How do I determine the population of living things in different habitats?

There are two main ways to estimate populations of organisms in habitats. For animals, because they move around, a tag and recapture method is best. A set number of the designated organism is captured and tagged. Then, to determine the population, a large number of the organisms are captured. The ratio can then give a good estimate of the actual number in the habitat: #originally tagged/estimated number = #captured with tags/total #captured. Cross multiply and solve to find the estimated number in the habitat.

For plants, or animals that don't move around but are in large numbers, set up a grid. Count the number of the organisms in a random sampling of the grid portions, and extrapolate to find the total. For example, if you count the number of organisms in 10 of 100 of the grid portions, you can get an average density of the population in the habitat.