OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(ii) the use of sampling and recording methods to determine the…
Full specification wording
Ecosystems: (ii) the use of sampling and recording methods to determine the distribution and abundance of organisms in a variety of ecosystems.
Sampling estimates organisms without counting everything.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 6 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Sampling estimates organisms without counting everything. Distribution is where organisms occur; abundance is how many or how much. In a habitat, a quadrat placed randomly (for example using random coordinates) gives percentage frequency, density per m² or percentage cover. A transect records how distribution changes along a line, with quadrats at intervals giving a kite diagram or belt transect data. For mobile animals, mark-release-recapture estimates population size using the Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number of marked recaptured. Methods must be appropriate to the ecosystem: quadrats suit plants and slow-moving organisms on rocky shores or grassland; sweep nets and pitfall traps suit invertebrates; kick-sampling suits streams. Random sampling avoids bias, and sufficient samples improve reliability.
What earns the marks
- States that quadrats estimate abundance (frequency, density or percentage cover) and transects show distribution along a gradient.
- Explains that random placement of quadrats reduces bias, and that larger numbers of samples improve reliability.
- Applies the Lincoln index correctly: estimated population = (marked in first sample × total in second sample) ÷ marked recaptured.
- Selects a method suited to the ecosystem, for example quadrats for plants on a rocky shore, pitfall traps for ground invertebrates, or kick-sampling for stream organisms.
- Recognises that consistent recording (same quadrat size, same cover scale) allows valid comparison between ecosystems.
Common mistakes and how to put them right
Thinking a single quadrat gives the abundance of the whole habitat.
Put it right: Many randomly placed quadrats are needed to estimate the population.
Confusing distribution with abundance.
Put it right: Distribution is where organisms are found, abundance is how many or how much.
Using mark-release-recapture for plants.
Put it right: It suits mobile animals, while quadrats suit plants and sessile organisms.
Exam tips
- Read the ecosystem named in the question and choose a sampling method that physically suits it before calculating anything.
- When using the Lincoln index, keep the two samples distinct and substitute values carefully, checking the arithmetic.
- Link each method to whether it measures distribution (transect) or abundance (quadrat counts, cover estimates).
Try the lesson
The start of the Ecosystems: (ii) the use of sampling and recording methods to determine the distribution and abundance of organisms in a variety of ecosystems. lesson, made from checked teaching notes for this specification point.
Have a go first
A student places 10 quadrats, each 0.5 m², in a meadow and counts a total of 40 buttercups. The meadow has an area of 200 m². Estimate the total number of buttercups in the meadow.
Listen
Sampling estimates organisms without counting everything. Distribution is where organisms occur; abundance is how many or how much. In a habitat, a quadrat placed randomly (for example using random coordinates) gives percentage frequency, density per m² or percentage cover. A transect records how distribution changes along a line, with quadrats at intervals giving a kite diagram or belt transect data. For mobile animals, mark-release-recapture estimates population size using the Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number of marked recaptured. Methods must be appropriate to the ecosystem: quadrats suit plants and slow-moving organisms on rocky shores or grassland; sweep nets and pitfall traps suit invertebrates; kick-sampling suits streams. Random sampling avoids bias, and sufficient samples improve reliability. A strong answer earns marks for these points. States that quadrats estimate abundance (frequency, density or percentage cover) and transects show distribution along a gradient. Explains that random placement of quadrats reduces bias, and that larger numbers of samples improve reliability. Applies the Lincoln index correctly: estimated population = (marked in first sample × total in second sample) ÷ marked recaptured. Selects a method suited to the ecosystem, for example quadrats for plants on a rocky shore, pitfall traps for ground invertebrates, or kick-sampling for stream organisms. A common slip: Thinking a single quadrat gives the abundance of the whole habitat. Instead, many randomly placed quadrats are needed to estimate the population. Exam tip: Read the ecosystem named in the question and choose a sampling method that physically suits it before calculating anything.
See it
Key words
Say what each word means before you turn the card.
Random sampling to estimate abundance
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Transects to record distribution
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Mark-release-recapture for mobile animals
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Matching method to ecosystem and consistent recording
Try answering aloud or in your head before turning the card.
See every definition
- Random sampling to estimate abundance
- Placing a quadrat randomly, for example using random coordinates, gives percentage frequency, density per m² or percentage cover, and avoids bias.
- Transects to record distribution
- A line along which distribution changes is recorded, with quadrats at intervals giving a kite diagram or belt transect data.
- Mark-release-recapture for mobile animals
- An estimate of population size using the Lincoln index: estimated population = (number marked in first sample × total in second sample) ÷ number of marked recaptured.
- Matching method to ecosystem and consistent recording
- Quadrats suit plants and slow-moving organisms, sweep nets and pitfall traps suit invertebrates and kick-sampling suits streams, while consistent recording allows valid comparison between ecosystems.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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