Numbers in plant populations often vary widely and, as yet, only a few studies describe adequately how they are determined. The importance of abiotic factors in establishment as well as in determining the carrying capacity of the habitat has been widely recognized. It is also accepted that intraspecific competition for limiting resources may limit numbers in a density-dependent way. In addition, both herbivores and pathogens may reduce numbers of plants through mechanisms which are either dependent or independent of population density.
Herbivory affects plants not only in the vegetative and reproductive phases but also in the pre-dispersal and post-dispersal phases of seeds. Because many studies on the population dynamics of plants have been confined to the former phases, the relative importance of herbivores is likely to have been underestimated for the latter. Furthermore, because herbivory often does not result in mortality but in reduced growth and seed production, its effects may be manifested only as reductions in the size of later generations. The effects of herbivory can therefore only be assessed properly if they are studied over several generations.
Examples are presented of plant populations in which numbers are determined by competition for limiting resources. However, other situations are recognized in which it is the activity of herbivores which appears to determine numbers. There is a continuing need for studies of population dynamics, but there is an urgent requirement for these studies to be related to a unifying conceptual base.