Vegetational diversity and arthropod population response

Contrasts mixed species plantings (polycultures) with bare-ground sole-crop fields (monocultures), the former being characterised by the simultaneous occurrence of herbivory with both inter- and intraspecific plant competition, and where plants are interspersed at spatial scales similar to the shorter movement scales of the arthropod herbivores and their natural enemies. The review examines whether generalities can be drawn from the arthropod response to polyculture, or whether these responses are idiosyncratic, depending on the particular combination of plants and arthropods. The diversity-stability hypothesis (the greater the biological diversity of a community the greater its stability) is evaluated. Herbivore response to polycultures might be expected to be determined by natural enemies: generalist and specialist natural enemies are more abundant in polycultures and therefore suppress herbivore populations to a greater extent. The resource concentration hypothesis argues that many herbivores, especially those with a narrow host range, are more likely to find and remain on host plants that are concentrated. Plants experience associational resistance, and arthropods can respond to plant-stand characteristics (resource concentration) or plant life history characteristics (plant apparency). Mechanisms underlying resource concentration are discussed. -P.J.Jarvis