Insect-plant interactions on a planet of weeds

Two conflicting views confront ecologists and evolutionary biologists on the degree of symmetry in interactions between plants and phytophagous insects. The symmetrical view holds that insects and plants have strong effects on one another's evolutionary and ecological dynamics. Thus, herbivores are regarded as a major influence on plant distribution and abundance in contemporary ecosystems, and coevolution is commonly invoked to explain adaptive radiation in plants and insects, host specialization in insects, as well as much of the morphological and chemical variety observed in plants. The asymmetrical view acknowledges that plants have major effects on insects, but claims that insects seldom impose significant effects on plants. Proponents of the asymmetric view tend to ignore or discount insect-plant interactions in communities and ecosystems altered by human impacts. If we recognize the scope and scale of human impacts, and ways in which these impacts change insect-plant interactions, then our views about symmetry or asymmetry in insect-plant interactions will change. To understand, predict, and manage insect herbivory we need to study it in all its manifestations. In particular, the study of interactions involving alien species is both an urgent priority for environmental management and potentially a source of ecological insights on the role of herbivores in plant population and community dynamics. A complete theory of insect/host plant interactions must explain and predict interactions both within and beyond the native range. Such a theory might guide efforts to deal with environmental problems stemming from rapid rates of extinction and homogenization of the world's biota.