What comes to your mind when you hear the word population? You must have always associated the word population with humans but did you know that even the dogs, cats, mango trees, pigeons, etc., in your area also have their own population. Population refers to the group of organisms of a particular species that live together in a particular geographical area at a particular time.
What would happen if one of the trees died in that population? Will it affect the population? Will it affect other populations? What if some adaptive changes happen to one individual of the population?
So, these are some tricky questions!! In fact, whatever happens to a single organism can actually affect the entire population and sometimes other connected populations too. No matter if it is a loss or adaptation, it will affect the whole ecosystem. An important branch of ecology which links ecology, population genetics and evolution is called population ecology.
Let’s explore more about it.
Population is one of the lowest levels of organisation of ecology. To understand the population dynamics various criteria are used at a population level. These criterias are termed as population attributes. The four major population attributes are Birth rate, Death rate, Sex ratio and Age pyramid. Let's discuss these one by one.
Birth rate is the birth per capita of a population which is expressed as the ratio between the number of individuals added (by birth) in an existing population and the initial population size, in a given time period. So equation of birth rate can be written as;
Birth rate (b) = Number of individuals added / Initial population.
Let's consider an example of lotus in the pond. In year 1, the number of lotus in the pond is 20 and in year 2, the number of lotus in the pond is 26. So what would be the birth rate?
In the given data,
In year 1, number of lotus = 20
In year 2, number of lotus = 26
Hence , Number of individuals added = 26 - 20 = 6
Initial population = 20
Therefore, Birth rate = 6/20
= 0.3 offspring per lotus per year
Death rate is the death per capita that is expressed as ratio between the number of deaths and the initial population size, in a given time period. So equation of death rate can be written as;
Death Rate = Number of individuals died / Initial population.
Let's consider an example of fruit flies in a flask. On day 1 the total number of fruit flies in flask is 40 and on day 7 total number of fruit flies in flask is 36. Then what would be the death rate?
In the given data,
On day 1, number of fruit flies = 40
On day 2, number of fruit flies = 36
Hence the Number of individuals died = 40 - 36 = 4
Therefore Death rate = 4/40
= 0.1 individuals die per fruit fly per week
Sex ratio can be defined as the ratio of male to female population of an area. So equation of sex ratio can be written as;
Sex ratio = Number of male / number of female X 100
Let’s consider two hypothetical countries, country X and country Y. For country X the number of males is 700 and number of females is 300. For country Y, the number of males is 250 and the number of females is 220. Then what would be the sex ratio for each country?
So let’s calculate the sex ratio for country X first.
Number of males = 700
Number of females = 300
Hence the sex ratio = 700/300 X 100
~ 233 (approx)
Which means country X has 233 males per 100 females. So the number of males >> number of females. Hence, it has poor sex ratio.
Now lets calculate the sex ratio for country Y.
Number of males = 250
Number of females = 220
Hence the sex ratio = 250/220 X 100
Which means country X has 113 males per 100 females. So the number of males ≈ number of females. Hence it has a healthy sex ratio.
If we take a population at any given point of time, it is composed of individuals of different ages. For ease of study, individuals can be divided into different age groups.
The three divisions of different age groups are: Pre-reproductive age group (0-14 yrs), Reproductive age group (15-45 yrs), and Post - reproductive age group (46 and above).
The pre-reproductive age group includes children and young adolescents.
The reproductive age group includes young adults and the working age population.It is commonly known as child bearing years.
The post - reproductive age group majorly includes the elderly population. The fertility of the females declines and bearing of child becomes highly unlikely.
Consider the total population of a country to be N and the respective population percentage of men and women in different age groups are as given in the table.
Age group (years)
Men (Population %)
Women (Population %)
16 - 45
46 and above
If the information of age distribution is plotted for this population, then a pyramid-like structure is obtained. It is known as the Age pyramid.
The pyramid obtained when the percent of individuals of a given age or age group is plotted for the population is called the Age pyramid. It can be of different shapes. Let's check those out.
Bell-shaped Age Pyramids
When the pre-reproductive and reproductive individuals are almost in equal numbers or percentages and the post-reproductive individuals are relatively fewer, such kinds of pyramids are called bell-shaped pyramids. Such a population is predicted to remain stable and indicates almost zero population growth. Example: Such an age pyramid can be seen in a country like USA.
