PopulationsChapter 5

Dr. Donna HowellBiology I Blacksburg High School

Characteristics of Populations• Three important

characteristics of a population:– Geographic

distribution (where located)

– Density (how many per unit of area)

– Growth Rate (how fast population is growing)

Population Size• Three factors influence

population size:– Number of births– Number of deaths– Number of

individuals that enter and leave a population

Carsey Institute. Data from Census Bureau and National Center for Heath Statistics. The map above shows the number of years between 1966 and 2009 that a county had more deaths than births — when there was a natural decrease in population. The counties in dark red had a natural decrease in 30 or more years during that time. The blank counties had no years of a natural decrease.

Population Size• Immigration –

the movement of individuals into an area

• Emmigration – the movement of individuals out of an area

Exponential Growth

• Exponential growth occurs when the individuals in a population reproduce at a constant rate.

• Only occurs when population has unlimited resources.

• Curve is “J” shaped.

Carrying Capacity

• Carrying capacity is the number of individuals that a given environment can support with its resources.

Limiting Factors• Limiting factors

are factors that cause a population’s growth to decrease.

• Two kinds:– Density-

dependent– Density-

independent

Density-Dependent Limiting Factor• A limiting factor

that depends on population size.

• Become limiting only when the population reaches a certain density.

• Include:– Competition– Predation– Parasitism– Disease

Competition• When populations

become crowded, organisms compete for food, water, space, sunlight, and other essentials.

Predation• When populations

are controlled by the predator-prey relationship.

• Example: snowshoe hare and lynx. As the population of hares increases, so does the population of lynxes. Eventually the lynxes will eat all the hares, and the hare population goes down in number.

Parasitism and Disease• Diseases and

parasites can sweep through a population and kill many. The more dense the population the faster the disease/parasite spreads.

• Ex: Black Plague

Density-Independent Limiting Factor• A limiting factor

in which population size does not matter.

• Some examples:– Natural

disasters– Clear cutting

of forests– Weather

patterns (drought)

Demography• Demography is

the study of human populations.

• Study things such as birth rate, death rate, and age structure (how many people of each age in a population)

• Age-structure diagrams are useful in studying age structure.

The End