Which of the following statements is incorrect?

The correct answer is Option (2) → The biomass of a species is expressed in terms of fresh or dry weight, and its measurement in terms of fresh weight is more accurate.

The standing crop of a trophic level is measured as the mass of living organisms (biomass) or as their number in a unit area. Biomass can be taken as fresh weight or dry weight. Fresh weight includes water, and the water content of living tissue keeps changing, so the dry weight gives the more accurate figure.

Option (1) Each trophic level has a certain mass of living material at a particular time, called the standing crop. — Correct statement. This is the NCERT definition of standing crop.

Option (2) The biomass of a species is expressed in terms of fresh or dry weight, and its measurement in terms of fresh weight is more accurate. — Incorrect statement. This is the changed line. Measurement of biomass in terms of dry weight, not fresh weight, is more accurate.

Option (3) Any calculation of energy content, biomass or numbers has to include all organisms at that trophic level. — Correct statement. Counting only a few individuals of a trophic level would give a wrong picture, so every organism at that level is included.

Option (4) The base of each ecological pyramid represents the producers or the first trophic level. — Correct statement. Producers form the broad base; the apex is the tertiary or top level consumer.

NCERT Extract:
Energy Flow – “… Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight. Measurement of biomass in terms of dry weight is more accurate.”
Ecological Pyramids – “… The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer. … Any calculations of energy content, biomass or numbers, has to include all organisms at that trophic level.”