Phenomenal and rapid increase of population in a short period is called

Assertion : In sigmoid growth curve, population finally stabilizes itself.

Reason : Finally, the death rate increases than the birth rate.

Which of the following is correct

$I.$ In a population, birth rate and death rate refer to per capita births and death respectively

$II.$ In nature, we rarely find isolated single individuals of any species

$III.$ The size of population for any species is stable phenomena

$IV.$ Ecological effects of any factors on a population growth are generally reflected in its size/population density