Population cycle
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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

H. Biology

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

  • 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

  • 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…

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)

  • Immigration and births cause an INCREASE in population

  • Emigration and death cause a DECREASE in population



even (uniform)


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

  • 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?

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 - 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

Exponential Growth (J-shaped)

Logistic (S-shaped)

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

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

  • Include:

    • Competition

    • Predation

    • Parasitism/disease

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

  • 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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