Age (as fraction of lifespan)

Age Structure

The distribution of individuals among different ages in a population is called age structure. Age structures are often presented in graphs, as in Figure 19-4. Many important population processes vary with age. In humans, very old individuals do not reproduce. If human populations have a high percentage of young individuals, they may have a greater potential for rapid growth.

Patterns of Mortality

The mortality data of different species tend to match one of three curves on a graph, as shown in Figure 19-5. These curves are called survivorship curves because they show the probability that members of a population will survive to a certain age. In humans or elephants, for instance, the likelihood of dying is small until late in life, when mortality increases rapidly. This pattern of mortality produces the Type I survivorship curve. For other organisms, such as some species of birds, the probability of dying does not change throughout life, giving a linear, or Type II, survivorship curve. Finally, many organisms are very likely to die when young. If an individual survives this early period, however, it has a good chance of surviving to old age. This type of survivorship curve, called Type III, is characteristic of animals such as oysters and salmon, and of many insects.

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