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Module #18

Population and Community Ecology

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Learning Objectives

  • Explain how nature exists at several levels of complexity.
  • Discuss the characteristic of populations.
  • Contrast the effects of density-dependent and density independent factors on population growth.

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The Texas Mosquito Mystery

  • What are two factors that allowed the population of mosquitoes to grow at exponential rates in the summer of 2012?
  • Why might this be important for population ecologists to know?
  • How might residents of Dallas benefit from knowing mosquito population trends?

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Texas Mosquito Mystery

  • 2012
  • West Nile Virus- 12 people died and 300 infected by mosquitoes
  • Significant increase in disease transmission from the previous year...why?

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New England Forests Come Full Circle

  • Early 1600s:
    • New England Forests dominated by trees (sugar maple, American beech, white pine and Eastern hemlock)
    • Settler clear cut the forests
  • Early 1800s, deforestation peaked, 80% of forests lost
  • 1850-1950, people move away and succession occurs
    • Seeds and grasses retake abandoned fields
    • Goldenrods and other flowers dominate and instigate return of herbivorous insects
    • Saplings begin to take hold
    • Fast-growing pine (conifers) dominate and then harvested for lumber
    • Deciduous trees take over

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Levels of Ecological Organization

  • Biosphere includes all life on Earth
  • Biomes are large scale, globally repeated ecosystems
  • Ecosystems are specific areas that include abiotic and biotic components
  • A community refers to all the various life forms in an ecosystem
  • A population refers to all of the same species living in an ecosystem
  • An individual refers to just one organism

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Species Dynamics

  • Community composition changes from ecosystem to ecosystem, regardless of whether they are the same biome or not
  • Population ecology is the study of how populations change over time
  • Population Size (N): the number of individuals in a specific area at a given time
  • Population Density: the number of individuals per unit area
  • Population Distribution: describes how individuals are distributed; density is not always the same
  • Sex ratio describes the number of males compared to females
  • Age Structure: describes the distribution of organisms of various ages in the population

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Distribution Models

  1. Top: Uniform Distribution
    1. Organisms spread out evenly
    2. Territorial birds tend to adopt this distribution
  2. Middle: Random Distribution
    • Typical of many (but not all) plant communities especially forests
    • No apparent pattern
  3. Bottom: Clumped Distribution
    • Typical of animals that live in groups such as meerkats

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Population Control

  • A limiting resource is something a population cannot live without and is too rare to permit unlimited growth of the population
  • Density-dependent factors influence a population’s ability to survive and is directly related to the population's size
    • Food is a typical density dependent factor
  • Density-independent factors influence the probability an organism can survive regardless of how large the population is
    • Natural disasters
  • The carrying capacity of an ecosystem refers to the maximum number of individuals the ecosystem can sustain

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Food: a Density-Dependent Factor