Lesson focus: An ecosystemAll the living things in an area together with their surroundings, such as the soil, water and air. includes interacting organisms and the abiotic environment, so representative sampling is needed to estimate patterns without counting everything.
Use quadratA square frame placed on the ground to count the living things inside it. and transectA line across an area along which you take samples, to see how living things change from place to place. to estimate abundanceHow many of a living thing there are in an area. and distribution, selecting sampling methods that match the ecological question.
Build on earlier learning
Disruption to food chains and websThe previous lesson predicted ecosystem change. Sampling provides evidence for testing whether those predicted changes occur.
Practical
Sampling with quadrats
- Use random coordinates to place a quadrat.
- Count the plants of one species inside it.
- Repeat for at least ten quadrats and calculate the mean per m².
- Multiply by the area of the field to estimate the population.
Record: Counts per quadrat, the mean and the estimate.
You will be successful when you can…
- Distinguish habitat, community and ecosystem.
- Choose random or systematic sampling appropriately.
- Calculate density and population estimates.
- Evaluate sampling uncertainty and bias.
Words that carry the science
- ecosystem
- A community of organisms interacting with each other and with the abiotic environment.
- habitat
- The place and conditions in which an organism lives.
- community
- All the populations of different species living and interacting in an area.
- quadrat
- A frame defining a known area for ecological sampling.
- transect
- A line or belt along which organisms or conditions are sampled.
- abundance
- How much of an organism is present, measured as count, density, frequency or cover.
Match the sampling design to the ecological question
01Ecological levels describe connected systems
A populationAll the living things of one kind (species) that live in one area. contains members of one species in an area. Different populations form a communityAll the different living things that live together in one area.. The communityAll the different living things that live together in one area. plus abiotic conditions and their interactions form an ecosystemAll the living things in an area together with their surroundings, such as the soil, water and air..
Boundaries are chosen for a question: a rotting log, pond and forest can each be treated as an ecosystemAll the living things in an area together with their surroundings, such as the soil, water and air. at a different scale.
02Random sampling estimates a wider area
Mark a study area with axes, generate random coordinates, place a quadratA square frame placed on the ground to count the living things inside it. at each coordinate and record a consistent abundanceHow many of a living thing there are in an area. measure. Random positioning reduces conscious placement bias.
Mean density can be multiplied by total habitatThe place where a living thing lives, such as a pond or a forest. area to estimate population size when the sampled patches are representative. Increasing sample number generally improves precision but does not remove systematic bias.
03Transects test change along a gradient
A line transectA line across an area along which you take samples, to see how living things change from place to place. records organisms touching a line; a belt transect uses contiguous or spaced quadrats along it. Measure an abiotic variable such as light, soil moisture or distance at matching positions.
Systematic sampling is appropriate for a gradient but should not be presented as a random estimate of the entire habitat. Correlation along a transect does not prove that the measured factor caused the distribution.
Estimating a population with quadrats
Count the daisies in 1 m² quadrats placed at random, find the mean per square metre, then multiply by the area of the field.
Controls
The field is illustrative. More quadrats give an estimate closer to the real number.
Estimate a daisy population
Ten 0.25 m² quadrats contain a total of 70 daisies in a 120 m² field.
- Total sampled area = 10 × 0.25 = 2.5 m².
- Mean density = 70 ÷ 2.5 = 28 daisies m⁻².
- Estimated population = 28 × 120.
- The estimate is 3360 daisies, with sampling uncertainty.
How to make the reasoning visible: Show sampled area before scaling and label the result as an estimate, not an exact count.
Compare two sampling designs
Choose the design that best answers each question.
| Question | Design | Measurement | Main limitation |
|---|---|---|---|
| Mean clover density in a field | Random quadrats | Count per known area | Patchiness |
| Change in lichens away from a road | Belt transect | Cover or frequency with distance | Other gradients may covary |
| Tree distribution across a reserve | Stratified random plots | Count or basal area by habitat | Requires correct habitat map |
| Mobile beetle abundance | Pitfall traps plus standard effort | Catch per trap-time | Activity affects catch |
| Grass cover | Point or percentage-cover quadrats | Percentage cover | Observer judgement |
Why may percentage cover be better than counting grass plants?
Why should random coordinates be generated before entering the field?
Check your understanding
Answer from memory, then check against the suggested answer.
What does a quadrat define?
When is a transect useful?
Why use random coordinates?
A student places quadrats only beside a path because access is easy. Explain the problem.
Explain why many biased samples can give a precise but inaccurate estimate.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student plans to estimate the number of dandelions in a field using three quadrats placed near the school gate.
Evaluate the plan. Identify two weaknesses.
(2)Describe an improved method.
(4)Total for this question: 6 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., identify how each design choice affects representativeness and give a practical correction.