Populations

Populations

• Populations: groups of individuals that belong to the same species and live in the same area

Characteristics Of A Population

• There are three important characteristics of a population: 1. Geographic Distribution, 2. Density, and 3. Growth- Geographic distribution, or range, is the area in which a population lives- Population density is the number of individuals per unit area. An example is the number of people per square kilometer.- Growth rate is how quickly a population increases or decreases in size.

Geographic Distribution Of Termites

Population Density

Population Density

• Dispersal patterns– Random– Clumped– Uniform

Random

Clumped

Uniform

Patterns

• Reproductive pattern = life-history pattern– Two main types of patterns:

• Rapid life-history patterns• Slow life-history patterns

Patterns

• Rapid life-history patterns– Changing or

unpredictable environment

– Small species– Mature rapidly– Reproduce early– Short life span

Patterns

• Slow life-history pattern– Large species– Stable environments– Reproduce slowly– Matures slowly– Long life span– Stay at or near carrying

capacity

Factors That Affect Population Size

• Three factors can affect population size: 1. the # of births, 2. the # of deaths, and 3. the # of individuals that enter and leave the population.

• Populations increase through births and immigration. Populations get smaller through deaths and emigration.– Immigration is the movement of individuals into an

area.– Emigration is the movement of individuals out of an

area

How Populations Grow• Exponential growth occurs when members of a

population reproduce at a constant rate. (This growth pattern is shown by a J-shaped curve.)– As the population grows, the number of reproducing

members keeps rising. Thus the population grows faster and faster.

– Under ideal conditions with unlimited resources, a population will grow exponentially.

– In nature, exponential growth does not go on for long. • Resources are used up in time. • This causes population growth to slow or stop. • Predators and disease also slow exponential

growth.

Exponential Growth Graph

How Populations Grow• As resources become less available, the growth of a

population slows or stops. This is called logistic growth. (Logistic growth is shown by an S-shaped curve.)– Logistic growth occurs when a population's growth

slows or stops following a period of exponential growth. Example, if food resources start to run out or space becomes limited, a population that was growing exponentially may start to exhibit logistic growth.

– The population size when growth stops is called carrying capacity.

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

Limits To Growth

• A limiting factor is anything that slows population growth. There are two kinds of limiting factors.– Density-dependent limiting factors – Density-independent limiting factors

Limiting Factors

• Density-dependent limiting factors rely on population size. They include competition, predation, parasitism, and disease.– Competition occurs when organisms are using the

same ecological resources at the same time.– Predation occurs when one organism captures and

feeds on another organism. The organism that gets eaten is the prey. The organism that eats the prey is the predator.

• Predator-prey relationships can affect the size of both the predator population and the prey population.

Limiting Factors

– Parasitism is a symbiotic relationship in which one organism benefits at the expense of another species

– Disease is an abnormal condition affecting the body of an organism.

• Diseases can come from bacteria, viruses, and genetics– Examples: cancer, heart disease, AIDS

Limits To Growth• Limiting factors continued

– Density-independent limiting factors do not rely on population size.

• They include natural disasters and human activities such as damming rivers.

• When such factors occur, many species show a rapid drop in population size.

Human Population

Human Population Growth

• The size of the human population tends to increase with time.• For most of human existence, the population grew slowly.• About 500 years ago the population began growing faster.

– Agriculture and later industry made food more available.– Improved sanitation and medicine reduced death rates.– However, birthrates stayed high in most places. This led

to exponential growth. Today, the human population continues to grow exponentially.

Agriculturebegins

Plowingand irrigation

Bubonicplague

IndustrialRevolutionbegins

Human Population Growth

Human Population Growth

Human Population Explosions

The Population Explosion– Around 1650, improvements in hygiene, diet, and

economic conditions further accelerated population growth.

– After World War II, the human population grew at the fastest rate in history, largely because of better sanitation and medical care in poorer countries.

Human Population Growth Due To Medicine Practices And Sanitation

Advances in Human Technology = Growth

Human Population Growth Vs Species Extinction

Study Of Population Growth

• Demography is the scientific study of human populations.

• Demographers try to predict how human populations will change over time.

• Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.

Study Of Human Population Growth

• Over the past century, population growth in the U.S., Japan, and much of Europe slowed greatly.

• Demographers call this shift a demographic transition.– The transition began as death rates fell, causing a

brief rise in population growth. Then birthrates fell, slowing population growth.

– Most people live in countries that still have high population growth.

Human Population Growth

Human Population Growth

Study Of Human Population Growth

• Demographers use age-structure diagrams to help predict population growth. An age-structure diagram is a bar graph.

• An age-structure diagram shows how many people of each gender are in each age group in the population.

• To predict how the world population will change, demographers need to think about the age structure of each country as well as the number of people with fatal diseases, including AIDS.

U.S. Population Rwandan PopulationMales MalesFemales Females

Age Distribution

YouMe