Three-trophic-level interactions affecting the success of biological control projects

Abstract

The thesis is developed that the detection of pattern in three-trophic-level systems of plants, insect herbivores, and natural enemies, is a necessary step towards the development of theory on natural systems and applied biological control. When patterns are discovered the underlying mechanisms can be revealed, and will contribute to synthesis and general theory. The strongest patterns are those imposed by vegetation passing through ecological succession over landscapes, providing a template on which insect herbivores and their natural enemies must function. Plants provide food and habitat for insect herbivores and therefore their distribution and abundance dictate distribution of organisms up the trophic system and a carrying capacity on herbivore populations. As plant vigor decreases through a successional gradient and biomass of food increases herbivore populations shift from specialists on vigorous plants with latent population dynamics to generalists on mature plants, many of which show eruptive population dynamics. A combination of many factors indicates that natural enemies are relatively ineffective in earlier stages of plant succession and their impact increases as habitats become more stable and extensive and herbivores become more generalized in host exploitation. The implications for applied biological control are principally that ephemeral habitats, such as in agricultural systems, are unfavorable for applied biological control and sophisticated husbandry of natural enemies will be essential before successes in such habitats outnumber failures.