Biological control of plant invaders: Regional patterns, field experiments, and structured population models

The traditional ways to design biological control systems for plant invader s include (in order of decreasing emphasis) introducing, augmenting, or con serving natural enemies, Manipulating consumer-resource relationships in th is way (1) emphasizes top-down control of the invader by consumers rather t han bottom-up control of the invader by limiting resources, and (2) contrib utes to a rising number of control organisms introduced to North America th at is creating complexity, redundancy, and risk. New concepts and methods h ave started to transform the way biological control organisms are found and developed by (I) combining herbivore and resource limitation of plant popu lation growth and (2) using targeted life-cycle disruption, which involves identifying plant life-cycle transitions that are both amenable to manipula tion and influential on population growth, and then targeting these for con trol. To illustrate these developments, we outline an experimental. and com putational approach for measuring how the processes of disturbance, coloniz ation, and organism interactions (plant competition and herbivory) manifest their influence on weed life cycles and population growth of ragwort Senec io jacobaea, a biennial or short-lived perennial herb. Manipulating these f orces may lead to designs of biological control systems that are parsimonio us, potent, and pose minimum risk to non-target organisms.