Relative roles of top-down and bottom-up forces in terrestrial tritrophic plant-insect herbivore-natural enemy systems

Whether resources (bottom-up forces), natural enemies (top-down forces), or both, determine the abundance of insect herbivore populations in plant-insect herbivore-natural enemy systems remains a major issue in population ecology. Unlike recent surveys of the tritrophic literature we do not seek to quantify whether top-down or bottom-up forces predominate in any given set of experimental systems. Acknowledging the dearth of empirical synthesis we employ two contrasting literature surveys to determine whether the plant-insect herbivore-natural enemy literature is currently adequate to form a conceptual synthesis of the relative roles of top-down and bottom-up forces. The emergence of a synthesis of the relative roles of top-down and bottom-up forces in plant-insect herbivore-natural enemy systems appears to have been largely prevented by (1) the absence of appropriate empirical data; (2) failure to appreciate the merits of existing data; (3) a continued desire to emphasise either top-down ol bottom-up forces to the exclusion of the other; and (4) confusion regarding which processes regulate and which influence the abundance of insect herbivores.