The given diagram of an age pyramid represents . . . . . . .

Read the given statements carefully and select the correct ones:
$(1)$ Individuals of the same species together form a population.
$(2)$ Mortality is an important factor for population growth.
$(3)$ Natality (birth rate) is an important factor for the immediate increase in population size.
$(4)$ In commensalism,both organisms benefit from the interaction.

The following figure shows the age pyramids for human populations. What types of populations are represented by $P$,$Q$,and $R$?
$P \quad\quad Q \quad\quad R$

The abundance of a species population within its habitat is called

If the age pyramid of a population is bell-shaped,it will show:

Which factors influence the change in population density?