Science 20F
Population Dynamics
Changes in population size occur when individuals are added to or removed from a
population. If increases in population are balanced by decreases in populations, the
population growth is zero. This balance is called dynamic equilibrium.
Factors that affect Population Sizes
1) Births – number of offspring of a species born in one year.
2) Deaths (mortality) – number of individuals of a species that die in one year.
3) Immigration – number of individuals of a species moving into an existing
population.
4) Emigration – number of individuals of a species moving out of an existing
population.
Open Populations – If all four factors mentioned above are acting on populations,
these populations are classified by Ecologists are open populations.
Closed Populations – If only births and deaths affect populations, these populations
are classified as closed populations.
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The global population of any species is considered a closed population.
The human population is not in dynamic equilibrium.
Population Histograms
Histograms are useful when studying populations of long-lived organisms such as
humans.
The shape of the histogram pyramid allows you to predict changes in the
population.
 A wide base indicates a rapidly growing population.
 When the base of the pyramid is narrower than the middle, a declining
population is indicated.
 A gradually growing histogram pyramid indicates a stabilized population.