In an $Urn$-shaped age pyramid of the population,the trend of growth is:

The different age groups in a population are classified by:

If there were $30$ lotus plants in a pond last year and through reproduction $10$ new plants were added, taking the current population to $40$, then the birth rate = $.........$ offspring per lotus per year.

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.

Identify the factor that has the most significant impact on the population size of a general population.

The tiger census in our national parks is often based on . . . . . . .