Sampling for Abundance and Distribution

Section 1 of 7Why sample?

Abundance

• Abundance Abundance means how many individuals of a species, or how many different species, are present in a habitat.

Distribution

• Distribution Distribution describes where in the habitat a species is found and how that changes from place to place.

Estimating the whole

• Estimating the whole In a large habitat you cannot find and count every organism,
  • so you count small sample areas and scale the result up to estimate the total.

Section 2 of 7Making a sample representative

Reliable sampling

Random positions

• Random positions Sample sites must be chosen at random, otherwise a student may pick easy or attractive spots and introduce bias.

Enough samples

• Enough samples The more samples taken, the better they represent the whole habitat and the more reliable the estimate.

Suitable quadrat size

• Suitable quadrat size A 1 m² quadrat suits small plants or limpets, whereas a much larger marked-out area is needed for trees.

Section 3 of 7Random quadrat sampling

1

Lay two long tape measures along the edges of the study area at right angles, like the axes of a graph.

2

Use a random number generator to pick two numbers, and use them as coordinates along the tapes.

3

Place the quadrat where the coordinates meet.

4

Record the abundance, for example the number of species, the number of individuals of one species, or the percentage cover of species such as grass.

5

Repeat with new random coordinates many times and calculate a mean per quadrat.

Section 4 of 7Transects for distribution

What a transect is

• What a transect is A transect is a line laid across the habitat, along which quadrats are placed at regular intervals.

When to use one

• When to use one Use a transect when conditions change gradually, for example moving away from a hedge, a path or the edge of a pond, or up a seashore.

Interpreting results

• Interpreting results Plotting abundance against distance along the line shows how a factor such as light, moisture or trampling affects where species live.

Section 5 of 7Choosing the method

Question	Method	Reason
How many daisies are in the whole field?	Random quadrats	Avoids bias and covers the whole area
How do plants change away from a woodland edge?	Transect with quadrats	Samples a gradient in a fixed order
Species that are hard to count as individuals	Percentage cover	Grass cannot be counted plant by plant

Section 6 of 7Key terms

Term	Meaning
Sampling	Collecting data from small parts of a habitat to represent the whole habitat.
Quadrat	A square frame of known area placed on the ground to sample organisms.
Transect	A line across a habitat along which samples are taken.
Bias	A systematic error that makes results unrepresentative, such as choosing sample sites by eye.
Percentage cover	The proportion of a quadrat covered by a species, used when individuals cannot be counted.

Section 7 of 7Exam technique

1

Throwing a quadrat over the shoulder is not truly random; use random number coordinates instead.

2

Taking too few samples and then treating the result as representative of the whole habitat.

3

Using a transect when the question is about total numbers across the habitat; random quadrats are better for that.

4

Counting only the individuals and forgetting that grass and similar plants need percentage cover.