A single plant is unlikely to have 'weed' status, but a population of the plants at a density that interferes with our use of the land or water will change that perception. An individual biological control agent may damage part of a weed, but this only becomes significant if the population of the agent reaches a density at which the total damage by all the individuals reduces the weed population. An understanding of how weed and biological control agent populations change to reach levels that are significant will help in management of weeds, and in biological control of weeds.

This section provides a brief checklist of factors and processes to be considered when interpreting changes in populations of weeds and biological control agents. More detailed explanations may be found in publications on ecology (eg. May 1981; Ricklefs 1990), population ecology (eg. Begon and Mortimer 1981; Harper 1977), insect-plant interactions (eg. Crawley 1983; Strong et al. 1984), and in some texts on biological control (eg. van Driesche and Bellows 1996). Aspects of the population ecology of weeds are discussed in the Biological Characters of Weeds section, and methods of investigation of populations of weeds are discussed in the Evaluation section.