Explain the characteristics of a population.

(N/A) Populations: Individuals of any species living in groups in a well-defined geographical area,sharing or competing for similar resources,and potentially interbreeding,constitute a population.
Population Attributes: $A$ population has certain attributes that an individual organism does not. For example,an individual may have births and deaths,but a population has birth rates and death rates. The birth and death rates are referred to as per capita births or deaths,respectively,which represent the increase or decrease in the number of individuals with respect to the members of the population.
Sex ratio is another attribute of a population. An individual may be male or female,but a population has a sex ratio.
$A$ population at any given time is composed of individuals of different ages. If the age distribution is plotted for the population,the resulting structure is called an age pyramid. The shape of the pyramid reflects the growth status of the population,which may be:
$(i)$ Expanding
$(ii)$ Stable
$(iii)$ Declining
Population size or population density $(N)$ is often measured in terms of the number of individuals,but this may sometimes not be appropriate.
The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets rather than direct counting.