S4.3.6 For the adult
Measuring populations
How can you find how many daisies are in a field, or snails in a garden, without counting every one?
What this chapter teaches
A population is estimated from a sample. Quadrats placed at random give a mean for each square metre, which is scaled up to the area. A transect shows change along a line. Capture and recapture estimates animals that move: marked in the first sample times the second sample, divided by the marked in the second sample.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why must the quadrats be placed at random?
- Why can one quadrat give a poor estimate?
- What would go wrong with capture and recapture if the marks came off?
After the chapter, the learner can
- Estimate a population using quadrats and a transect
- Use capture and recapture to estimate a number of snails or insects
What they copy into their notebook
Count a random sample and scale up. Mean per quadrat, divided by its area, times the field area. A transect shows change along a line. Capture and recapture: marked first times second sample, divided by marked in the second.