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Witton Middle School



LO: Investigate population sampling


We know that it is important to maintain biodiversity but how can we measure this? One thing we can do is estimate the size of the population.


TASK ONE - plants

It is relatively easy to estimate plant numbers as they don’t move. You are going to estimate how many wildflowers are in your garden or a local green space.


Every May, Plantlife run a citizen science competition to estimate the number of wild flowers in our lawns and create a National Nectar Score so that we can support bees. Look at the results from last year’s survey. On a piece of paper or in a blank word document, write down:

  • 3 interesting facts
  • 2 questions you have about the results
  • 1 thing we can do to support bees


To estimate the number of wildflowers:

  • Estimate the area of your grass in m2 (for a rectangle area = width x length)
  • Gently throw a ball or pen over your shoulder to pick a random area
  • Mark out a 1m x 1m square around this mark
  • Create a tally chart and record the number of dandelion, daisy and clover flowers
  • For a large area, you may want to repeat this and then find an average
  • Scale it up. If there were that many flowers in 1m2, how many are on the whole lawn?


This video explains the process


Why is it a good idea to select your area randomly?

Would your results be an under- or an overestimate if you deliberately chose an area with lots of flowers?


TASK TWO – animals

It is harder to estimate the population of animals because they move. One method is to sample the population:

  1. capture a certain number, tag them and release them into the wild.
  2. capture another sample and see how many were in the original sample.
  3. estimate the population using the equation:

population estimate = size of sample 1 x size of sample 2 ÷ number recaptured


We are going to investigate how sample size affects the reliability of your estimate. You need a large collection of things. This could be: paper circles from your hole punch, pieces of gravel, Lego bricks.

  • Randomly choose 5 of your objects (size of sample 1 = 5)
  • Write 5 on each object and put them all back
  • Mix up your objects
  • Randomly choose 5 of your objects (size of sample 2 = 5)
  • Count how many have 5 written on them (= number recaptured)
  • Estimate the population using the equation above
  • Repeat the process but this time choose 10 objects each time and write 10.
  • Repeat three more times: use 15, 20 or 25 objects.


You could record your results like this:

size of sample 1 size of sample 2 number recaptured population estimate
5 5    
10 10    
15 15    
20 20    
25 25    


When you have finished, count how many objects there actually were.

Which of your estimates was the closest?

How does the sample size affect the reliability of the estimate?


EXTENSION (optional)

If you enjoyed this task, you could take part in these citizen science project to estimate the populations: