Energy and Ecosystems — AQA A-level Biology

SPEC 3.5.3 - ENERGY TRANSFERS IN ECOSYSTEMS

Energy & Ecosystems

Biomass, gross primary production, net primary production, net production of consumers and farming efficiency

Energy Flow in Ecosystems

All energy in an ecosystem ultimately comes from the Sun. Plants capture light energy via photosynthesis and store it as chemical energy in biological molecules — this forms the biomass of producers.

Biomass

The mass of biological material in organisms or a trophic level. Measured as dry mass of tissue per unit area (e.g. g m⁻²) or mass of carbon per unit area. Dry mass is used to remove the variable contribution of water.

Why dry mass?

Water content varies between organisms and conditions. Removing water gives a consistent measure of the actual organic material. Dry mass is obtained by heating samples at ~80°C until constant mass is reached.

Examiner Alert

Biomass is measured as dry mass, not fresh (wet) mass. Stating "mass of water" or "wet mass" is incorrect. The spec explicitly states dry mass of tissue or mass of carbon per given area.

Gross Primary Production (GPP)

The total chemical energy stored in plant biomass, in a given area or volume, per unit time. It represents the total amount of energy fixed by photosynthesis — all the organic compounds produced before any are used in respiration.

GPP is expressed in units such as kJ ha⁻¹ yr⁻¹ or kJ m⁻² yr⁻¹.

What happens to GPP?

Most of the sugars synthesised by plants are used as respiratory substrates (energy released as heat — lost to the environment). The remainder is used to build other biological molecules and forms new plant biomass available to other trophic levels.

Calorimetry

The chemical energy stored in dry biomass can be estimated using calorimetry — burning a known dry mass of material and measuring the heat released. This gives energy values in kJ g⁻¹.

Net Primary Production (NPP)

The chemical energy stored in plant biomass after respiratory losses to the environment have been taken into account. NPP is the energy available for plant growth and reproduction, and for other trophic levels (herbivores and decomposers).

NPP = GPP − R

where R = respiratory losses to the environment by the producers

What NPP represents

NPP is the energy that actually becomes new plant biomass. It is what herbivores, decomposers, and ultimately all consumers in the ecosystem depend on. Only NPP — not GPP — is available to the next trophic level.

Common Confusion

GPP is the total fixed energy. NPP is what remains after the plant's own respiration. Always subtract R from GPP to get NPP. Do not confuse these — GPP > NPP always.

Net Production of Consumers

Net Production of Consumers (N)

The energy available for growth and reproduction in consumers (animals). It is what remains from ingested food after faecal and urinary losses and respiratory losses.

N = I − (F + R)

I = energy ingested | F = energy lost in faeces and urine | R = respiratory losses

Breaking down the equation

I (ingested): total energy taken in when food is eaten

F (faeces/urine): energy in undigested/unabsorbed material — never assimilated

I − F = assimilated energy: what actually enters the organism's cells

R (respiration): energy released as heat — lost to the environment

N = assimilated − R: energy stored as new biomass (growth + reproduction)

Worked Calculations

Example 1 — NPP

A grass ecosystem has a GPP of 87 000 kJ m⁻² yr⁻¹. Respiratory losses (R) = 28 000 kJ m⁻² yr⁻¹.

NPP = GPP − R = 87 000 − 28 000 = 59 000 kJ m⁻² yr⁻¹

Example 2 — Net production of a consumer

A rabbit ingests 10 000 kJ. Faecal + urinary losses = 4 000 kJ. Respiratory losses = 4 500 kJ.

N = I − (F + R) = 10 000 − (4 000 + 4 500) = 10 000 − 8 500 = 1 500 kJ

Example 3 — Efficiency of energy transfer

Energy at trophic level 2 = 59 000 kJ m⁻² yr⁻¹. Energy at trophic level 1 = 590 000 kJ m⁻² yr⁻¹.

Efficiency = (59 000 ÷ 590 000) × 100 = 10%

Always include units

Units (kJ ha⁻¹ yr⁻¹ or kJ m⁻² yr⁻¹) are frequently omitted and cost marks. Show all working. AQA mark schemes specifically award a mark for correct units.

Why Energy Transfer Is Inefficient

Only approximately 10% of energy at one trophic level is available to the next. The other ~90% is lost because:

Reason for lossExplanation

Respiratory heat lossEnergy used in cellular respiration is released as heat — cannot be used by the next trophic level

Not all consumedSome biomass (e.g. roots, bones, bark) is not eaten — passed to decomposers instead

Faecal lossesNot all ingested material is digested and absorbed — undigested material is egested

Urine lossesNitrogenous waste excreted — represents energy-containing molecules lost from the consumer

Exam Tip

The question will often ask you to "explain why energy transfer between trophic levels is inefficient." Give at least two distinct reasons with the mechanism — e.g. "energy is lost as heat in respiration" AND "not all biomass is consumed." Don't just say "energy is lost."

Energy Flow Through a Trophic Level

Farming: Increasing Energy Transfer Efficiency

Farmers aim to maximise the energy available to humans by reducing losses at each step. AQA requires two broad strategies:

Strategy 1 — Simplifying Food Webs

Reducing energy losses to non-human food chains by eliminating organisms that compete with humans or livestock:

Pesticides: kill pest organisms that consume crop plants — keeps more NPP in the human food chain

Herbicides: kill weeds that compete with crops for light, water and minerals — more energy fixed by the crop

Strategy 2 — Reducing Respiratory Losses

Reducing the energy livestock use in their own respiration, so more ingested energy becomes net production (N increases):

Keeping animals indoors / insulating barns: reduces energy used maintaining body temperature — less heat lost to environment

Restricting movement: reduces energy spent on locomotion — more energy available for growth

Shorter Food Chains = More Efficient

Each additional trophic level loses approximately 90% of energy. Shorter food chains therefore transfer more energy to humans.

