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Ecosystems and the stability of populations

  • To give a broad view of the levels of ecological organisation, the following definitions are important:


  • Part of the environment occupied by an organism.
  • If particular region an individual organism is normally confined to is extremely small, it is known as a microhabitat.


  • Position organism occupies in it's habitat.
  • Determined by sum of characters required for survival,eg:
    • Physical,
    • Chemical,
    • Spatial factors.
  • No two species occupy the same niche, in the same environment in each other's presence.


  • Total number of particular species in a habitat.


  • The total of all populations of different species of organisms, that live and interact in the same habitat.
  • based on Dynamic Feeding Relationships within a food chain.


  • An settled, stable unit of nature that includes all the organisms (biotic component) and the non-living (abiotic) components of the environment.
  • Ecosystems are very variable in size.
  • Ecosystems can support a limited population size of any single species.
  • Influencing factors:
    • Abiotic Factors
    • inter-organism interactions
    • inter-organism competition
    • predation


  • A tropical rainforest is an ecosystem, in which one of the habitats within it as a teak tree. On the tree is a community of insects, which consists of an ant population, a bee population and a beetle population. The niche of the ant is to be the primary consumer by consuming food it locates on the tree, eg. leaves and seeds.

People change communities.

  • The introduction of new plant/animal species to different countries changes the stability of native species.
  • Changes may influence whole:
    • populations
    • communities
    • ecosystems.
  • We must be able to evaluate evidence and make balanced judgements between meeting human demands and the need to conserve the environment.

Winners and Losers

  • An alien species is a type of organism that has been introduced to an area it does not occur naturally by humans.
  • May be very invasive and cause considerable ecological damage.
  • Domesticated species may also give rise to problemsto native species.
  • Introduced species may affect ecosystems by competing with native species.
    • Examples:
      • domestic cats,
      • grey squirrels,
      • Floating fern (Salvinia molesta),
  • These may eventually replace the native species by interspecific competition.
  • The floating fern was introduced around the world as a pond or aquarium plant but has damaged many lakes in the tropics due it its rapid growth.
  • It has been controlled by the introduction of another alien species - salvinia weevil (Cyrtobagus salviniae).
  • An example of biological control.
  • Humans may influence ecosystems in other ways.
  • Growth of the urban environment has increased the habitats and niches for:
    • foxes
    • rats
    • pigeons
    • cockroaches and others.

GM Organisms

  • The creation of genetically modified organisms and their release may have unexpected and undesirable consequences on native populations.
  • Environmental Impact Assessment report must be undertaken to minimise impact of large-scale introduction of GM organisms.
  • An EIA is an assessment of the possible impact (positive or negative) that a proposed project may have on the environment.
  • Some examples of genetic modification are controversial.
  • Maize crops are often damaged by corn borer insects.
  • A gene from a bacterium (Bacillus thuringiensis) has been transferred to maize.
  • The gene codes for a bacterial protein called Bt toxin that kills corn borers feeding on maize.

Potential benefits of Bt maize

Possible harmful effects of Bt maize

Less pest damage produces higher crop yields

Humans or animals that eat the GM maize may be harmed by bacterial DNA or toxin.

Less landed needed for crop production - possibly freeing land for conservation projects

Insects that are not pests may be killed.
Maize pollen containing the toxin is blown onto wild plants growing near the maize. Insects feeding on the wild plants are therefore affected even if not feeding directly on the GM maize.

Less use of insecticides - less expensive, may be harmful to farm workers and wildlife.

Populations of wild plants may be changed.
Cross-pollination will spread Bt gene into some wild plants but not others. These plants producing Bt toxin now have a survival advantage over the others.