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Populations. Honors Biology – Chapter 5. What is a Population?. A group of the same species living in a given area It is populations or organisms that interact in ecosystems Example:. An Introduction – Sea Otters of the Pacific Northwest. Live in kelp forest community.

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Honors Biology – Chapter 5

What is a population
What is a Population?

  • A group of the same species living in a given area

  • It is populations or organisms that interact in ecosystems

  • Example:

An introduction sea otters of the pacific northwest1
An Introduction – Sea Otters of the Pacific Northwest

  • Sea urchins as food

    • Sea urchins eat kelp

  • Sea otters nearly eliminated due to hunting

  • Sea urchin populations rise

  • Kelp forests decline

  • Restoring balance: Sea otters declared endangered – protected from hunting

Predator prey popluation relationships
Predator/Prey Popluation Relationships

  • Hare / Lynx

  • Hare population goes up

    • Lynx population goes up

      • Lots of food (hares) available

    • Causes hare population to go down

      • More lynx = more predation on hares

    • Causes lynx population to go down

      • Hare population decrease means less food for lynx and they die

4 characteristics of populations
4 Characteristics of Populations

  • Geographic distribution

    • Range

    • Area inhabited by a population

  • Density

    • Number of individuals per unit area

  • Growth rate

  • Age structure

Population growth
Population Growth

  • 3 factors affect population size

    • Number of births

      • natality

    • Number of deaths

      • mortality

    • Number of individuals that enter or leave a population

      • Immigration - enter

      • Emigration - leave

Population growth1
Population Growth

  • A population will increase or decrease in size depending on how many individuals are added to it or removed from it

  • Growth occurs if

    • More individuals are born than die

    • If birth rate equals death rate the population will stay near the same size

Movement into or out of a population
Movement into or out of a population

  • May be caused by

    • Young animals mature and leave the area

    • Shortage of food in an area

Exponential growth
Exponential Growth

  • Individuals in a population reproduce at a constant rate

    • Initially, growth is slow

    • Overtime the population becomes larger and large until it approaches an infinitely large size

    • Ideal conditions with unlimited resources required


  • Fastest rates of reproduction among living things

  • 20 minutes for population to double

  • Why don’t they take over the Earth?

    • NOT unlimited resources

Logistic growth
Logistic Growth

  • Growth pattern that occurs when a population’s growth slows or stops following a period of exponential growth.

  • Things that slow a population’s growth

    • Natality (birth rate) decreases

    • Mortality (death rate) increases

    • Rate of immigration decreases

    • Rate of emigration increases

Logistic growth1
Logistic Growth

  • Also, when birth and death rates are the same, the population growth will slow or even stop for a time

  • Even when a population stops “growing”, the population is still rising and falling somewhat – though the ups and downs average out to the same population size.

Logistic growth curve
Logistic Growth Curve

  • Slow growth

  • Exponential growth

  • Growth levels off

  • Carrying Capacity

    • K

    • Dotted line

    • The maximum number of individuals that an environment can support

Limits to growth
Limits to Growth

  • Limiting Factors

    • Factors that cause population growth to decrease

      • Competition

      • Predation

      • Parasitism

      • Drought

      • Human disturbance

There are two types of limiting factors
There are TWO types of limiting factors

  • Density Dependent

  • Density Independent

Density dependent limiting factors
Density Dependent Limiting Factors

  • Depends on the size of a population

  • Usually greatest impact when the population is large and dense

  • Include

    • Competition

    • Predation

    • Parasitism

    • Disease

Density independent limiting factors
Density Independent Limiting Factors

  • Affects all populations (large or small) in similar ways.

    • Unusual weather

    • Natural disasters

    • Human activities

      • Like Habitat Destruction

Human population growth
Human Population Growth

  • Like other populations of organisms, the human population tends to increase with time.

Human population growth curve
Human Population Growth Curve

  • Initially slow

    • Food scarce

    • Life harsh

  • 500 years ago

    • More rapid growth

      • Currently EXPONENTIAL

    • Why?

      • Agriculture and industry advances

        • Food supply increased

        • Improved sanitation

        • Medicine


  • The study of human populations

  • Examines human populations and tries to predict how they will change over time.

Demographic transition
Demographic Transition

  • Population growth in some countries has slowed

    • U.S.

    • Japan

    • Much of Europe

  • Why?

    • Hypothesis: These countries have completed the “demographic transition

Demographic transition1
Demographic Transition

  • A dramatic change in birth and death rates

  • What happens

    • Initially both birth and death rates high

    • Advances in nutrition, etc. lower death rate

    • Population increases rapidly

    • Societies modernize – increase education, etc.

    • Families have fewer children

    • Birthrate falls to meet the death rate

    • Population growth stops

Populations that have not completed the demographic transition
Populations that have NOT completed the demographic transition

  • Most countries

    • India

    • China

Age structure diagrams
Age Structure Diagrams transition

  • Shows how many people of different ages and genders are in a population

  • Useful in predicting future population growth

Predicting future population growth
Predicting Future Population Growth transition

  • Factors to consider

    • Age structure of each country

    • Prevalence of life-threatening diseases

  • Predictions for 2050

    • 9 billion people on Earth

Effects of human population growth
Effects of Human Population Growth transition

  • Some scientists suggest if human population growth rate does not become LESS than 0 (deaths outnumber births)

    • Serious damage to environment

    • Serious damage to global economy

  • Some economists say that technology advances and changes in societies can control negative impacts