Biology · Organisms and Populations · NEET
No. NCERT clearly states density (N) 'need not necessarily be measured in numbers only.' Total number is the most appropriate measure in most cases, but it is sometimes meaningless or too hard to get. For a huge banyan tree with a large canopy versus 200 small carrot grass plants, percent cover or biomass tells the real story better than plain numbers. For NEET, remember: numbers are usual, but not the only measure.
Because counting gives a wrong picture of importance. If an area has 200 carrot grass (Parthenium) plants but only ONE giant banyan tree with a huge canopy, saying banyan density is 'low' underestimates its enormous role in the community. So NCERT uses percent cover or biomass for such cases. This exact banyan-vs-carrot-grass example is a favourite NTA test point.
Absolute density is the real, actual number of individuals per unit area or volume. Relative density is an indirect measure that works just as well for comparison, without knowing the exact total. NCERT's example: the number of fish caught per trap is a good enough measure of the total fish population density in a lake. You do not count every fish; the catch-per-trap tells you if density is high or low.
By indirect estimation. NCERT says: 'We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them.' The tiger census in national parks and reserves is often based on pug marks (paw prints) and fecal pellets (droppings). This is a classic NEET one-liner — pug marks AND fecal pellets, both.
By an indirect or sample measure, not by counting every cell. NCERT poses this exact question: a dense laboratory culture of bacteria in a petri dish is impossible to count individually, so total number is not adoptable. This shows why we need alternatives like relative density or sampling when populations are huge and counting is impossible or very time-consuming.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Population density is designated as N in NCERT. It stands for population size — the number of individuals of a species in a defined area or volume at a given time.
Four ways: (1) total number (most common), (2) percent cover or biomass (for a huge banyan tree or plants where counting misleads), (3) relative density such as number of fish caught per trap, and (4) indirect estimation such as tiger census by pug marks and fecal pellets.
The number of fish caught per trap is a good enough measure of the total fish population density in a lake. It is a relative measure — you compare catch per effort instead of counting every fish.
Because a single huge organism (like a banyan tree) contributes far more to the community than its low count suggests. Percent cover or biomass captures its true ecological role, whereas total number would underestimate it.
No. NCERT states that for some ecological investigations there is no need to know absolute densities; relative densities serve the purpose equally well, and populations are often estimated indirectly without counting or seeing the individuals.
