Many plants spend a significant fraction of their resource and energy budgets on defensive chemicals and structures. Despite this commitment, insects consume approximately 20% of all new plant growth each year, while pathogens destroy a substantial additional amount. The magnitude of this loss means that pathogens and herbivores must be a potent force in both ecology and evolution, limiting the success of some species while allowing some of their competitors to prosper. In nature, plant fitness may be strongly influenced by the capacity to deter herbivores and resist pathogens.

Interactions between plants and their consumers are thought to have influenced patterns of plant diversification. Just like pollinating insects, plant-eating insects diversified along with angiosperms (the flowering plants). The coincidence of timing is consistent with an important role for plant–insect coevolution in generating diversity. Further evidence that defense has influenced plant diversification comes from plant genes and secondary compounds. For example, the R genes that are instrumental in pathogen recognition form one of the largest gene families in plants, while chemical defenses against herbivores include about 6000 alkaloids and more than 10,000 terpenoid compounds. Such observations suggest that plants are locked in an evolutionary arms race with herbivores and pathogens. This arms race is very much in evidence in the efforts by farmers to limit crop losses by reducing the numbers of the pathogens and herbivores in fields and orchards.