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Biology · Communities in Ecosystems

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Biology · OCR Paper 2

Communities in Ecosystems

Levels of Organisation in an Ecosystem

Living things are organised into organisms, populations of one species, communities of many species, and ecosystems that include the abiotic environment.

  • Organism A single living thing.
  • Species A group of similar organisms that can breed together to produce fertile offspring.
  • Population All the organisms of one species in an area.
Four levels of organisation: a single organism, a population of one species, a community of many species, and an ecosystem that includes the non-living surroundings.
Each level of organisation contains the one before it.

Community and ecosystem

  • Community All the living things in an area; Many populations of different species; Only the biotic parts
  • Ecosystem The community and the abiotic factors; Living and non-living parts together; Materials and energy flow through it

Abiotic and Biotic Factors

Abiotic factors such as temperature, light intensity, moisture level and soil pH.

  • Abiotic factor A non-living factor that affects organisms, such as temperature.
  • Biotic factor A living factor that affects organisms, such as a predator.
  • Light intensity How bright the light is.
Abiotic factors, light intensity, temperature, moisture level and soil pH, and biotic factors, predators, food availability, disease and competition.
Abiotic factors are non-living, and biotic factors are living.

Abiotic and biotic factors

  • Abiotic factors Non-living; Temperature, light intensity, moisture level, soil pH; Affect plants directly, and animals directly and through plants
  • Biotic factors Living; Predators, food, disease, competition; Change when the populations of other species change

Interdependence and Competition

Species in a community are interdependent through predation, mutualism and parasitism, and they compete for limited resources.

  • Interdependence The way in which species in a community depend on each other.
  • Predation The hunting and eating of one animal by another.
  • Mutualism An interaction in which both organisms benefit.
Predation, competition, mutualism and parasitism, with plus and minus badges to show which organisms benefit and which are harmed.
In predation and parasitism one species benefits and one is harmed, in mutualism both benefit, and in competition both are harmed.

Interactions between two species

  • Predation and parasitism One species benefits; The other is harmed; A predator kills its prey
  • Mutualism and competition Mutualism: both benefit; Competition: both are harmed; Mutualism helps both species to survive

Trophic Levels, Pyramids of Biomass and Efficiency (Separate only)

Biomass falls at each trophic level because much of it is lost by respiration, egestion, excretion and parts that are not eaten, so the efficiency of each transfer is low.

  • Trophic level The position of an organism in a food chain.
  • Producer An organism that makes its own food, such as a plant.
  • Primary consumer An animal that eats producers.
A food chain from a plant to a caterpillar to a thrush to a sparrowhawk, with each organism's trophic level labelled.
The arrows show the direction in which biomass is transferred.

Eating plants and eating meat

  • Eating plants Short food chain; Fewer trophic levels; Less biomass lost
  • Eating meat Longer food chain; An extra trophic level; More biomass lost by the animal