Ecosystems and interaction between organisms

• Levels of organisation in ecosystems

  • Individual - Single organism
  • Population - Number of organisms of same species living in a habitat
  • Community - All the organisms (different species) living in a habitat
  • Ecosystem - Community of organisms along with all the non-living (abiotic) and living (biotic) conditions
• Habitat


  The place where an organism lives, e.g. a rocky shore or a field
• Species


  A group of similar organisms that can reproduce with each other to give fertile offspring
• Organisms compete for resources to survive
• Things organisms need from their environment and other organisms to survive and reproduce


  • Plants need light, space, water and minerals from the soil, as well as seed dispersers (e.g. animals that eat fruit and spread the seeds in their droppings) and pollinators (e.g. bees)
  • Animals need space (territory), shelter, food, water and mates
• The size of a population is limited by competition for resources as well as predation
• Organisms compete with other species (and members of their own species) for the same resources
• Competition with the grey squirrels for resources in some areas means there's not enough food for the red squirrels, so the population of red squirrels is decreasing
• How environmental changes affect communities

  • For some species population size may increase
  • For others it may decrease
  • The distribution of populations (where they live) may change
• Abiotic factors that affect communities

  • Environmental conditions - e.g. the distribution of bird species in Germany appears to be changing because of a rise in average temperature
  • Light intensity (plants only)
  • Moisture level
  • Soil pH
  • Chemical pesticides or fertilisers
• Bioaccumulation


  Where, at each stage of the food chain, concentration of a pesticide increases, so organisms at the top of the chain receive a toxic dose
• Eutrophication


  Excess fertilisers released into lakes and ponds cause increased growth of algae, which block sunlight from plants, causing them to die. Microorganisms feeding on the dead plants use up oxygen in the water, leading to the death of other organisms (e.g. fish)
• Biotic factors that affect communities

  • Availability of food
  • Number of predators
  • Presence of pathogens
  • Presence of competitors
• If the number of lions (predator) decreases then the number of gazelles (prey) might increase because fewer of them will be eaten by the lions
• Biotic factors:

  • availability of food
  • number of predators
  • presence of pathogens
• Abiotic factors:

  • Environmental conditions
• Ecosystems level of organisatio:

  • individual
  • population
  • community
  • ecosystem
  • habitat
  • species