A population has certain attributes that an individual organism does not. An individual may have birth or death but a population may have birth rate or death rate. 

Birth rate: The no. of births in a definite time period is called the birth rate, It is a positive factor to the population.

Death rate: The no. of deaths in a population in an interval of time period is called the death rate. It is called the negative factor. Another attribute characteristic of a population is sex ratio. An individual in either a male or a female but a population has a sex ratio. Again the size of the population gives us its status in the habitat. 

population of any species in nature has its disposal unlimited resources to permit exponential growth. This leads to competition between individuals for limited resources. Here the fittest individual will survive and reproduce. In nature, a given habitat enough resources to support a maximum possible number, beyond which no further growth is possible. This is known as nature’s carrying capacity (K) for that species in that habitat. 

A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches, the carrying capacity. A plot of population density (N) in relation to time (t) results in a sigmoid curve. This type of population growth is called verhulst – Pearl logistic Growth and described by the following equation. 

dN/dt = rn (N –N/k)

Where, N = Population density at time t. 

r  = intrinsic rate of natural increase. 

K = carrying capacity