¨       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.