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.
There are a few ways that scientists determine the population of living things in a habitat. One method is by using quadrats - this method works best with smaller organisms. The process is as follows:•Quadrats are randomly placed in the ecosystem •Several species or a single species are counted within the quadrat •This is repeated several times within the ecosystem •An average of all of the species is taken •The average is then multiplied by the size of the ecosystem being studied With larger organisms, scientists will complete arial counts - they will count the number of organisms being studied from an airplane. Tagging animals is also useful for getting population numbers in an ecosystem. With some shark species, like leopard sharks, many individuals are captured and fitted with a tag and released. Other useful information can be obtained at the same time, such as sex and size. This process is repeated, often over a period of many years, to gather data about population numbers. Satellite tagging of organisms also gives scientists a great deal of information about organisms, but this is typically limited to information such as migration routes and feeding areas rather than population numbers. It is important to note that whichever method is being used to calculate population numbers, they are an estimate of the population in any given area. For example, you could use the quadrat method to count the number of blades of grass in your yard, but this nuber would not be the exact number of blades of grass.
It would depend to a great deal on what living things you were counting. If it for example were elk, a simple visual count may do. If, however, you're looking at ants you would need to come up with some reasonable method to estimate the numbers in a given area and then extrapolate.