echoesof.science · Ages 15–16
Sampling a habitat
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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.
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.
Percentage frequency = (number of quadrats containing the species ÷ total number of quadrats) × 100. It estimates the probability of finding the species in one quadrat.
Experiment: Quadrat survey: trampled versus untrampled grass
- 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.
| Species | Trampled: quadrats present (of 10) | Trampled: frequency % | Untrampled: quadrats present (of 10) | Untrampled: frequency % |
|---|---|---|---|---|
| Grass | ||||
| Daisy | ||||
| Dandelion | ||||
| Clover | ||||
| Plantain |
Questions
Clover was found in 6 of 15 quadrats. Calculate its percentage frequency.
Explain why percentage frequency results can only be compared if the same size of quadrat is used.
Plan an investigation to test whether trampling affects the plants on a school field. Include your hypothesis, how you will sample randomly, and two variables you cannot control.
Explain how the behaviour of woodlice helps them survive in their habitat.
Sources for every fact on this sheet: echoesof.science/school/habitats/sources/