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

Plant secondary metabolites are often acutely toxic to animals, but the inf luence of this property on interactions with herbivores has been debated vi gorously. In this paper we apply a compartmental model, developed to invest igate the movement of insecticides through the bodies of animals, to quanti fy the effect of natural plant toxins. A simulation based on the model is a lso used to examine the degree of tolerance necessary for a herbivore to av oid 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 ada ptive 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 d iffer considerably in efficiency, with desensitization at the molecular sit e of action cited as particularly cost effective. However, such efficient m echanisms are often toxin-specific, and more likely to evolve in herbivores which display a high degree of host-plant constancy. It should therefore b e less costly to achieve tolerance in oligophagous herbivores than polyphag ous ones. Reduced concentrations of secondary metabolites in cultivated spe cies could curtail their effectiveness dramatically and universally.