16.3 Distribution and Abundance

A free GCSE Biology lesson for AQA (8461) from OBA School: quadrats, random sampling, mean, median and mode, estimating population size, and transects. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain why ecologists use samples and why they must be random
• Use quadrats to count organisms or estimate percentage cover
• Calculate the mean, median, mode and range of a set of counts
• Estimate the size of a population from quadrat results
• Use a transect to see how distribution changes across a habitat

Key words

• Quadrat: A square frame (often 0.5 m × 0.5 m or 1 m × 1 m) placed on the ground to sample organisms.
• Random sampling: Choosing sample positions by chance (e.g. random coordinates), so the results are not biased.
• Abundance: The number of organisms of a species in an area.
• Distribution: Where organisms are found in a habitat.
• Mean: Add all the values and divide by how many there are.
• Median: The middle value when the values are in order.
• Mode: The value that appears most often.
• Transect: A line across a habitat along which samples are taken at regular intervals.

Sampling with quadrats

It is usually impossible to count every plant in a field, so ecologists count organisms in samples. If the samples are typical of the whole area, they can be scaled up.

Quadrats must be placed randomly. If you choose where to put them, you may pick areas with lots of plants, and the results would be biased.

• Lay two tape measures at right angles along two edges of the area.
• Use a random number generator to pick two coordinates, e.g. (6, 3).
• Place the bottom-left corner of the quadrat at that point and count the organisms inside.
• Repeat for at least 10 quadrats. More quadrats give a more representative sample.

For plants that are hard to count, such as grass or moss, estimate the percentage cover: the percentage of the quadrat area covered by that species. A quadrat divided into 100 small squares makes this easier.

Mean, median, mode and range

Ecologists summarise their counts with an average. You need to be able to calculate all three averages.

Worked example: counts of plantain plants in 10 quadrats

Counts: 3, 7, 0, 5, 7, 2, 4, 7, 6, 9
Mean = (3 + 7 + 0 + 5 + 7 + 2 + 4 + 7 + 6 + 9) ÷ 10 = 50 ÷ 10 = 5
In order: 0, 2, 3, 4, 5, 6, 7, 7, 7, 9. Median = (5 + 6) ÷ 2 = 5.5 (two middle values)
Mode = 7 (appears three times). Range = 9 − 0 = 9

Answer: mean 5, median 5.5, mode 7, range 9

Key fact: Include quadrats with a count of 0 when you work out the mean. Leaving them out makes the mean too big.

Estimating population size

To scale up from your quadrats to the whole area:

estimated population = mean number per quadrat ÷ area of one quadrat (m²) × total area (m²)

Worked example: plantains in a field

The 10 quadrats above were each 0.5 m × 0.5 m = 0.25 m². The field is 40 m × 25 m.
Number per m² = 5 ÷ 0.25 = 20 plants per m².
Area of field = 40 × 25 = 1000 m².
Estimated population = 20 × 1000 = 20 000 plantains.

Answer: about 20 000 plantains

Exam tip: Check the quadrat area. A 50 cm quadrat is 0.5 m × 0.5 m = 0.25 m², not 0.5 m².

Transects

Random quadrats tell you how many. To find out how a species’ distribution changes across a habitat, for example from a wood into a field, or from the low-tide line up a beach, use a transect.

• Line transect: record every organism that touches the tape at regular points.
• Belt transect: place a quadrat at regular intervals along the tape (e.g. every 2 m) and count or estimate percentage cover.
• Measure abiotic factors (light intensity, soil moisture, pH, temperature) at each quadrat, so you can link the changes in distribution to a factor.

Common mistakes

• Placing quadrats where the plants are. This gives biased results; use random coordinates.
• Leaving out zeros when calculating the mean.
• Forgetting to put values in order before finding the median.
• Using 0.5 m² as the area of a 0.5 m × 0.5 m quadrat. It is 0.25 m².
• Using random quadrats when the question asks how distribution changes across a habitat – that needs a transect.

Revision questions

Try these, then check your answers below each one.

Question 1. Why should quadrats be placed using random coordinates? (Choose from: To avoid bias in the sample, So that more plants are counted, To save time, So every quadrat contains the species.)

Answer: To avoid bias in the sample

Working: Random placement means the person cannot pick “good” spots, so the sample is not biased and is more representative.

Question 2. A student counts clover plants in six quadrats: 2, 5, 3, 8, 4, 8. Calculate the mean.

Answer: 5

Working: (2 + 5 + 3 + 8 + 4 + 8) ÷ 6 = 30 ÷ 6 = 5.

Question 3. Counts of a moss in seven quadrats: 3, 9, 4, 1, 6, 4, 8. What is the median?

Answer: 4

Working: In order: 1, 3, 4, 4, 6, 8, 9. The middle (4th) value is 4.

Related lessons

• Previous lesson: 16.2 Organisms in Their Environment
• Next lesson: 16.4 Competition in Animals
• In this chapter: 16.1 The Importance of Communities
• In this chapter: 16.5 Competition in Plants
• In this chapter: 16.6 Adapt and Survive

Continue in the full course

This lesson is section 16.3 (AGCBH1603) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.