Learn how quadrats sample non-mobile organisms, then carry out a random quadrat sample yourself.
This is an important skill for the IA, especially if you want to investigate how plants or other non-mobile organisms change across different locations.
In exams, you may also be asked to comment on the abundance of a species and what this could mean for an ecosystem, describe a method for measuring population abundance, choose an appropriate sampling technique, or interpret quadrat data.
You therefore need to understand both how the sample is collected and how the data are calculated and interpreted.
A quadrat is a frame of a known size placed over part of a habitat. It may be divided into smaller grid squares to make measurements such as percentage cover and percentage frequency easier.
Decide what is being measured: abundance, population density, percentage frequency or percentage cover.
Choose a quadrat size suitable for the organisms.
Choose a sampling method appropriate to the investigation.
Take enough quadrat samples to make the results representative.
| 10 × 10 cm | 0.01 m² | Very small organisms, e.g. lichens or algae |
|---|---|---|
| 0.5 × 0.5 m | 0.25 m² | Grasses, herbs, small shrubs, mussels or limpets |
| 1 × 1 m | 1 m² | Medium-sized plants and larger herbs |
| 5 × 5 m | 25 m² | Larger vegetation such as mature trees |
Click each method to see how the sample positions are chosen and when the method is useful.
Add all counts and divide by the number of quadrats.
Divide abundance by the area sampled.
The share of sampling units that contain the species.
Estimate the proportion of quadrat area covered.
This 1 m² quadrat is divided into 25 equal squares. Seven individual clover plants occur across six occupied grid squares.
Example 1 m × 1 m quadrat
This is random sampling. The study area is divided into 30 numbered squares. A random number is generated to decide which square must be sampled, so the student does not choose the location themselves. This helps reduce selection bias.
The 10 × 10 grid gives 100 equal squares. Each grid square represents 1% of the quadrat area.
Save the sample, return to the meadow and generate another random coordinate.
A disease affects the daisy population in this meadow. Explain why a larger population of daisies, representing a larger biological storage within the system, could make the ecosystem more resilient to this disturbance.