Biogeographic Dynamics of Insect-Host Plant Communities

Associations of insect herbivores with hosts can be dynamic. One scale of this change is coevolutionary, involving the reciprocating adaptations be­ tween herbivores and plants (27), which have helped to produce the bewil­ dering phytochemical diversity (24) as well as the complementary tolerances of insects to the chemical protections of plants (84). The major and recognizable attributes of coevolution probably occur slowly, over at least thousands of generations, and the tight taxonomic associations be­ tween highly coevolved herbivores and hosts can persist through even geo­ logically long periods of time (44, 65, 66). More quickly, however, ecological and biogeographic changes in insect-herbivore associations can occur without obvious coevolution. A substantial fraction of plant-eating insect species can include more than one plant species in its diet (69, 73, 111), and many probably can complete life cycles on any of several host taxa. Thus, the commonness of polyphagous species creates potential for frequent host shifts. This potential has been appreciated almost exclusively in agriculture, which, albeit unintentionally, is an experiment with the dynamics of herbivore-host associations of the grandest dimensions. The results of the experiment are clear: Pests soon colonize plants that are cultivated extensively (122, 123), plant species recruit different pest species in different regions (108), and associations of insects with plants are often more casual, fortuitous, and labile than those usually interpreted as coevo­ lutionary. A set of questions follows. How common are these more casual associa­ tions in nature? At what rates are insect species lost and gained by plant populations in non-agricultural situations? Is turnover of herbivore species