5.4B: Distribution and biodiversity using quadrats

LEARNING GOALS

By the end of this lesson

Use a transect with quadrats to investigate distribution.
Record species presence, abundance and abiotic factors to compare biodiversity.
Interpret distribution data and evaluate the conclusions.

BIG QUESTION

Why do different plants grow at different distances from a path, a hedge or the shore?

A belt transect links changes in species to changes in an abiotic factor, but correlation needs careful interpretation.

Choose a transect for a gradient

A is a line across a habitat where conditions change, such as from a tree into open grass or from the edge of a path into a field. Placing quadrats at regular intervals along it shows how the of species changes.

Systematic sampling is appropriate here because the aim is to follow a gradient. Choose the transect position at random within the area to avoid bias.

Method

Lay a tape measure across the gradient. Place a quadrat (for example 0.5 m × 0.5 m) at fixed intervals, such as every 2 m. In each quadrat, identify every species present and record number or of each.

At each quadrat, measure abiotic factors such as light intensity (light meter), soil moisture and soil pH, and temperature. Repeat with two more parallel transects to check that the pattern is consistent.

The is distance along the transect and the dependent variable is the abundance of each species. Keep quadrat size, sampling interval, time of day and the criteria used to estimate cover the same (control variables).

Measuring biodiversity with quadrats

Record the number of species per quadrat (species richness) and the abundance of each to judge evenness. Compare the mean number of species per quadrat in different zones or areas.

Use a key or identification guide, and record unknown species as “species A” consistently rather than guessing names.

Data handling and interpretation

Plot abundance of each species against distance along the transect, with the abiotic factor on the same distance axis. Kite diagrams or bar charts make zones visible.

Illustrative pattern: moss cover falls from 60% to 5% as light intensity rises away from a tree. This is consistent with light limiting competitors, but moisture or trampling may change at the same time. A correlation does not prove one factor is the cause.

Safety and limitations

Assess the site for hazards such as water, tides, slopes, sharp objects or stinging plants; work in groups, wash hands and return organisms undisturbed.

Limitations include one visit in one season, misidentification, subjective percentage-cover estimates, small numbers of transects and several abiotic factors varying together.

Graph skills: SLAPUK and choosing a scaleTwo data sets on one graph need a key (SLAPUK).

Random sampling versus transects

Choose the method that fits the question.

Quadrat sampling strategies
Feature	Random quadrats	Belt transect
Best for	Estimating population size or comparing two areas.	Showing change in distribution along a gradient.
Quadrat placement	Random coordinates.	Regular intervals along a tape.
Abiotic data	Optional.	Measured at every quadrat.
Main risk	Missing patterns across a gradient.	Bias if the transect line is chosen subjectively.
Output	Mean per quadrat and population estimate.	Zones of species linked to changing conditions.

KEY TERMS

Language that earns precision

distribution: Where organisms are found within a habitat.
transect: A line across a habitat along which samples are taken.
systematic sampling: Taking samples at regular intervals.
correlation: A relationship in which two variables change together.

Common misconceptions

Claim: If a plant is more common where light is brighter, light causes the change.

Correction: This is a correlation. Other factors may vary along the transect, so causation needs further testing.

Claim: One transect describes the whole habitat.

Correction: Repeat transects are needed to show that the pattern is consistent.

Claim: Percentage cover always adds up to 100%.

Correction: Plants overlap in layers and bare ground exists, so totals may be above or below 100%.

EXAM LANGUAGE

Match the answer to the command word

Describe

Name the tape, interval, quadrat size, what is recorded and which abiotic factor is measured at each point.

Evaluate

Separate correlation from cause and suggest controlled follow-up work or more transects.

CHECK YOUR UNDERSTANDING

Recall, apply, then justify

01Describe4 marks

Describe how a student could investigate the distribution of plant species from the base of a tree into an open field.

Hint

Tape, interval, quadrat, record species and a factor.

Suggested marking points

Lay a tape measure (transect) from the tree into the field.
Place quadrats at regular intervals along the tape.
Identify and count (or estimate percentage cover of) each species in each quadrat.
Measure an abiotic factor such as light intensity at each quadrat.

02Explain2 marks

Moss cover decreased as light intensity increased along a transect (illustrative data). Explain why the student cannot conclude that light intensity alone caused the change.

Hint

Correlation and other variables.

Suggested marking points

The data show a correlation, not causation.
Other factors, such as soil moisture or trampling, may also change along the transect.

STRETCH AND CHALLENGE

Design an improvement to the tree-to-field investigation that would give stronger evidence that light intensity affects moss distribution.

Planning hint

Replication, measuring other factors and a controlled manipulation.

What a strong design includes

Use several transects at randomly chosen positions.
Measure other abiotic factors, such as soil moisture and pH, at each quadrat.
Sample on several days or seasons.
Compare places with similar moisture but different light.
Use shading over marked plots in the open field as a controlled field experiment.
Keep unshaded plots nearby as a control group.
Compare moss growth over weeks between shaded and unshaded plots.

END OF THE LESSON

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