An inverted pyramid of biomass can be found in which ecosystem?

Correct — option (1), Marine.

A pyramid of biomass shows the standing crop of each trophic level: the mass of living material at a particular time, with producers at the base. It is inverted when the base holds less biomass than the level feeding on it.

NCERT's Class XII Biology (2021-22), Chapter 14 Ecosystem, states: "The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton."

Its Figure 14.4 (c) shows an inverted pyramid of biomass in which a small standing crop of phytoplankton supports a large standing crop of zooplankton.

How can a small base feed a larger level? OpenStax Biology 2e explains its inverted English Channel pyramid by the high turnover rate of phytoplankton. They are consumed rapidly, which keeps their biomass low at any moment, but they reproduce quickly.

Its figures for that ecosystem: primary producers 4 g/m², primary consumers 21 g/m². So the answer is Marine.

The idea to remember: a biomass pyramid is a snapshot of standing crop, so fast-reproducing producers with a small mass can support a larger mass of consumers.

Why the others are wrong

• (2)
  Grassland — Grassland is NCERT's example of a pyramid of numbers with a broad base. Its Figure 14.4 (a) runs from nearly 6 million plants at the base to only three top carnivores.

  OpenStax likewise describes a typical grassland in summer as having a base of many plants, with numbers decreasing at each trophic level. NCERT's example for an inverted biomass pyramid is the sea.

• (3)
  Forest — A forest is where the pyramid of numbers can invert. OpenStax notes that in a temperate forest in summer the base consists of few trees compared with the number of primary consumers, mostly insects.

  NCERT raises the same point among its exceptions to upright pyramids, asking readers to count the insects feeding on a big tree. The pyramid it states is generally inverted for biomass is the one in the sea.

• (4)
  Tundra — Tundra is a biome where tree growth is hindered by frigid temperatures and short growing seasons, of three types: Arctic, alpine and Antarctic. Arctic tundra supports low-growing plants such as moss and heath.

  NCERT's chapter says that in most ecosystems the pyramids of number, energy and biomass are upright. The exception it gives for biomass is the sea, not tundra.

An ecological pyramid shows a quantity at each trophic level, with producers at the base and the top consumer at the apex. NCERT names three: the pyramids of number, biomass and energy.

Standing crop is the mass of living material at a trophic level at a particular time, measured as biomass or as number in a unit area. NCERT notes that biomass measured as dry weight is more accurate.

NCERT states that the pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat at each step.

The pyramids of number and biomass can be inverted: among its exceptions NCERT asks readers to count the insects feeding on a big tree, and it states that the pyramid of biomass in sea is generally inverted.

NCERT also lists limits of pyramids: they do not account for a species at two or more trophic levels, assume a simple food chain, and give saprophytes no place.

Key facts

• NCERT Class XII Biology (2021-22): the pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton.
• NCERT: the pyramid of energy is always upright, because some energy is always lost as heat at each step.
• Standing crop is the mass of living material at a trophic level at a particular time, measured as biomass or number per unit area.
• OpenStax Biology 2e, English Channel: primary producers 4 g/m² and primary consumers 21 g/m², an inverted pyramid of biomass.
• OpenStax: in a temperate forest in summer, few trees support many primary consumers, mostly insects, so the pyramid of numbers inverts.

Common traps

• Carrying the forest's inverted pyramid of numbers over to biomass. Few trees supporting many insects inverts a count; NCERT's example for biomass is the sea.
• Expecting every aquatic ecosystem to invert. OpenStax's Silver Springs aquatic ecosystem has an upright biomass pyramid, while its English Channel example is inverted.
• Inverting the energy pyramid. NCERT states the pyramid of energy is always upright and can never be inverted.

UnlockIAS will link similar questions from RAS Pre 2013 here once that paper is published on this site.

• practice — not a real PYQ

  Which one of the following ecological pyramids is always upright?

  • (a)Pyramid of numbers
  • (b)Pyramid of biomass
  • (c)Pyramid of energy
  • (d)None of these
  Answer(3) — Some energy is always lost as heat at each step, so a higher level never holds more energy than the one below it. Option (1) can invert, as with insects on a tree; option (2) can invert, as in the sea; option (4) fails because the energy pyramid is always upright.
• practice — not a real PYQ

  In a sea, the standing crop of phytoplankton at a given time is 4 g/m² and that of the zooplankton feeding on them is 21 g/m². These figures describe –

  • (a)An upright pyramid of biomass
  • (b)An inverted pyramid of biomass
  • (c)An inverted pyramid of energy
  • (d)An upright pyramid of numbers
  Answer(2) — The figures are masses per square metre, so they describe biomass, and the consumer level (21 g/m²) is larger than the producer level (4 g/m²). Option (1) would need the producers to be heavier; option (3) is not possible; option (4) needs counts of organisms, which the figures do not give.