Investigating Population Size of an organism

A population is all the organisms of one species that live in the same place at the same time — for example, all the daisies on a school field.

Imagine trying to count every single daisy on a large field. It would take far too long, and you would lose track. Instead, ecologists take a sample: they count the organisms in a few small areas and use this to estimate the total for the whole area.

The tool for sampling plants (and slow-moving or non-moving organisms) is the quadrat. A quadrat is a square frame, often 0.5 m × 0.5 m, which marks off a known small area of ground:

Quadrat side	Area of quadrat
0.5 m × 0.5 m	0.25 m²
1 m × 1 m	1 m²

You place the quadrat down, count how many of your chosen organism are inside it, then move it and count again — building up several samples.

Exam tip. The word estimate matters. Because you only count a sample, your figure for the whole field is an estimate, not an exact count. Examiners expect you to say "estimate the population", not "count the population".

If you choose where to put the quadrat yourself, you might (without meaning to) drop it where there are lots of plants, or avoid muddy patches. That is bias — and it makes your sample unrepresentative, so your estimate is wrong.

To make sampling fair, quadrats must be placed randomly. A common method:

1. Lay two tape measures at right angles along the edges of the area to make a grid (like the x and y axes of a graph).
2. Use a random number generator (or numbers from a calculator) to choose a pair of coordinates.
3. Place the quadrat where those coordinates cross.
4. Count the organisms inside, then repeat with new random coordinates.

Random placement means every part of the area has an equal chance of being sampled, so no human choice can bias the result.

You should also take enough samples. One or two quadrats could land on an unusual patch by chance. Taking many quadrats and finding the mean makes the estimate more reliable (closer to the true value if repeated).

Exam tip. "Random" is a key marking word. If asked why quadrats are placed randomly, the answer is to avoid bias and make the sample representative/fair. Saying "to be fair" alone is weak — link it to bias and representative sampling.

Once you have counted the organisms in several quadrats, you turn the counts into an estimate for the whole area in two steps.

Step 1 — Find the mean number per quadrat.

$\text{mean per quadrat} = \frac{\text{total number counted in all quadrats}}{\text{number of quadrats}}$

Step 2 — Scale up to the whole area.

$\text{estimated population} = \text{mean per quadrat} \times \frac{\text{total area}}{\text{area of one quadrat}}$

The fraction $\dfrac{\text{total area}}{\text{area of one quadrat}}$ simply tells you how many quadrats would fit into the whole area.

Worked example. A student counts daisies in five 0.25 m² quadrats and gets 4, 6, 3, 5 and 2.

• Total counted = 4 + 6 + 3 + 5 + 2 = 20
• Mean per quadrat = 20 ÷ 5 = 4 daisies per quadrat
• The field is 200 m², and each quadrat is 0.25 m².
• Number of quadrats that fit = 200 ÷ 0.25 = 800
• Estimated population = 4 × 800 = 3200 daisies

To compare two different areas, you repeat the whole method in each area and compare the estimates (or the mean per quadrat if the areas are the same size). The area with the higher mean per quadrat has the larger population density.

Exam tip. Show your working clearly: total → mean → scale-up. Keep your units consistent — if the quadrat area is in m² then the total area must be in m² too. Calculation marks are often awarded for the method even if the final number is slightly off.

Random quadrats tell you how many organisms there are, but not where they are spread. Sometimes a species is not spread evenly — it changes across the area because a factor changes. For example, a plant might grow more in the light than in the shade, or more away from a trampled footpath.

To show this distribution along a gradient, you use a transect:

1. Stretch a tape measure in a straight line across the area, running along the gradient (e.g. from the shady edge of a wood out into open ground).
2. Place a quadrat at regular intervals along the line (e.g. every 2 m).
3. At each point, count the organism (or record the percentage cover if the plants are hard to count individually).
4. Plot the count against distance along the transect.

The graph then shows how the organism's numbers change as you move along the gradient — for instance, daisies increasing as you move from shade into bright light.

This is different from random sampling: a transect is placed deliberately along the gradient (not randomly), because the whole point is to study how the species changes with position.

Exam tip. Be ready to explain the difference: random quadrats estimate population size/density, while a transect shows distribution/how a species changes along a gradient. Naming the factor that causes the change (e.g. light intensity, trampling) often earns an extra mark.

A good practical investigation is planned and evaluated, not just done.

Variables. When you compare the population of an organism in two areas, the thing you are testing is the area (the independent variable) and the thing you measure is the number of organisms (the dependent variable). To make it a fair comparison, you should keep the method the same in both areas:

• same size of quadrat,
• same number of quadrats,
• same way of placing them randomly,
• counting at the same time of day / season where possible.

Fair sampling. Sampling is fair when quadrats are placed randomly (no bias) and enough samples are taken so the mean is reliable. Too few quadrats, and one unusual patch can distort the estimate.

Results. Record counts in a clear table, calculate the mean per quadrat for each area, and present comparisons in a bar chart (or a line graph for a transect).

Evaluation. When you evaluate, you judge how good the investigation was and suggest improvements:

• Did you take enough quadrats? Taking more samples makes the estimate more reliable.
• Was placement truly random, or could bias have crept in?
• Are there anomalous results (odd values) that should be checked or repeated?
• Some organisms are hard to count (they overlap) — using percentage cover can help.
• Quadrats are not suitable for fast-moving animals, which would move in or out as you count.

Exam tip. Evaluation marks reward specific, sensible improvements linked to reliability — e.g. "take more quadrats and recalculate the mean". Vague answers like "be more careful" score nothing.