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How are ecosystems organised, and how do we estimate the number of organisms in an area?

Describe the levels of organisation in an ecosystem, how communities are affected by abiotic and biotic factors, interdependence including parasitism and mutualism, and sampling with quadrats and transects.

A focused answer to Edexcel GCSE Biology 9.1 to 9.6, covering the levels of organisation in an ecosystem, abiotic and biotic factors, interdependence, parasitism and mutualism, and estimating population size with quadrats and transects.

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  1. What this dot point is asking
  2. Levels of organisation
  3. Abiotic and biotic factors
  4. Interdependence
  5. Sampling with quadrats and transects
  6. Try this

What this dot point is asking

Edexcel statements 9.1 to 9.6 want you to describe the levels of organisation in an ecosystem, explain how communities are affected by abiotic (non-living) and biotic (living) factors, describe interdependence including parasitism and mutualism, and use quadrats and belt transects to estimate the number of organisms (statement 9.5 is the core practical).

Levels of organisation

Abiotic and biotic factors

What lives where in a community depends on two kinds of factor:

Interdependence

In a community, species depend on one another for food, shelter, pollination and seed dispersal, so a change in one population affects others. This is interdependence. Two special relationships:

  • Parasitism: one organism (the parasite) benefits while the other (the host) is harmed. Examples: fleas and tapeworms, which take nutrients from their host.
  • Mutualism: both organisms benefit. Examples: nitrogen-fixing bacteria in the root nodules of legumes (the bacteria gain sugars, the plant gains nitrogen compounds), and cleaner fish that remove parasites from larger fish (the cleaner gets food, the larger fish gets cleaned).

Sampling with quadrats and transects

To estimate the number of organisms without counting every one, ecologists sample an area:

  • A quadrat is a square frame of known area. Placing quadrats at random positions and counting the organisms inside gives a mean, which is scaled up to the whole area. Random placement avoids bias.
  • A belt transect is a line across an area, with quadrats placed at intervals along it. It is used to study how the distribution of organisms changes across a gradient, for example from the shade of a wood into open ground.

To make sampling reliable, take many samples, place quadrats randomly (for population size) or systematically (for a transect), and calculate a mean.

Try this

Q1. Define a "community" in an ecosystem. [1 mark]

  • Cue. All the populations of different species living and interacting in the same area.

Q2. State whether you would use a quadrat or a belt transect to study how plant distribution changes from a path into a meadow, and why. [2 marks]

  • Cue. A belt transect, because it samples along a line to show how distribution changes across the gradient.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20194 marksA student uses a 0.25 m squared quadrat and counts daisies in 10 places in a field. They find a mean of 6 daisies per quadrat. The field is 800 m squared. Estimate the total number of daisies in the field. Show your working.
Show worked answer →

A 4-mark calculation rewards scaling the mean up to the whole field correctly.

First find the mean number per square metre: each quadrat is 0.25 m20.25\ m^{2}, so 66 daisies per 0.25 m20.25\ m^{2} is 6÷0.25=246 \div 0.25 = 24 daisies per m2m^{2}.

Then multiply by the field area: 24×800=19 20024 \times 800 = 19\,200 daisies.

Markers reward converting the quadrat count to a per-square-metre figure (or scaling by the ratio of areas) and multiplying by the total area to get about 19 20019\,200. Forgetting that the quadrat is 0.25 m20.25\ m^{2}, not 1 m21\ m^{2}, is the usual error.

Edexcel 20213 marksExplain the difference between parasitism and mutualism, giving an example of each.
Show worked answer →

A 3-mark explain question rewards a clear contrast plus correct examples.

In parasitism, one organism (the parasite) benefits while the other (the host) is harmed, for example fleas or tapeworms living on or in a host and taking its nutrients.

In mutualism, both organisms benefit, for example nitrogen-fixing bacteria in the root nodules of legumes (the bacteria get sugars and the plant gets nitrogen compounds), or cleaner fish and the larger fish they clean.

Markers reward one harmed and one benefiting for parasitism, both benefiting for mutualism, and a valid example of each. Saying both are harmed, or muddling the examples, loses marks.

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