Introduction

A thought experiment: take two animals, one male and one female. Provide their offspring with unlimited food and allow them to breed. If animals die only of old age, how many animals will you have after 2 years, 5 years, 10 years, 20 years or 50 years? Figure 4.1 shows the results of this experiment using the intrinsic rate of population increase (rm, see Table 4.1, p. 80, for a definition) for six primate species. It is clear from this that some animals can increase their population size more rapidly than others. The bushbaby (Galago moholi) population contains over 2000 females after only 12 years and 3600 million (3.6 × 1014) individuals after 50 years, whereas the gorilla population contains only 29 females after 50 years.

The intrinsic rate of population increase depends on three variables: age at first reproduction (AR), birth rate (b) and age at last reproduction (L). Birth rate has an important influence on the rate of population increase, which is illustrated by the examples given of Macaca silenus and Macaca sylvanus the females of which start breeding at about five years of age. However, in Macaca sylvanus, birth rate is higher, leading to a higher intrinsic rate of population increase. Animals that start breeding at an early age also show a higher intrinsic rate of population increase than do those that start breeding later. This has a knock-on effect over the generations, as a female that breeds early produces female infants that also breed early, thus increasing the total number of females that are producing infants. For example, both Macaca sylvanus and Erythrocebus patas breed annually, but E. patas reaches maturity nearly two years before M sylvanus.