According to the passage, what is called a “population” in biology ----.

A population is a group of individual organisms of the
same kind that are limited to some particular space.
The most familiar example is the human population,
but there are also populations of animals and plants
everywhere on Earth. In fact, scientists regard a
population as a biological unit that has both structure
and function. The parts of a population are its
individual members. The functions of a population are
similar to those of other biological units: growth,
development, and self-maintenance in a changing
environment. Individuals enter a population by birth
and by moving in, that is, by immigration. Individuals
leave a population by death and by moving out, that
is, by emigration. If the environment of a population
remains the same, loss and replacement of members
are in balance. The population will be able to survive
in that particular environment. If the environment
changes, however, loss or addition of members
increases or decreases the size of the population.