The number of individuals in the population is the population's size. If a population is small enough you will be able to determine the size by counting the individuals.
Emigration.
The group of individuals used to represent a population is called the sample. It should have the same statistics as the population, though be of a smaller size.
Over population.
genetic variation
emigration
dispersion
Moving out of a population is called emigration. (Moving into a population is called immigration.)
Individuals of a species that live in an area are called population.
population
sample.
its called competition