An inverted pyramid of biomass is characteristic of:

An aquatic (sea/pond) ecosystem with phytoplankton

Which of the following statements about energy and nutrients in an ecosystem is correct?

Energy flows one-way and is lost; nutrients cycle and are reused

Both energy and nutrients cycle endlessly

Both energy and nutrients flow one-way and are lost

Nutrients flow one-way; energy cycles

The phosphorus cycle differs from the carbon and nitrogen cycles in that phosphorus:

Has no significant gaseous phase; its reservoir is rock (sedimentary)

Has a large atmospheric gaseous reservoir

Cannot be used by living organisms

Net primary productivity (NPP) is defined as:

Gross primary productivity minus respiration

Gross primary productivity plus respiration

The total energy fixed by producers

A pyramid of numbers is inverted (narrow base, wide top) in the case of:

A single large tree supporting many insects and birds

A grassland with grass, deer and lions

A pond with algae, small fish and big fish

Ecosystem — Energy Flow, Ecological Pyramids and Nutrient Cycling

An ecosystem is where physical and living components meet, and the whole chapter is organised around one grand principle: energy flows in one direction and is lost at every step, while nutrients cycle round and round. Fix that contrast and the details slot in. The most examined number is the 10% law — only about a tenth of the energy at one trophic level passes to the next — which explains why food chains are short and why the pyramid of energy is always upright. The other pyramids (number, biomass) can sometimes be inverted, and knowing which is a favourite trap. Learn energy-versus-nutrients as the organising axis, memorise the 10% law and the invertible pyramids, and the chapter is very scoring.

Trophic Levels and Energy Flow

The trophic levels: producers (T1) — green plants, the only ones that capture solar energy; primary consumers/herbivores (T2); secondary consumers/carnivores (T3); and so on. Decomposers break down dead matter, returning nutrients to the soil.
Energy flow is one-way: sunlight → producers → consumers → decomposers, never back. At every transfer, most energy is lost as heat (respiration), which is why so little reaches the top.
The 10% law (Lindeman): only about 10% of the energy at one trophic level is available to the next; the other ~90% is lost. This is why food chains rarely have more than 4–5 links.

🎯 Why food chains are short — the 10% law does the explaining

Here is a puzzle the 10% law answers neatly: why do you almost never see a food chain with eight or ten links? Why is there no predator that eats the animal that eats the lion? The reason is energy accounting. Green plants capture only a small fraction of the sunlight falling on them, and then at every transfer up the chain, roughly 90% of the energy is lost — spent on the organism's own respiration, movement and heat, and never passed on. Only about 10% is stored as body tissue that the next level can eat. So if producers hold, say, 1000 units of energy, herbivores get about 100, the carnivores that eat them about 10, and the next level only about 1. After four or five steps there is simply too little energy left to support another level of consumers — the top predator would starve. This is why food chains are short, why there are far fewer top carnivores than herbivores, and why the pyramid of energy is always upright: each level necessarily contains less energy than the one below it, with no exceptions, because you cannot create energy on the way up. When a question asks why long food chains do not exist, or why top carnivores are rare, the 10% law is the answer.

Ecological Pyramids

The three pyramids — and which can be inverted
Pyramid of	Usually	Can be inverted?
Energy	Upright	Never — always upright (energy is lost each step)
Number	Upright	Yes — inverted for a big tree with many insects/birds
Biomass	Upright (on land)	Yes — inverted in the sea (tiny phytoplankton feed larger fish)

⚠️ Only the energy pyramid can never be inverted

This is one of ecology's most reliable trick questions, and the safe rule is short: the pyramid of energy is always upright; the pyramids of number and biomass can sometimes be inverted. The energy pyramid cannot invert because of the 10% law — each level, by physical necessity, holds less energy than the one below it, so a higher level can never contain more energy than a lower one. But number and biomass count things, not energy, and things can be arranged top-heavy. The pyramid of numbers is inverted for a single big tree: one tree (one producer) supports thousands of insects (many primary consumers) and then birds — so the base is narrow and the upper levels wide. The pyramid of biomass is inverted in the sea: at any instant the tiny, fast-reproducing phytoplankton have less standing biomass than the larger, longer-lived fish and zooplankton feeding on them, because the phytoplankton are eaten as fast as they grow. So when a question asks "which pyramid is always upright", the answer is energy, and when it asks for an inverted pyramid, reach for the big tree (numbers) or the sea (biomass). Never say the energy pyramid is inverted — that is the trap the question is built to catch.

Nutrient Cycles and Ecosystem Services

Unlike energy, nutrients cycle between the living and non-living components (biogeochemical cycles). The carbon cycle runs through photosynthesis and respiration; the phosphorus cycle is sedimentary (its reservoir is rock, with no significant gaseous phase — a common exam point).
Ecological succession is the orderly change of communities over time toward a stable climax community. Primary succession starts on bare rock (pioneers are lichens); secondary succession starts where soil already exists (e.g. after a fire) and is faster.
Productivity: GPP (gross primary productivity) is total energy fixed by producers; NPP = GPP − respiration is what is left for consumers. Standing biomass and the rate of decomposition also define an ecosystem.

