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9.1.6Ecosystems and Material Cycles

Sampling Organisms

5 min read 8 parts Lesson 7 of 15 Unit quiz

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Section 1 of 8Why we sample

It is usually impossible to count every organism in a large area such as a field. Instead, ecologists take samples from small parts of the area and use them to estimate the whole. For a sample to be reliable it must be representative, which means large enough and taken without bias.

Section 2 of 8Using a quadrat to estimate population size

Estimating population size using quadrats
Students using a gridded quadrat to survey a grassy area

A quadrat is a square frame, often 0.5 m by 0.5 m or 1 m by 1 m, placed on the ground to mark out a sample area. The method for estimating the population of a plant such as daisies in a field is:

1
Mark out the study area with two long tape measures at right angles to each other.
2
Use a random number generator to pick pairs of coordinates along the tapes.
3
Place the quadrat at each coordinate and count the number of individuals of the species inside it.
4
Repeat at least 10 times; the more samples, the more reliable the result.
5
Calculate the mean number per quadrat.

Random placement is essential to avoid bias, such as only choosing places where plants look plentiful. To estimate the total population, calculate the mean number per square metre and multiply by the total area:

Population estimate = mean number per m2 × total area in m2

You can also estimate percentage cover for plants that are hard to count individually, such as grass, by estimating the percentage of each quadrat covered by the species.

Section 3 of 8Using a transect to study distribution

Table of abiotic factors and how each is measured

To find out how the distribution of a species changes across a habitat, such as from a hedge into a field or from the shade of a tree into the open, a transect is used.

Using a transect to study distribution
A tape measure is laid in a straight line across the area, and quadrats are placed at regular intervals along it, for example every 2 m.
The species in each quadrat are recorded.
A line of quadrats like this reveals a gradient,
  • and the results can be compared with abiotic factors measured at the same points,
  • such as light intensity or soil moisture.

Section 4 of 8Reliability and animals

Animal Cell and Plant Cell
Reliability and animals
To make results reliable and valid: use random positions, take many samples, use the same size of quadrat each time,
  • and measure abiotic factors using the same method.
  • To compare two areas, such as a shaded and an open field, you must keep every other variable the same.
Quadrats suit plants and slow-moving animals.
  • Other methods are used for animalspitfall traps for ground invertebrates, sweep nets for insects in long grass,
  • and pooters for small insects.
  • Mobile animals can be sampled with a capture-mark-recapture technique, but this is more advanced than the GCSE requirement.
Data from sampling feed into the study of biodiversity and the effect of abiotic factors,
  • and are the starting point for judging human impact on ecosystems.

Section 5 of 8Worked example: interpreting results

Suppose a student places ten quadrats along a transect that begins at a hedge and ends 20 m into a field, and records the number of dandelions.

1
Near the hedge there are none, then the numbers rise steadily to a maximum at 10 m and level off.
2
The student also measures light intensity and finds it is low near the hedge and rises along the transect.
3
A reasonable conclusion is that dandelion abundance increases as light intensity increases,
  • which fits the idea that dandelions need high light for photosynthesis.
4
The student should add that this is only a correlation, since soil moisture or grazing may also change along the transect.

The standard way to improve such an investigation is to take more transects and average them, use more quadrats at each point, and measure the other abiotic factors. Calculating a mean reduces the effect of anomalies, and identifying an anomaly (for example a count that does not fit the pattern) and repeating it improves reliability.

Safety and ethics also appear in exam questions. Wear gloves when working with soil, avoid damaging the habitat by trampling it, return any animals to the place where they were collected and wash your hands after the fieldwork.

Section 6 of 8Exam-style questions

Pitfall trap and line transect sampling methods
Two students placing a gridded quadrat on grassland and recording

  1. A student counted daisies in ten 1 m2 quadrats placed randomly in a field of area 800 m2. The total number counted was 45. Calculate the estimated population of daisies in the field. [3]3 marks

  2. Describe how you would investigate the effect of light intensity on the distribution of moss from the base of a large tree outwards. [4]4 marks

Section 7 of 8Key terms

TermMeaning
QuadratA square frame used to mark a small sample area.
TransectA line along which samples are taken to show a gradient in distribution.
Random samplingChoosing sample positions by chance to avoid bias.
Percentage coverThe proportion of a quadrat covered by a species, as a percentage.
RepresentativeTypical of the whole area being studied.
MeanThe total of the values divided by the number of values.
Pitfall trapA container sunk in the ground to catch small ground-dwelling animals.

Section 8 of 8Exam technique

1
Placing quadrats by choosing the best spots. Use random coordinates to avoid bias.
2
Forgetting to multiply the mean per m2 by the total area, or using the total count instead of the mean.
3
Using a transect to estimate population size. Random quadrats estimate size; transects show a change in distribution.
4
Counting too few quadrats. A small sample is not reliable.
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