Mark-release-recapture

Use of quadrats is only successful for species that don't move (plants) or very slow animals (snails).
Animals of a particular species are collected (usually by a netting or trap technique) and counted.
The animals are then marked in some way (paint or tagged).
The animals are then released and left to redistribute into the population.
A second recapture is carried out and the total number caught are recorded as well as how many of those captured are marked.
The population size is calculated as:

N1 is the number caught the first time
N2 is the number caught the second time
Nm is the number caught second time which were marked.