Density, birth rate and death rate

A population is more than many animals

Think of all the lotus plants in a pond. Each plant is born and dies. But only the whole group of plants has a birth rate, a death rate and a density. These are the attributes of a population.

A population is a group of individuals of the same species. They live in a well defined area, they share or compete for similar resources, and they can breed with each other. NCERT gives these examples: all the cormorants in a wetland, the rats in an abandoned house, the teakwood trees in a forest, the bacteria in a culture plate and the lotus plants in a pond.

Density

The population density is the size of the population in a unit of area or of volume. It is often a number, for example 200 plants in a hectare. But a number is not always the best measure.

• A forest may have one huge banyan tree and 200 small herbs. Counting each as one tells little. The biomass is better.
• Some organisms are too many to count, such as the tiny algae in a pond. The percent cover or the biomass is better.
• Some animals cannot be counted easily. For a tiger census, the number is found from indirect signs, such as pug marks and fecal pellets.

Four processes change the density

Over a period of time, the size of a population goes up and down. NCERT names four processes.

• Natality (births, B): the number of births in the period. It adds to the density.
• Mortality (deaths, D): the number of deaths in the period. It takes away from the density.
• Immigration (I): the number of individuals of the same species that come in from elsewhere. It adds to the density.
• Emigration (E): the number of individuals that leave. It takes away from the density.

If the population is $N_t$ at time $t$, then at time $t + 1$ it is

Within a population, growth depends on the food, the pressure of predation and the weather. When the food is rich and the weather is good, the density goes up.

Birth rate and death rate for each individual

It is better to give births and deaths for each individual of the population. These are the per capita rates.

Birth rate: the number of births divided by the population. Death rate: the number of deaths divided by the population.

NCERT gives two examples.

• A pond has 20 lotus plants last year. By reproduction, 8 new plants are added, and the population becomes 28. The birth rate is $8 \div 20 = 0.4$ offspring for each plant in a year.
• A lab population of 40 fruit flies has 4 flies die in a week. The death rate is $4 \div 40 = 0.1$ for each fly in a week.

Notice the units. A rate is always for each individual and for a stated time.

You should find that the population goes up when births plus immigrants are more than deaths plus emigrants. If the two sums are equal, the size stays the same.

Try it in Python

The program follows a population for five years. The birth rate and the death rate are for each individual, so the number of births and deaths grows with the population.

Practise with new numbers

Common mistakes

• Saying that a single animal has a birth rate. The rates belong to the population.
• Adding emigrants to the population. Emigrants leave, so they are taken away. Immigrants are added.
• Forgetting that a rate needs a time. "0.1 flies" means nothing. "0.1 for each fly in a week" does.
• Counting only the births and the deaths. Immigration and emigration change the density too.

Remember these

• A population: the same species, one area, shared resources, able to breed together.
• Four processes: natality and immigration add. Mortality and emigration take away.
• $N_{t+1} = N_t + (B + I) - (D + E)$.
• Birth rate $= B/N$ and death rate $= D/N$ for each individual in a stated time.
• Density can be a number, a biomass or a percent cover. Indirect signs, such as pug marks, are used for tigers.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four processes that change the size of a population. Which two add to it?

2. A population has 100 individuals. In a year there are 20 births, 10 deaths, 5 immigrants and 15 emigrants. What is the population at the end?

3. Why is the biomass sometimes a better measure of density than the number?

4. 30 flies of a lab population of 150 die in a week. What is the death rate?

5. How is the number of tigers in a forest found?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 11, Organisms and Populations, section on populations.
• OpenStax Biology 2e (2018), chapter 45, Population and Community Ecology, section on population demography.
• Campbell Biology, 12th edition (2021), Pearson, chapter 53, Population Ecology.