Ecology in Further Detail ❮ 223Sex ratio Proportion of males and females in a population.
Generation time Time needed for individuals to reach reproductive maturity.
Age structure Statistic that compares the relative number of individuals in the
population from each age group (Figure 18.2).
Immigration rate Rate at which individuals relocate into a given population.
Emigration rate Rate at which individuals relocate out of a given population.
All these statistics together determine the size and growth rate of a given population.
Obviously, a higher birth rate and a lower death rate will give a faster rate of population
growth. A high female sex ratio could lead to an increase in the number of births in a pop-
ulation (more females to produce offspring). A short generation time allows offspring to be
produced at a faster rate. An age structure that consists of more individuals in the middle
of their reproductive years will grow at a faster rate than one weighted toward older people.Population Growth and Size
Biotic potentialis the maximum growth rate of a population given unlimited resources,
unlimited space, and lack of competition or predators. This rate varies from species to
species. The carrying capacityis defined as the maximum number of individuals that a
population can sustain in a given environment.
If biotic potential exists, then why isn’t every inch of this planet covered with life?
Because of the environment in which we live, numerous limiting factorsexist that help
control population sizes. A few examples of limiting factors include predators, diseases,
food supplies, and waste produced by organisms. There are two broad categories of limiting
factors:Figure 18.2 A typical age structure chart.Age
80+
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
Below 5
6 5 4 3 2 1 0 1 2 3 4 5 6
%Males % FemalesLiz (college
freshman):
“Know how to
read these
charts.”
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