Answer:
The methods used to estimate the population size are the following:
Census: Count of the total number of individuals in a population
Density sampling: The number of individuals per spatial unit is studied. This data multiplied by the extension of the area occupied by the population gives an estimate of the total number of individuals.
Capture / recapture method: Individuals are captured and tagged and returned to their environment. After a certain period of time a new capture is made and the ratio between marked and unmarked animals is established. Knowing the number of initially marked individuals, the size of the population can be determined from this proportion.