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Population Cycle. H. Biology. Population Ecology. Population ecology = study of populations in an environment, including environmental influences on density and distribution, age structure, and population size

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Population Cycle

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Population cycle

Population Cycle

H. Biology

Population cycle

Population Ecology

Population ecology = study of populations in an environment, including environmental influences on density and distribution, age structure, and population size

Population = group of same individuals of a single species living in the same general area at the same time and can interbreed



  • Three key features of populations:

    • Size – number of individuals in an area

    • Density – measurement of population per unit area or unit volume

    • Dispersion - describes their spacing relative to each other

      • Clumped, even/uniform, random

Measuring density

Measuring Density

  • Population Density = individuals

    unit area

  • Unit area= length x width

    Ex: What is the density of trees in 1 km2 in there are 1000 trees in 50 km2?

    1000 trees = 20 trees

    50 km2 km2

Factors that affect populations

Factors that Affect Populations

  • Available resources (ex. food, water, shelter)

  • Activities of other organisms (ex. predators, disease-causing parasites)

  • Organism's own characteristics (ex. gestation period, number of young produced, nurturing of young, migratory)

  • Time of day or year (ex. tides, seasons, nocturnal or diurnal)

  • Weather (ex. amount of rainfall, cyclone, drought)

Size depends on

Size depends on…

Growth Rate:

Birth Rate – Death Rate = Rate of natural increase (b - d = r)



  • Population density = number of individuals / unit of area

    • Dp=N/s



  • Density is affected by…

    1. Immigration- movement of individuals into a population

    2. Emigration- movement of individuals out of a population

    3. Density-dependent factors- Bioticfactors in the environment that have an increasing effect as population size increases

      - Ex. Disease, competition, parasites

    4. Density-independent factors-Abioticfactors in the environment that affect populations regardless of their density

    - Ex. Temperature, storms, habitat destruction,


Population density growth

Population Density (Growth)

  • Immigration and births cause an INCREASE in population

  • Emigration and death cause a DECREASE in population




even (uniform)


What is dispersion

What is Dispersion?

  • Clumped Dispersion

    • Populations found in tight clusters, dispersed across a large landscape.

    • In between very few to no individuals are usually found.

    • Grouped possibly for protection or survival

  • Random Dispersion

    • Populations found randomly about their habitat.

    • In immobile species, this is usually caused by their ability to live anywhere in a given habitat, except, they are limited to growing wherever they are first set root.

    • In motile populations, individuals are able to move about their habitat, so that at any given instance, they can be found anywhere about their environment.

  • Uniform Dispersion

    • Populations found evenly distributed about their habitat.

    • This is generally caused by a species ability to survive anywhere in their habitat - they use the resources found immediately around them, and spread out as to use all of the available resources.

Population growth

Population Growth

  • Exponential Growth =

    • population grows as if there are no limitations to how large it can get (biotic potential)

    • J-curve

    • A population increases slowly at first (the "lag phase") and then grows increasingly rapidly with time ("log phase”)



What are other factors that affect population growth

What are Other Factors That Affect Population Growth?

Limiting factor-any biotic or abiotic factor that restricts the existence of organisms in a specific environment.

EX.- Amount of water, Amount of food, Temperature

Carrying capacity

Carrying Capacity

  • Carrying Capacity - the maximum population size that can be supported by the available resources

    • There can only be as many organisms as the environmental resources can support

Population curves

Population curves

Exponential Growth (J-shaped)

Logistic (S-shaped)

Factors that affect the carrying capacity

Factors That Affect the Carrying Capacity

  • Limiting Factor - any biotic or abiotic factor that restricts the existence of organisms in a specific environment

    - Ex: Amount of water, Amount of food, Temperature

  • Two types:

  • Density-dependent limiting factor

  • Density-independent limiting factor

Density dependent limiting factors

Density-dependent limiting factors

  • Factors that are affected by the number of individuals in a given area

  • Include:

    • Competition

    • Predation

    • Parasitism/disease

Density independent limiting factors

Density-independent limiting factors

  • Aspects of the environment that limit a population’s growth regardless of population density

  • Include:

    • Unusual weather

    • Natural disasters

    • Human activities (chopping down a forest)

Limiting factors control population sizes

Limiting Factors Control Population Sizes

  • Density-dependent factors come into play when population approaches and/or passes the carrying capacity

    • Food supplies, waste, population-crowding diseases, H2O, mates, predators

  • Density-independent factors have nothing to do with the population size


    • Floods, droughts, earthquakes, other natural disasters and weather conditions

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