The numbers of individuals in populations are broadly regulated by four factors, viz., (1) Reproduction, (2) Mortality of individuals within populations and (3) Immigration, and (4) Emigration between different populations. These factors are influenced by climatic parameters as well as by agencies intrinsic to the population such as the density of individuals already present (Law, 1981). Plant population biologists have mainly worked on the effects of population density on reproduction and mortality.
The chief conclusion has been that rates of reproduction decline with increasing density. However, one limitation of this approach derives from the fact that plant populations are often spatially heterogeneous in density. Some regions of low density are usually interspersed among other regions of high density.
Consequently, those characteristics of populations which are determined by density often show significant within-population variation. Law (1981) has pointed out how spatial patterns of developing populations affect their temporal patterns and how the temporal patterns in turn affect the spatial patterns. In plant populations, the spatial and temporal dimensions are strongly interdependent.