My 6th grade classes have started ecology, and this section is on determining how many organisms are in a population. As a warm up for the section, each table had 2 beakers, one filled with dry beans. They also had a ruler. They had 2 minutes to estimate how many beans they had. The trick was that they couldn’t just count them. They had to estimate.

The kids came up with varying methods of estimating. One of the better ones being to count how many beans you can put in a layer, then seeing how many layers there were and multiplying. One group was only off by 2.

This was a good introduction to some of the ways of counting populations.

**Direct Observation** Just count them all. Not always feasible.
**Indirect Observation** Frequently you can’t count the organisms, but you can count the signs of them. For example, you can count how many bird families are in a tree by counting the nests.
**Sampling** If the population is homogeneously spread, you can overlay a grid, and count how many in one grid square, then multiply to get the final estimate.
**Mark and Recapture** Sometimes ecologists carefully capture a group of organisms and mark them (tags, paint). Then they release them back to the wild. Later they capture more organisms, and count how many of them are marked. Then a simple formula tells the population size.

*Related*