2.2 Communities and ecosystems

Significant ideas:

  • The interactions of species with their environment result in energy and nutrient flows.
  • Photosynthesis and respiration play a significant role in the flow of energy in communities.
  • The feeding relationships of species in a system can be modeled using food chains, food webs and ecological pyramids.

Knowledge and understanding:

  1. A community is a group of populations living and interacting with each other in a common habitat.
  2. An ecosystem is a community and the physical environment with which it interacts.
  3. Respiration and photosynthesis can be described as processes with inputs, outputs and transformations of energy and matter.
  4. Respiration is the conversion of organic matter into carbon dioxide and water in all living organisms, releasing energy. Aerobic respiration can be represented by the following word equation. glucose + oxygen –> carbon dioxide + water
  5. During respiration, large amounts of energy are dissipated as heat, increasing the entropy in the ecosystem while enabling organisms to maintain relatively low entropy and so high organization.
  6. Primary producers in most ecosystems convert light energy into chemical energy in the process of photosynthesis.
  7. The photosynthesis reaction is can be represented by the following word equation. carbon dioxide + water –> glucose + oxygen
  8. Photosynthesis produces the raw material for producing biomass.
  9. The trophic level is the position that an organism occupies in a food chain, or the position of a group of organisms in a community that occupy the same position in food chains.
  10. Producers (autotrophs) are typically plants or algae that produce their own food using photosynthesis and form the first trophic level in a food chain. Exceptions include chemosynthetic organisms that produce food without sunlight.
  11. Feeding relationships involve producers, consumers and decomposers. These can be modelled using food chains, food webs and ecological pyramids.
  12. Ecological pyramids include pyramids of numbers, biomass and productivity and are quantitative models that are usually measured for a given area and time.
  13. In accordance with the second law of thermodynamics, there is a tendency for numbers and quantities of biomass and energy to decrease along food chains; therefore, the pyramids become narrower towards the apex.
  14. Bioaccumulation is the build-up of persistent or non-biodegradable pollutants within an organism or trophic level because they cannot be broken down.
  15. Biomagnification is the increase in concentration of persistent or non- biodegradable pollutants along a food chain.
  16. Toxins such as DDT and mercury accumulate along food chains due to the decrease of biomass and energy.
  17. Pyramids of numbers can sometimes display different patterns; for example, when individuals at lower trophic levels are relatively large (inverted pyramids).
  18. A pyramid of biomass represents the standing stock or storage of each trophic level, measured in units such as grams of biomass per square metre (g m–2) or Joules per square metre (J m-2) (units of biomass or energy).
  19. Pyramids of biomass can show greater quantities at higher trophic levels because they represent the biomass present at a fixed point in time, although seasonal variations may be marked.
  20. Pyramids of productivity refer to the flow of energy through a trophic level, indicating the rate at which that stock/storage is being generated.
  21. Pyramids of productivity for entire ecosystems over a year always show a decrease along the food chain.

Applications and skills:

  • Construct models of feeding relationships – such as food chains, food webs and ecological pyramids – from given data.
  • Explain the transfer and transformation of energy as it flows through an ecosystem.
  • Analyse the efficiency of energy transfers through a system.
  • Construct system diagrams representing photosynthesis and respiration.
  • Explain the relevance of the laws of thermodynamics to the flow of energy through ecosystems.
  • Explain the impact of a persistent or non-biodegradable pollutant in an ecosystem.


  • Ecosystems such as lakes and forests can exist across political boundaries.

Theory of knowledge:

  • Feeding relationships can be represented by different models—how can we decide when one model is better than another?


  • Energy and equilibria (1.3)
  • Sustainability (1.4)
  • Climate change—causes and impacts (7.2)
  • Water pollutiion (4.4)
  • Terrestrial food production systems and food choices (5.2)
  • Biodiversity and conservation (topic 3)