Biology 2014-2015

Section 5-1: How Populations Grow
SWS Biology
SWS Life Science

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.


Population Growth

       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)



Exponential Growth

       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 increases slowly.

       Overtime, the population becomes larger and larger until it approaches an infinitely large size.


Logistic Growth

       As resources become less available, the growth of a population slows or stops.

       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 reached zero.

       Carrying capacity is the largest number of individuals that a given environment can support.

       In the natural world, most populations follow a logistic growth curve.