The process of change in populations, including weeds and biological control agents, can be summarised by the equation:

population change = (births + immigrants) - (deaths + emigrants).

All species have genetically determined limits to the rates of these processes (birth, death, migration), and environmental factors limit the processes or modify the rates within the genetically determined limits.

Some environmental factors cannot be controlled, but their effects on population change may be significant. Other factors can be controlled, so manipulation of these factors may be included in management of the weed (See Biological Characters of Weeds).

The objective of pest management is to reduce the population growth rate of the pest, to levels at which its impact is insignificant, by:

Biological control of weeds is usually aimed at reducing birth rate or increasing death rate. Migration rates could be affected by biological control agents that interfere with the biology or mechanics of seed or fruit dispersal, but the aims of weed biological control are usually more direct.

Populations of insects and plants differ in that all adult insects in a population are usually the same shape and roughly the same size, but plants of the same age within a population are highly variable in shape and size. It is useful to think about plant populations in two ways. Taking the broader view, plant populations are made up of individual plants. At a more detailed level, individual plants are made up of populations of modules (eg. shoots on trees, leaf and bud units on annuals, tillers on grasses). The 'birth' and death of these modules lead to the growth, and decline, of individual plants. Biological control agents affect specific parts of plants and hence influence the births and deaths of these modules.

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