Ch. 5 Populations

Ch. 5 Outline• 5-1: How Populations Grow– Characteristics of Populations– Population Growth– Exponential Growth– Logistic Growth

• 5-2: Limits of Population Growth– Density-Dependent Factors– Density-Independent Factors

• 5-3: Human Population Growth– Patterns of Population Growth

Characteristics of Populations• Geographic Distribution

• Density

• Growth Rate

• Age Structure

• Geographic Distribution (or range) is the area inhabited by a population

• Population density: the number of individuals per unit area

Population Growth• Three factors can affect population

size:– The number of births– The number of deaths– The number of individuals leaving or

entering the population

• A population is growing when its birthrate is greater than its death rate.

Population Growth• The movement of Individuals also

affects population size.– Immigration: The movement of

individuals to an area– Emigration: The movement of

individuals out of an area

• What might cause the movement of individuals either to or from and area?– Food Supply– Finding a mate

Exponential Growth• Exponential growth: occurs when

individuals in a population reproduce at a constant rate– A population will grow exponentially

under ideal conditions:• No predation• Plenty of Food and Space

• The graph of a population growing exponentially:– J-shaped Curve

• Bacteria often grow exponentially

Exponential Growth Curve

Logistic Growth• Logistic Growth: occurs when a

population’s growth slows or stops following a period of exponential growth– Takes into account a limited amount of

resources

• Graph of Logistic Growth:– S-shaped Curve

• The largest number of individuals that a given environment can support is called the carrying capacity.

Limits to Population Growth• What causes population size to

decrease?– Limiting Factors: a factor that causes

population growth to decrease

• Examples of Limiting Factors:– Competition– Predation– Parasitism– Drought (or other climate extremes)– Human Disturbances

Density-Dependent Factors• A limiting factor that depends on

population size is called a density-dependent factor.– They only become limiting when the

population density reaches a certain level. (They don’t affect small, scattered populations)

Examples of Density-Dependent Factors• Competition– When populations are crowded, organisms

begin competing for food, space, mates, sunlight, etc…

– Competition between two different species also exists leads to adaptations and natural selection (EVOLUTION!!!)

• Predation– The predator-prey relationship is one of the

best known mechanisms for population control.• Ex. Moose-wolf population near Lake Superior

Examples of Density-Dependent Factors• Parasitism and Disease– Parasites take nourishment at the

expense of their host, often weakening them (leading to death).

Density-Independent Factors• Density-Independent Factors: affect

all populations in similar ways, regardless of the population size.

• Examples:–Weather/seasonal changes– Natural disasters– Human Activities

Historical Overview• For most of history, the human

population has grown very slowly

• About 500 years ago, agriculture and industry made life easier and the population growth rate increased.

• With the Industrial Revolution came improved medical care and increased sanitation, which lead to exponential human population growth.

Patterns of Population Growth• The Earth cannot sustain exponential

population growth forever. It DOES have a carrying capacity. (Resources are limited!)

• Demography: the study of human populations– Attempts to explain how they will change over

time

• Birthrates, death rates, and age structures of populations help predict why some countries have high growth rates while others have low growth rates.

Patterns of Population Growth• In the last century, Japan, the United

States, and much of Europe have slowed their population growth.–Why?• With improved nutrition, sanitation, and

medical care, the death rate was slowing and the birthrate was still high (cause an increase in the population). BUT, as societies become more modern, families are tending to have less children (as education level and standards of living increase).

Patterns of Population Growth• However, the worldwide population

is still growing exponentially.

• Population growth also depends on age structure.– How many people of different ages

make up a population?

• Age-Structure Diagrams: Graph the number of people in different ages in a population– Ex: USA vs. Rwanda.