2.5 Investigating ecosystems

Significant ideas:

  • The description and investigation of ecosystems allows for comparisons to be made between different ecosystems and for them to be monitored, modelled and evaluated over time, measuring both natural change and human impacts.
  • Ecosystems can be better understood through the investigation and quantification of their components.

Knowledge and understanding:

  1. The study of an ecosystem requires that it be named and located; for example, Deinikerwald in Baar, Switzerland—a mixed deciduous–coniferous managed woodland.
  2. Organisms in an ecosystem can be identified using a variety of tools including keys, comparison to herbarium or specimen collections, technologies and scientific expertise.
  3. Sampling strategies may be used to measure biotic and abiotic factors and their change in space, along an environmental gradient, over time, through succession, or before and after a human impact (for example, as part of an EIA).
  4. Measurements should be repeated to increase reliability of data. The number of repetitions required depends on the factor being measured.
  5. Methods for estimating the biomass and energy of trophic levels in a community include measurement of dry mass, controlled combustion and extrapolation from samples. Data from these methods can be used to construct ecological pyramids.
  6. Methods for estimating the abundance of non-motile organisms include the use of quadrats for making actual counts, measuring population density, percentage cover and percentage frequency.
  7. Direct and indirect methods for estimating the abundance of motile organisms can be described and evaluated. Direct methods include actual counts and sampling.
  8. Indirect methods include the use of capture–mark–recapture with the application of the Lincoln index.
    • n1 is the number caught in the first sample
    • n2 is the number caught in the second sample
    • nm is the number caught in the second sample that were marked
  9. Species richness is the number of species in a community and is a useful comparative measure.
  10. Species diversity is a function of the number of species and their relative abundance and can be compared using an index. There are many versions of diversity indices, but students are only expected to be able to apply and evaluate the result of the Simpson diversity index as shown below. Using this formula, the higher the result (D), the greater the species diversity. This indication of diversity is only useful when comparing two similar habitats, or the same habitat over time.
    • D is the Simpson diversity index
    • N is the total number of organisms of all species found
    • n is the number of individuals of a particular species

Applications and skills:

  • Design and carry out ecological investigations.
  • Construct simple identification keys for up to eight species.
  • Evaluate sampling strategies.


  • The use of internationally standardized methods of ecological study are necessary when making comparisons across international boundaries.

Theory of knowledge:

  • When is quantitative data superior to qualitative data in giving us knowledge about the world?
  • Controlled laboratory experiments are often seen as the hallmark of the scientific method, but are not possible in fieldwork—to what extent is the knowledge obtained by observational natural experiment less scientific than the manipulated laboratory experiment?


  • Sustainability (1.4)
  • Biodiversity and conservation (topic 3)