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7.2.2Ecology

Required Practical: Ecosystems

5 min read 9 parts Lesson 7 of 14 Unit quiz

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Section 1 of 9The aim

This required practical measures the population size of a common species in a habitat and investigates how the distribution of organisms is affected by an environmental factor. Because it is impossible to count every organism in a large area, you take samples and scale up.

Section 2 of 9Estimating population size with random quadrats

Estimating population size using quadrats
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Mark out the study area, for example a field, with two long tape measures at right angles.
Random numbers
Use random numbers from a calculator or table to generate coordinates, and place a 0.25 m2 quadrat at each. This avoids bias.
Count the number of individuals of the species inside each quadrat.
Take at least ten samples, because larger samples reduce the effect of chance.
Calculate the mean number per quadrat.
Estimate the population: mean per m2 × total area of the habitat.

For example

  • mean of 6 daisies per 0.25 m2 = 24 per m2.
  • In an area of 500 m2, the estimate is 24 × 500 = 12 000 daisies.

For plants that are difficult to count, such as grass, estimate the percentage cover of each quadrat.

Section 3 of 9Investigating distribution with a transect

Investigating the Effect of Light Distance on Photosynthesis — Full 2D Simulation
Line transect and belt transect sampling in a habitat

Where an environmental factor changes gradually, such as light moving from a hedge into an open field, use a belt or line transect.

1
Lay a tape measure in a straight line across the change.
2
Place a quadrat at regular intervals along the line, for example every 2 m.
3
At each point count the organisms (or percentage cover) and measure the abiotic factor,
  • such as light intensity with a light meter, soil moisture with a probe,
  • or soil pH.
4
Repeat the transect in other places, and calculate means.

Plot the abundance against distance, or against the value of the factor, and describe the pattern.

Section 4 of 9Reliability and sources of error

1
Random placement avoids deliberately choosing areas with more or fewer organisms (bias).
2
The quadrat must be the same size each time and laid flat.
3
Take enough samples; the more samples, the more reliable the mean.
4
Be consistent in counting plants that lie on the edge, such as counting only those touching two sides.
5
Conditions such as the weather and time of day should be noted since they may affect abiotic readings.
6
Correlation does not prove cause: other variables may differ along the transect.

Section 5 of 9Safety and ethics

Take care in the field

  • wear suitable clothing, avoid contact with poisonous plants, wash hands, and do not damage habitats.
  • Leave the area as you found it and avoid collecting organisms unnecessarily.

Section 6 of 9Worked analysis of field data

Pitfall trap and line transect sampling methods
Animal Cell and Plant Cell
Animal Cell and Plant Cell

This section shows how results from a quadrat investigation are processed and evaluated.

1
Suppose a student sets out a 20 m transect running from the edge of a hedge into the middle of a lawn, and places a 0.25 m2 quadrat every 4 m.
2
The number of plantain plants in each quadrat is 2, 3, 6, 9, 11 and 12, and the light intensity increases from 150 to 900 lux over the same distance.
3
A line graph of abundance against distance shows a positive relationship between light and the number of plants.
4
A suitable conclusion is that plantain is more abundant in brighter positions,
  • which suggests that light is a limiting factor,
  • but other things such as soil moisture might also change from the hedge to the lawn,
  • so it is not proven.

To calculate a mean density, divide the total number of organisms by the total area sampled.

Worked analysis of field data
For the data above the total is 43 plants in 6 quadrats, which is 1.5 m2, so the density is 43 ÷ 1.5 = 28.7 plants per m2.
If the whole lawn is 200 m2,
  • then the estimated population is about 5700,
  • but this assumes the plants are evenly spread,
  • which is not the case on a transect with a gradient,
  • so a random sample would be a better basis for estimating the population.

The choice of quadrat size depends on the organisms. A small quadrat is suitable for small, dense plants, whereas a larger one is needed for sparse plants such as trees. A pilot study can show how many quadrats are required: when the mean stops changing as more quadrats are added, the sample is large enough.

For animals that move, such as beetles, quadrats are not suitable and capture methods such as pitfall traps are used, but at GCSE the emphasis is on quadrats and transects for plants and slow-moving animals.

Section 7 of 9Exam-style questions

Two students placing a gridded quadrat on grassland and recording

  1. Describe how you would estimate the number of dandelions in a field of area 800 m2 using 1 m2 quadrats. [5]5 marks

  2. In ten 0.25 m2 quadrats a student counted a total of 45 buttercups. Estimate the number in a 600 m2 field. [3]3 marks

  3. Explain why a transect is better than random sampling for investigating the effect of shade on moss growth. [2]2 marks

Section 8 of 9Key terms

TermMeaning
QuadratA square frame of known area used for sampling.
TransectA line along which samples are taken to show change in distribution.
Random samplingChoosing sampling positions by chance to avoid bias.
Percentage coverThe proportion of the quadrat area covered by a species.
Population sizeThe total number of individuals of one species in an area.
Abiotic factorA non-living part of the environment.

Section 9 of 9Exam technique

1
Placing quadrats where the plants look dense; this introduces bias, so use random numbers.
2
Forgetting to convert the quadrat area to m2 before scaling up.
3
Using too few quadrats and expecting reliable results.
4
Saying a transect gives the population size; it shows how distribution changes with a factor.
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