¨ 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.

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### 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.