Estimating the population size of each species is known as the abundance of a specie.
The best measure to measure abundance is to count every individual in the population. This is difficult so there's other ways:
Quadratting---> a quadrat is a frame of known area which is placed on the ground and an estimate of the population sizes within the quadrat are made. Quadrants must be placed at random. Placing measuring tapes alongside the side of the area and generating random co-ordinaries from a computer or random number tables. Each square is 1%.
Systematic sampling---> a transect is a line across the area and sampling is done at set predetermined intervals along the line. Sampling may consist of recording what species are touching the line at each sampling point. Placing two parallel lines across and recording what species are found between the two lines (a belt transect). This has to be random and several repeats must be done.

Investigating Populations