Coevolution of Poisonous Plants and Large Herbivores on Rangelands

Abstract

Early literature generally described toxic plant substances as waste products. However, more recent publications in entomology, plant biochemistry, and other fields suggest that toxic secondary compounds in plants may be defense systems against insects and other herbivores. The pertinence of these discussions to range management is the subject of this paper. If plant poisons have evolved as defense mechanisms, various ways that they might function include: (1) extreme toxicity; (2) poisonous properties linked with palatability; and (3) aversive conditioning, i.e., animals "learn" that a plant will make them ill and avoid that plant. All could reduce consumption of poisonous plants by herbivores, thus making the plants more competitive in natural communities. If plant poisons are defense mechanisms, it would be logical to assume that coevolution has occurred in herbivores to prevent their being poisoned by plants. Some of the possible evolutionary adaptations in large herbivores include: (1) a generalized diet that reduces the probability of eating a toxic amount of any one species; (2) ability to detect and avoid poisonous plants; and (3) ability to detoxify plant poisons. All appear to operate, at least to a degree, in both domestic and wild herbivores. Ability to detect and to detoxify poisons varies among species and appears to be more prevalent in native animals that coevolved with the vegetation than in domestic animals. Native big-game animals are occasionally poisoned by plants but large losses usually occur in overpopulated or overgrazed areas where nonpoisonous species have been depleted.