There are multiple ways to count or estimate the population of a species and these depend upon available resources for the study and the type of species that you're trying to estimate. The most obvious way to get an estimation of a population is to simply count all of the species in an area. This process is often assisted by aerial flyovers or aerial photography.
One other method is known as the drive approach, where a line of individuals walks across an area and counts every individual organism of the particular species in question. The use of complete counts is often rare. This is because of the great number of resources and personnel that are usually required to complete them.
As an alternative, indirect and estimated... counts are usually done and there are a number of different ways and methods to complete these estimates. We will look at the mark recapture method, the quadrate sampling method, and the transect sampling method.
The mark recapture method is usually used for species that are mobile. This method goes by several other names one of which is the capture recapture method. In the mark recapture method traps or other methods are used to capture animals. Once animals are captured they are are marked with a distinguishing feature such as a tag or a band.
These animals are then released into the natural habitat and back into the population. Animals are then captured again on a random basis and special note is taken of those that are marked. Then the ratio of those that are marked to those that are unmarked is used to estimate the total size of the population.
The Quadrant Sampling Method is an additional technique for estimating populations of a certain size. species. It is usually reserved for species with limited mobility such as plants or even snails. This method uses the count of organisms in a species found inside of a quadrant to then extrapolate to a larger area.
A quadrant is a simple device that is used to randomly select an area where an organism count should be done. Once the organism count is completed inside of the quadrant This data can then be extrapolated to the larger area. For example, if you counted organisms inside of a one square meter quadrant, this data could then be extrapolated very easily to a much larger area that was measured in square meters.
Estimating populations is called transect sampling. This process uses lines that transect or are drawn across a particular area. Individuals will then count the observed organisms that are found while walking along this transect. In some variations of this method, the distance an organism is from this transect line is also recorded and used in the calculations.
Each transect line is then used to get an estimate of the total area covered. And the number of observed species along each transect can then be used to get a total population estimate. There are a number of different variations and methods that can be used to estimate populations, but these are a few examples of common methods and techniques that are used.