Triangular-shaped Age Pyramids
The triangular-shaped age pyramid is the pyramid of an expanding population that has a broad base as the number of pre-reproductive individuals is higher when compared to other age groups. The reproductive individuals are moderate in number and individuals of the post-reproductive age group are less. This population will grow in the future. Example: population of India
Urn-shaped age pyramids
Population size is also known as population density and is represented as ‘N’. Population density is generally calculated in terms of number, but sometimes measuring population density in terms of number does not make sense and is not the most appropriate approach. For example, let us consider that in a particular area there are 200 carrot grass plants, but only a single banyan tree with a large canopy.
If the population density is considered in terms of number, then the density of carrot grass plants is certainly higher than the banyan tree. But the ecological role of the banyan tree in that area is much higher than that of carrot grass..
Therefore, measuring the number of individuals may not always be apt. Using biomass as a unit for measuring population density is more meaningful in this case. If we take the biomass of a single Banyan tree, it is far more than the 200 carrot grass plants.
In some cases, instead of absolute population densities, i.e instead of determining exact number of organisms in a particular habitat, relative population densities are employed. If we take an example of the number of fish caught per trap, it will help us to estimate the population of fishes in the lake at a time.
Population density of tigers in an area is estimated differently. It is estimated based on the pugmarks and faecal pellets of the tiger population.
Factors affecting population density
There are three major factors that can affect the population density.
Predation pressure : Increase in number of predators decreases prey population and vice versa.
Availability of food : Decrease in amount of food decreases size of species population, which may lead to competition.
Question1. If 6 individuals in a population of 30 frogs died in a year, what would be the death rate in the population during that period?
Answer: Initial population of frogs = 30, Number of frogs that died in a year = 6. The formula used to calculate the death rate of a population = Number of individuals died/Initial population size. Death rate = 6/30 = 0.2.
Hence the correct option is c.
Question2. The basic unit of ecological hierarchy exhibits which of the following attributes?
A. Birth rate
C. Sex ratio
D. Both a and b
Answer: An individual organism is the basic unit of ecological hierarchy. Birth rate, death rate and sex ratio are attributes of the population. These attributes (features) are not exhibited by the individual organisms. The characteristic features of the organisms are births and deaths. Therefore, the basic unit of ecological hierarchy is organisms. Births and deaths are the characteristic features of an organism.
Hence the correct option is b.
Question3. Calculate the number of individuals added to a population of rats if its initial density is 20 and birth rate for a specified period of time is 0.3.
Answer: Initial number of rats in the population = 20, Birth rate = 0.3. The formula used to calculate the birth rate of a population is, Number of individuals got added/Initial population.
The number of individuals added (by births) to the population in the specified time = Birth rate X Initial number of individuals present in the population = 0.3 x 20 = 6.
Hence the correct option is c.
Question4. Population ‘X’ has 700 males and 300 females. Population ‘Y’ has 250 males and 220 females. Calculate the sex ratio for the population‘X’ and population ‘Y’ and then select the correct option.
A. Population ‘Y’ has approximately 113 males per 100 females
B. Population ‘Y’ has approximately 233 females per 100 males
C. Population ‘X’ has approximately 233 males per 100 females
D. Both a and c are correct
Hence the correct option is d.
Question1. What were the birth and death rates in India in the year 2021?
Answer: For the yer 2021, the birth rate of India was estimated to be roughly around 17.1 births per 1000 people of the population and the death rate was estimated to be around 7.1 deaths per 1000 people of population.
Question2. If a population has less people in the pre-reproductive stage compared to that in the reproductive or post-reproductive age, what would it signify?
Answer: A population with more people in the pre-reproductive stage compared to that in the reproductive or post-reproductive age is surely to decline as with time the number of people capable of reproduction will reduce. This will result in gradual decrease in birth rate.
Question3. What are the probable implications of the unbalanced sex ratio in India?
Answer: India is a country with poor sex ratio as the number of males is much more compared to that of the number of females. Some of the probable reasons for this imbalance are facts such as female foeticides, preference for a son, etc. A high male to female ratio can reflect in the form of reduced birth rates in the country. It can also have many social implications such as suppression of women, sexual harassment and violence, female trafficking, etc.
Question4. What happens in case of a dramatic increase in the population density of predators?
Answer: Drastic increase in the prey population will result in rapid decline of the prey population. As the prey population declines, the predators will eventually run out of food and the predator population will also start declining.