Determination of Population Size

The method for determining population size is dependent on the type of organism, whether stationary or mobile and on the type of habitat under study. 

The most accurate way to determine the population size in a population is to count all the individuals. This approach would be cumbersome when there are large populations or extensive habitats.

Alternatively, scientists can determine population size by sampling which involves counting individuals within a certain area or volume that is part of the population’s habitat. Analyses of sample data enable scientists to determine the population size and densityDensity is the measurement of how tightly a material is packed together i.e. how closely the particles are packed in the material. The tighter the material is packed the more its... More of the entire population.

A. Complete census: This involves counting all individuals because they are large, comparatively few and the habitat is small.

B. Sampling methods: Samples are usually taken when the individuals/specimens are usually small in size and many in number.

Some sampling methods are described below.

i. Quadrat Method:

A quadrat is a square that encloses an area within a habitat. Quadrats are of different sizes (0.5 m by 0.5, 1.0 m by 1.0).

The first step involves defining or mapping out an area. This involves obtaining an overview of the area by identifying and marking its boundaries, noting its topographyTopography can be defined as the study of the shape and features of land surfaces. The topography of an area refers to the surface shapes and features on the ground and... More and geography, and creating a base map. The area is staked out using sticks and string, or using a square made of wood, plastic, or metal placed on the ground.

The quadrat is thrown in a random manner on the plot. After each throw, the number of species under study is counted and recorded. After about 10 or more throws, the average number of individuals of a species per unit area (i.e. 1/m²) is calculated. From this, the population of individuals in the habitat can be estimated.