Biology · Organisms and Populations · NEET
'r' is the intrinsic rate of natural increase. It is the net per capita rate at which a population grows, calculated as r = b − d, where b is the per capita birth rate and d is the per capita death rate. NCERT calls it a very important parameter used to assess the impact of any biotic or abiotic factor on population growth.
No. Birth rate (b) counts only new individuals added per member. 'r' is the leftover after you subtract deaths, r = (b − d). A population can have a high birth rate but a low or even negative r if the death rate is also high. Never treat b and r as the same value.
When r = 0, births exactly balance deaths (b = d), so dN/dt = rN = 0. The population size does not change over time. This is exactly what a NEET numerical tests: equal per capita birth and death rates give r = 0 and a stable, unchanging population.
Yes. If the death rate d is greater than the birth rate b, then r = (b − d) is negative. A negative r means the population is shrinking (declining). A positive r means it is growing, and r = 0 means it is stable.
It is 'intrinsic' because it reflects the population's own inherent capacity to grow under ideal, unlimited conditions — plenty of food, no competition, no predators. It is a property built into the species, which is why fast breeders like the flour beetle (r = 0.12) have a much higher r than slow breeders like the Norway rat (r = 0.015).
NCERT gives three: Norway rat r = 0.015, flour beetle r = 0.12, and the human population in India in 1981 had r = 0.0205. These exact numbers are frequently asked directly, so memorise them.
Consider a population of 10 million cells with a per-capita birth rate of 0.002 (per unit time) and a per-capita death rate of 0.002 (per unit time). The expected number of cells after 10 generations is ______.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
r = (b − d), where b is the per capita birth rate and d is the per capita death rate. It is used in the exponential growth equation dN/dt = rN, and its integrated form Nt = N0 e^(rt).
NCERT states that in 1981 the r value for the human population in India was 0.0205.
The one with r = 0.12 (the flour beetle) grows much faster than one with r = 0.015 (the Norway rat), because a higher r means a higher net per capita growth rate under unlimited conditions.
No. r is a per capita rate and is independent of N. Two populations of the same species can differ in size but have the same r under the same conditions.
NCERT calls r a very important parameter for assessing how any biotic or abiotic factor affects population growth, and NEET tests it through the b − d calculation and the dN/dt = rN equation.
