Barrel of Beasts

A fake stone wall in the background with a blue information panel displayed. In the foreground is a brown table with an orange cylinder on top laying on it's side with a steel spindle through it's centre. There are controls in front of this on the top of the table.
A fake stone wall in the background with a blue information panel displayed. In the foreground is a brown table with an orange cylinder on top laying on it's side with a steel spindle through it's centre. There are controls in front of this on the top of the table.

By comparing the ratio of tagged to untagged animals, a reasonably accurate estimate of the total population of animals can be calculated.

How it works

Capture and count balls in a barrel to model estimation of animal populations.

Things to try or ask around the exhibit

  • How many balls are in this barrel?
  • Would it be okay to only do one or two samples, or should we do lots of samples?

Background

Sampling a population is more accurate if you repeat measurements and average your results. Ecologists use capture, tag, release and recapture techniques to estimate how many animals exist within an ecosystem's population.

For example, in the first round. an ecologist would capture a small number of animals (say 10 marsupials of a particular species), tag them with a plastic marker (or radio transmitter chip) and release the marsupials back to mix with other animals in the population.

After a few weeks, the ecologist might capture 60 marsupials and find that 3 of those marsupials are wearing a tag. So from the second sample, the ratio of tagged to untagged marsupials is 3/60 or 1/20, meaning one in twenty marsupials in the resampled group were tagged.

The ecologist would continue to capture, count and release the particular species of marsupial over a series of weeks and months, to obtain an average number of tagged versus untagged marsupials and use this ratio to calculate the total population of marsupials in the ecosystem.

Finding the science in your world

Measuring animal population numbers is very important for monitoring the health of an ecosystem and its biodiversity.

Researchers sometimes track where animals go after they are released as well as tagging for numbers. Elephant seals are tracked by gluing radio transmitters on their heads!