Almost all relations in this book are derived from and applied to individual animals. The ecological implications of biological scaling are, therefore, primarily autecological. Such information is valuable and important, but many processes depend on other members of the population or the community. For example, excretion, ingestion, growth, and reproduction are each physiological processes that scale to body size. These autecological rates are given new significance when multiplied by the number of individuals in the population or community. Excretion becomes nutrient regeneration, ingestion might represent prey mortality, and growth and reproduction become production. Each of these population and community processes, and many others, can be calculated from allometric equations if we know population size and community size structure (i.e., the number of organisms of each size, regardless of species, in the community). Such composite values may even be better than individual predictions, because they average over many different members of the population or over many different species and individuals.

Ideally, figures for abundance would be measured accurately and precisely, but such estimates are often unavailable for financial, practical, and biological reasons. One then searches the literature for approximations, values from similar sites and similar organisms. This book is dedicated to the premise that size is a major criterion of similarity, consequently this chapter summarizes the literature relating animal abundance to individual size as a necessary step toward the application of allometry to higher ecological levels.