ℹ️ Why nutrients cycle but energy does not

The single most important contrast in this chapter is that energy flows through an ecosystem in one direction and is lost, while nutrients cycle within it and are reused — and understanding why explains a great deal. Energy enters as sunlight, is captured by producers, passes up the trophic levels, and at every step a large share is converted to heat and radiated away, never to be recaptured by the ecosystem; so energy must be constantly resupplied by the sun, and its flow is a one-way street ending in heat. Matter is different. The atoms of carbon, nitrogen, phosphorus and the rest are not destroyed when an organism uses them; when the organism dies, decomposers break the body down and return those atoms to the soil, water and air, where producers take them up again. So the same phosphorus atom can pass from rock to plant to herbivore to carnivore to decomposer and back to the soil, round and round indefinitely. This is why decomposers are so important — they close the loop, releasing locked-up nutrients for reuse — and why a nutrient cycle is called biogeochemical: it moves between the living (bio), the earth (geo) and chemical forms. The exam framing to carry is: energy flows and is lost (needs constant solar input); matter cycles and is conserved (reused via decomposers).

Key takeaways

Energy flows one-way and is lost as heat each step; nutrients cycle (biogeochemical) and are reused via decomposers.
The 10% law: only ~10% of energy passes to the next trophic level, so food chains have only 4–5 links.
Pyramid of ENERGY is always upright; pyramids of number and biomass can be inverted.
Inverted number pyramid = one big tree with many insects/birds; inverted biomass pyramid = the sea (phytoplankton).
Producers (green plants) alone capture solar energy; decomposers return nutrients to the soil.
The phosphorus cycle is sedimentary (reservoir = rock, no gaseous phase); carbon cycle runs via photosynthesis/respiration.
Succession: primary starts on bare rock (lichen pioneers); secondary starts where soil exists (faster), ending in a climax.
GPP = total energy fixed; NPP = GPP − respiration = energy available to consumers.

Practice questions (8)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Which ecological pyramid is always upright and can never be inverted?
1. The pyramid of energy
2. The pyramid of numbers
3. The pyramid of biomass
4. All three are always inverted
Show answer
Answer: A — The pyramid of energy
Because energy is lost at every transfer (the 10% law), each level always holds less energy than the one below, so the energy pyramid is always upright. Pyramids of number and biomass count things and can be inverted, as with a big tree or ocean plankton, so they are not the always-upright one.
According to the 10% law, if the producers of an ecosystem contain 10,000 kcal, the energy available to secondary consumers is about:
1. 100 kcal
2. 1000 kcal
3. 10,000 kcal
4. 10 kcal
Show answer
Answer: A — 100 kcal
Producers 10,000 → primary consumers ~1000 (10%) → secondary consumers ~100 (10% again). The 1000 figure stops one level too early, at the herbivores, which is the trap for anyone who applies the 10% only once.
An inverted pyramid of biomass is characteristic of:
1. An aquatic (sea/pond) ecosystem with phytoplankton
2. A grassland
3. A tropical forest
4. A desert
Show answer
Answer: A — An aquatic (sea/pond) ecosystem with phytoplankton
In water the tiny, rapidly reproducing phytoplankton have less standing biomass than the larger fish feeding on them, so the biomass pyramid is inverted. Forests and grasslands have huge producer biomass at the base, giving the usual upright shape instead.
Which of the following statements about energy and nutrients in an ecosystem is correct?
1. Energy flows one-way and is lost; nutrients cycle and are reused
2. Both energy and nutrients cycle endlessly
3. Both energy and nutrients flow one-way and are lost
4. Nutrients flow one-way; energy cycles
Show answer
Answer: A — Energy flows one-way and is lost; nutrients cycle and are reused
Energy enters as sunlight, is lost as heat at each step and needs constant resupply, whereas atoms of nutrients are returned to the environment by decomposers and reused. The last option exactly reverses this — energy does not cycle and nutrients do — so it is the classic inversion trap.
The phosphorus cycle differs from the carbon and nitrogen cycles in that phosphorus:
1. Has no significant gaseous phase; its reservoir is rock (sedimentary)
2. Has a large atmospheric gaseous reservoir
3. Cannot be used by living organisms
4. Is not recycled at all
Show answer
Answer: A — Has no significant gaseous phase; its reservoir is rock (sedimentary)
The phosphorus cycle is sedimentary: its main reservoir is rock and it has essentially no gaseous phase, unlike carbon and nitrogen. It is still recycled and used by organisms, so the options claiming it cannot be used or recycled misdescribe the cycle.
Net primary productivity (NPP) is defined as:
1. Gross primary productivity minus respiration
2. Gross primary productivity plus respiration
3. The total energy fixed by producers
4. The energy lost by decomposers
Show answer
Answer: A — Gross primary productivity minus respiration
NPP is the energy left after producers use some of their gross production for their own respiration, so NPP = GPP − respiration, and it is what is available to consumers. The total energy fixed with no subtraction is GPP itself, which the third option describes instead.
The pioneer organisms in primary succession on bare rock are usually:
1. Lichens
2. Large trees
3. Grasses only
4. Fish
Show answer
Answer: A — Lichens
Lichens colonise bare rock first, secreting acids that weather it and begin soil formation, paving the way for mosses and later plants. Large trees appear only much later once soil has built up, so they cannot be the pioneers on bare rock.
A pyramid of numbers is inverted (narrow base, wide top) in the case of:
1. A single large tree supporting many insects and birds
2. A grassland with grass, deer and lions
3. A pond with algae, small fish and big fish
4. Any ecosystem, always
Show answer
Answer: A — A single large tree supporting many insects and birds
One big tree is a single producer supporting thousands of insects and then birds, so the base is narrow and the higher levels are wide — an inverted number pyramid. A grassland has many grass plants at a broad base, giving the usual upright pyramid instead.