Biology · Organisms and Populations · NEET
Always divide by the STARTING (original) population, not the final one. Per capita birth rate = (new individuals added) / (original number). In the NCERT lotus example, 20 plants become 28 (8 added), so birth rate = 8/20 = 0.4, NOT 8/28. This is the single most common mistake NEET students make.
New individuals added = 2500 - 250 = 2250 (only the NEW births count, not the total). Birth rate = 2250/250 = 9 offspring per snail per year. If you wrongly use 2500/250 you get 10, which is the trap option. The number added is final minus initial, then divide by initial.
The method is identical: death rate = (number of deaths) / (original population). NCERT: 4 fruitflies die out of 40, so death rate = 4/40 = 0.1 individuals per fruitfly per week. Same per capita logic — only the word changes from births to deaths.
The unit is 'individuals (or offspring) per individual per unit time' — for example 0.4 offspring per lotus per year, or 0.1 individuals per fruitfly per week. It is a per capita rate, so 'per individual' and a time period (year, week) are always part of the answer.
Yes. Natality = birth rate and mortality = death rate. NCERT uses both terms interchangeably. A population has birth RATES and death RATES, while a single individual only has births and deaths — this per-population idea is why it is a population attribute.
If '8' Drosophila in a laboratory population of '80' died during a week, the death rate in the population is __ individuals per Drosophila per week.
If there are 250 snails in a pond, and within a year their number increases to 2500 by reproduction. What should be their birth rate per snail per year?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Per capita birth rate = number of new individuals added divided by the original population size, measured over a stated time period (e.g. per year).
Because a rate measures growth relative to who was already present. Dividing by the starting number gives the true per-individual (per capita) contribution to the population.
Natality is the birth rate (individuals added by reproduction) and mortality is the death rate (individuals lost by death). Both are calculated per capita over a time period.
Yes. An individual only has a birth and a death, but a population has birth RATES and death RATES, so they are attributes of the population, not the individual.
Per capita birth/death rate numericals are repeatedly asked — NEET 2022 and 2023 both had one-step numericals. They are easy guaranteed marks if you remember to divide by the original population.
