Teacher Resource 4: Capture-Recapture

Teacher Resource 4

Capture-recapture

Once you have decided on a method to catch the organism you want to investigate, the capture-recapture method is used to estimate the population size of more mobile organisms.

This activity models the process using counters/beads as the 'population'.

Each group of 2 or 3 learners will need one bag (any material but it should not be transparent - learners mustn't be able to see the beads/counters when picking them out), between 120-160 counters/beads and either a marker pen (that can be removed from the counters/beads afterwards) or small stickers.

Instructions

Learners work in groups of 2 or 3 and receive a bag of counters/beads and a marker pen/stickers.

Each counter/bead represents one organism and the bag represents the habitat. Learners use the capture-recapture method to sample the population and estimate the population size:

Each group removes 10 counters from their bag and marks each one with a sticker/pen.
The marked counters are returned to the bag.
The bag is shaken and 20 counters are taken out of the bag at random.
Learners record the number of marked and unmarked counters.
Return the counters to the bag and repeat stages 3 and 4 at least 3 more times.
Calculate the average number of marked and unmarked counters.

Introduce the learners to the calculation needed to take their data and estimate the total population size:

Learners can then count the actual number of counters/beads in their bag and discuss how close their estimates were along with reasons why they may not have been accurate and what issues may arise when carrying out this method with 'live' organisms.

Discuss any variables learners may want to control if using live organism e.g. collecting organisms for the same amount of time, same time of day, similar weather etc. and what assumptions we make when carrying out this method of sampling e.g. no immigration to or emigration from the population, the marks don't affect the organism's chance of survival, the marks don't come off between sample collections.

On the classroom board write the question:

A learner collected woodlice for 30 minutes in a woodland area next to her school. 24 woodlice were caught and a mark painted on to their upper surface before being released back into their habitat. At the same time the next day 30 woodlice were captured in 30 minutes, 6 of these had a mark on them. Estimate the size of the woodlouse population in this habitat.

As homework, you may ask learners to plan an investigation into the population size of two organisms in two areas such as millipedes in leaf litter ina woodland compared to under a bush on the edge of a field. Learners must include step-by-step instructions on how to catch the organisms, factors they must control and how to estimate the population size using the techniques they have learnt so far.