The composition of a spider population in terms of species and numbers is influenced by many environmental factors. Examples of faunal differences between simple, uniform habitats and those which are much more diverse are discussed. This paper examines the characteristics of populations in different habitat types, some stable and others subject to use which causes disturbance. When adequate and accurate samples are taken of spider populations in a defined habitat it is usual to find a few Abundant species, a large number of Scarce, and a third category which is classified as Frequent. The species—numbers relationship can then be illustrated by calculating the proportion of the total population represented in each of the three groups. When graphed, an L-shaped curve is produced, which varies according to the structure and environmental conditions of the habitat, as well as to the sampling method used. A number of examples are described, and possible ecological interpretations are discussed.