Effects of Population Density on Growth Rates of Animal Populations

The growth rate of a population at time t is defined as the change in numbers per unit population at that time period (rₜ = dn/Nₜd). The rate usually changes with time as the population increases or decreases. The objective of this study was determine whether the growth rate of animal populations is or is not a function of the population density. In most mathematical models of populations whose changes resemble those of actual populations, the growth rate is a decreasing function of density, meaning that as the population increases the growth rate decreases and vice versa. Records of actual populations can be used to test the relation between growth rate and density by calculating correlation coefficients for these two characteristics. The methods of doing this are described and the limitations on the data are explained. The records of 111 different populations representing 71 species were analyzed by these methods. Of the 71 species, 7 were eliminated from further analysis because their census records were not significantly different from a series of random numbers, 47 had correlation coefficients that were negative and significantly different from zero, 16 had coefficients not significantly different from zero (all but one of these estimated coefficients were negative), and one, the world's human population, had a positive coefficient significantly different from zero. There were no differences between taxonomic groups (insects other invertebrates, fish, birds, mammals). The primary conclusion is that in most animal species a population's growth rate is a decreasing function of density. The explains the relative stability of animal populations, which never continue to increase at rates their fertility would allow, and rarely decrease to extinction. Tentative conclusions are presented regarding the processes regulating population numbers. Populations of herbivorous insects at low and moderate levels are regulated by disease, parasites, and predators causing mortality that is an increasing function of density. Favorable conditions may allow an increase in insect numbers so rapid that the population temporarily escapes regulation by its enemies. Nonterritorial species of vertebrates are normally controlled by predation and, when that fails, by competition; in both cases juvenile individuals are most affected. In territorial vertebrates competition for suitable territories determines the size of the breeding population. Populations of vertebrate species (expecting man) are in general regulated by the production of adult individuals being a decreasing function of population density.