mechanisms differ considerably in efficiency, with desensitisation at the molecular site of action cited as particularly cost effective. However, such efficient mechanisms are often toxin‐specific, and more likely to evolve in herbivores which display a high degree of host‐plant constancy. It should therefore be less costly to achieve tolerance in oligophagous herbivores than polyphagous ones. Reduced concentrations of secondary metabolites in cultivated species could curtail their effectiveness dramatically and universally.