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Sampling a habitat
How ecologists estimate what lives in a habitat: random quadrat sampling, percentage frequency and cover, sample size and bias. With a field investigation, calculations and a look at animal behaviour and habitat loss.
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A habitat must meet all the environmental conditions an organism needs. Because we cannot count every plant in a field, ecologists use random sampling to estimate the populations of different species in an area.[2][6]
A frame quadrat is a square frame (for example 0.25 m × 0.25 m or 0.5 m × 0.5 m) that outlines a sample area. To place it randomly, lay two tapes at right angles and use random numbers as coordinates; place it, rather than throw it.[7][6]
Percentage frequency = (number of quadrats containing the species ÷ total number of quadrats) × 100. It estimates the probability of finding the species in one quadrat.[7]
Percentage cover estimates how much of the quadrat area each species occupies. It is faster but more subjective: it tends to overestimate flowering plants and underestimate low-growing ones. Ten quadrats is the absolute minimum for each area; more reduce the effect of anomalies.[7]
Behaviour also decides where animals live. Woodlice lose water quickly and their breathing system stops working if they dry out. They move more in dry conditions and slow down in damp ones, a reflex response that makes them collect in damp places; experiments suggest damp matters more to them than darkness.[5]
Habitat loss is probably the greatest threat to the variety of life today: WWF reports that it is a main threat to 85% of the threatened species on the IUCN Red List. Polar bears, for example, depend on sea ice to hunt seals, and disappearing sea ice threatens them.[8][4]
Try it: Quadrat survey: trampled versus untrampled grass
You need
- Frame quadrats (0.5 m × 0.5 m, ideally divided into 100 squares with string)
- Two long tape measures
- Random number tables or a random-number app
- Simple plant identification key for playing fields
- Recording sheet and pencil
- Hand-washing facilities or sanitiser
Safety
Teacher checks the site for hazards (broken glass, holes, dog waste) and does a risk assessment. Cover cuts, do not eat outdoors, wash hands after handling plants and soil, use sun protection, and be aware of stings and allergies. Place quadrats; never throw them.
What to do
- Choose two areas of the school field: one near a path or goal (trampled) and one away from paths (untrampled). Write a hypothesis.
- In each area lay two tapes at right angles along two edges.
- Use two random numbers to find the coordinates of each quadrat. Place the quadrat there (do not throw it).
- Record which species are present in each quadrat (presence/absence). Optionally count in how many of the 100 small squares each species appears.
- Take at least 10 quadrats in each area.
- Calculate the percentage frequency of each species in each area and draw a bar chart. Discuss the reliability of your results and the sources of bias.
What happens, and why?
Random coordinates avoid choosing 'interesting' spots, so the sample is not biased. Percentage frequency = quadrats with the species ÷ total × 100. With at least 10 quadrats per area, one unusual quadrat has less effect on the result. A difference between the areas may be linked to trampling, but other factors (light, soil moisture) could also differ, so a correlation alone does not prove the cause.[6][7]
Play: Good sampling or bias?
Is this good sampling practice, or does it make the results biased or unreliable?
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Quiz
Other ages: Ages 6–7 · Ages 10–11