A sample of 39 ground beetles was captured from an area of waste ground measuring 100 × 25 metres. Each animal was marked and then released. A second sample of 35 was caught on the following day. Of these, 20 had been marked.

a. Use these results to estimate the number of ground beetles in the population. Show your working.

b. State three assumptions that must be made in order to make this estimate.

c. Describe a method that you could use to check that the mark–release–recapture method gives a valid estimate of the ground beetle population in the area of waste ground.