In 1898 two young botanists in Nebraska, Roscoe Pound and Frederic Clements, published a way to answer a deceptively simple question: how many of each plant are there on a prairie? Looking wasn't enough: after years on the prairies, they found the showiest flowers were often not the most common. So they marked out plots five metres square, counted every plant of each of the less dominant (secondary) species inside, and averaged the counts from many plots. They are usually credited with bringing the quadrat into ecology. Clements went on to become a leading American plant ecologist; Pound became better known as a legal scholar. Their idea is the one you use on a school field.
This lesson covers required practical 9 for AQA GCSE Biology Topic 7 (Ecology): measure the population size of a common species in a habitat, and use sampling techniques to investigate the effect of a factor on the distribution of this species. It's also useful for Combined Science.
Part 1: random quadrats and population size
AQA's handbook suggests a quadrat of 25 cm × 25 cm or 50 cm × 50 cm.
- Lay two tape measures at right angles to mark two sides of the survey area, for example a 20 m × 20 m square.
- Pick two random numbers, such as from two bags of numbers. Use them as coordinates along the two tapes.
- Place the quadrat at that point and count the chosen species inside it, such as daisies or plantains.
- Repeat until you have counted ten or more quadrats.
Random placement matters. If you choose where to put the quadrat, you might pick the patches that look interesting, and your sample would be biased.
Estimating the population
Estimated population size = (total area ÷ area sampled) × number counted.
Worked example: ten 0.5 m × 0.5 m quadrats each cover 0.25 m², so 2.5 m² is sampled. A 20 m × 20 m field is 400 m². If you count 30 buttercups in total, the estimate is 400 ÷ 2.5 × 30 = 4800. Check your units: both areas must be in m².
You may also be asked for the mean, median or mode of the quadrat counts. The mean is the total divided by the number of quadrats; the median is the middle value in order; the mode is the most common value.
Part 2: a transect and a factor
To see how distribution changes, lay a 30 m tape in a straight line, for example from the base of a tree out to open grass. Place the quadrat with one corner at 0 m, count the plants and measure the light intensity with a light meter. Then move the quadrat 5 m along the line and repeat, all the way to the end.
- Independent variable: distance along the transect, and with it light intensity.
- Dependent variable: number of plants, or percentage cover for plants like grass that are hard to count one by one.
- Abiotic factors you could measure instead include temperature, moisture levels and soil pH.
Plot the number of plants against light intensity and describe the pattern. A pattern is a correlation. Soil moisture and other factors may also change under a tree, so be careful about saying light alone is the cause.
Safety
Check the area for hazards beforehand, avoid very wet grass, lock tape measures so they don't recoil, place quadrats carefully rather than throwing them, and wash your hands if you touch soil.
Common exam mistakes
- Forgetting to convert quadrat size to m². A 50 cm quadrat is 0.25 m², not 0.5 m².
- Saying random sampling makes results "accurate". Say it avoids bias and gives a representative sample.
- Mixing up the two methods. Random quadrats estimate population size; a transect shows how distribution changes.
- Claiming cause from a correlation. Name another factor that could also change along the transect.
- No counting rule. Decide before you start how to count plants on the edge of the quadrat.
Frequently asked questions
How do you calculate population size from quadrats?
Divide the total area by the area you sampled, then multiply by the number of organisms you counted in all your quadrats.
What is the difference between a quadrat and a transect?
A quadrat is the square frame you count inside. A transect is a line across a habitat along which you place quadrats at regular intervals.
Why are quadrats placed randomly?
To avoid bias. Random placement means the sample is more likely to represent the whole area.
How many quadrats should you use?
AQA's student sheet uses ten. More quadrats give a more representative mean.
Try it
Say each answer before you flip the card. Then put yourself in Pound and Clements's place on the Nebraska prairie, with a frame and a notebook. What would you have tested next?
Braineos is not affiliated with or endorsed by AQA.