Characteristics of Populations
¨ Three important characteristics of a population are its geographic
distribution, density, and growth rate.
¨ Geographic distribution, or range, is a term that describes the
area inhabited by a population.
¨ Range can vary from a few cubic centimeters occupied by bacteria
in a rotting apple to the millions of square kilometers occupied by migrating whales.
¨ Population density is the number of individuals per unit area.
¨ Three factors can affect population size: the number of births, the number of
deaths, and the number of individuals that enter or leave the population.
¨ Populations grow if more individuals are born than die in any period of time.
¨ Immigration is the movement of individuals into an area. (population will increase)
¨ Emigration is the movement of individuals out of an area. (population will decrease)
¨ If a population has abundant space and food, and is protected from
predators and disease, then organisms in that population will multiply.
¨ Exponential growth occurs when the individuals in a population reproduce at a constant rate.
¨ The pattern of growth is a J-shaped curve.
¨ The number of individuals in an exponentially growing population
¨ Overtime, the population becomes larger and larger until it approaches
an infinitely large size.
¨ As resources become less available, the growth of a population slows
¨ Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth.
¨ The pattern of growth is an S-shaped curve.
¨ Population growth may slow down when the birthrate decreases, when
the death rate increases, or when both events occur at the same time.
¨ When the population has leveled off, the average growth rate has
¨ Carrying capacity is the largest number of individuals that a given environment can support.
¨ In the natural world, most populations follow a logistic growth