Numerical example

Start with 10 000 kJ fixed by plants:

After 1 transfer (plant → human): ~1 000 kJ available

After 2 transfers (plant → cow → human): ~100 kJ available

Eating plant crops directly (one transfer) provides 10× more energy to humans than eating animals that fed on those crops (two transfers). This is why plant-based food production is more efficient per hectare of land.

Synoptic Link — Biodiversity (3.4.6)

Intensive farming practices (pesticides, monocultures) that increase energy transfer efficiency can reduce biodiversity. AQA Paper 3 essays often ask students to evaluate the tension between maximising food production and maintaining ecosystem biodiversity.

Productivity

The rate of production of new biomass. Measured as biomass (or energy) produced in a given area in a given time.

Units: kJ ha⁻¹ yr⁻¹ or g m⁻² yr⁻¹

Primary productivity

Rate at which producers (plants) convert light energy into chemical energy in biomass. GPP and NPP are both measures of primary productivity.

Secondary productivity

Rate at which consumers build new biomass. Calculated using N = I − (F + R), where N is the energy stored as new consumer biomass per unit time.

Exam Tip

When asked to calculate productivity or efficiency, always state the formula first, substitute values with units, then give the answer with units. Missing units in a calculation question can lose a mark even if the number is correct.

DEEP Framework: Extended Answer

A common AQA extended response: "Explain why energy transfer between trophic levels is inefficient and describe how farmers can improve this."

DEEP Chain — Energy Transfer Efficiency

D

Define: Net primary production (NPP = GPP − R) is the energy available to the next trophic level. Net production of consumers (N = I − (F + R)) determines how much energy becomes new biomass at each level. Only approximately 10% of energy is transferred between trophic levels.

E

Explain mechanism: Energy is lost at each trophic level because: (1) organisms use energy in respiration — released as heat and unavailable to the next level; (2) not all biomass is consumed — some passes to decomposers; (3) not all ingested material is assimilated — undigested matter is egested in faeces; (4) energy is lost in urine.

E

Evidence/Example: Starting with 10 000 kJ fixed by grass, only ~1 000 kJ is available to a herbivore and ~100 kJ to a carnivore. Eating meat requires ~10× the land area of eating crops directly for the same calorie intake.

P

Practical/Farming: Farmers reduce respiratory losses by housing livestock indoors (reducing thermoregulatory costs) and restricting movement. They simplify food webs using pesticides and herbicides, keeping more NPP within the human food chain. Shorter food chains deliver more energy per hectare — eating crops directly is approximately 10× more energy-efficient than eating animals fed on those crops.

Practice Question

A wheat field has a gross primary production of 64 000 kJ m⁻² yr⁻¹. Respiratory losses by the wheat are 24 000 kJ m⁻² yr⁻¹. A farmer keeps cattle that graze on the wheat. Each cow ingests 45 000 kJ yr⁻¹, loses 18 000 kJ yr⁻¹ in faeces and urine, and 21 000 kJ yr⁻¹ in respiration.

(a) Calculate the NPP of the wheat field. [1 mark]

(b) Calculate the net production of each cow. [2 marks]

(c) Explain one way the farmer could increase the net production of the cattle. [2 marks]

a:

NPP = GPP − R = 64 000 − 24 000 = 40 000 kJ m⁻² yr⁻¹

b:

N = I − (F + R) = 45 000 − (18 000 + 21 000) = 45 000 − 39 000 = 6 000 kJ yr⁻¹ per cow

c:

Housing the cattle indoors in insulated barns reduces heat lost to the environment via thermoregulation — so less energy is lost in respiration and more ingested energy is available for growth (net production increases). Alternatively: restricting movement reduces respiratory losses from locomotion, increasing net production.

Examiner Alerts: Common Errors

Error 1: Confusing GPP and NPP

GPP is the total energy fixed. NPP is what remains after plant respiration. GPP is always larger than NPP. Only NPP is available to herbivores — not GPP.

Error 2: Missing units in calculations

Always include units: kJ m⁻² yr⁻¹ or kJ ha⁻¹ yr⁻¹. Writing a number without units in a productivity calculation loses the mark for that answer. State the formula before substituting values.

Error 3: Vague reasons for inefficiency

Do not write "energy is lost at each trophic level" without specifying how. Must state: "energy is released as heat in respiration" and/or "not all biomass is consumed" and/or "not all ingested food is assimilated — some is egested in faeces."

Error 4: Farming question — wrong mechanism

When explaining how housing animals indoors increases net production, students often write "so they get more food." The correct mechanism is: reduces respiratory heat loss → less energy lost in respiration → more energy available for growth (N increases).

Synoptic Links

Synoptic Link — Respiration (3.5.2)

The R in both NPP = GPP − R and N = I − (F + R) refers to energy lost as heat via aerobic respiration. Understanding why respiratory heat is irrecoverable (second law of thermodynamics; heat is not chemical energy) connects directly to Lesson 5.04's coverage of oxidative phosphorylation efficiency.

Synoptic Link — Nutrient Cycles (3.5.4)

The biomass not consumed by the next trophic level does not disappear — it enters the decomposer food chain. Saprobionts break down dead organic matter, recycling nutrients back into the ecosystem. This connects directly to Lesson 5.06 (nitrogen and phosphorus cycles).

Synoptic Link — Homeostasis & Thermoregulation (3.6.4)

Endothermic (warm-blooded) animals use a large proportion of their assimilated energy maintaining constant body temperature via respiration. This is why R is large in mammals and birds relative to ectotherms — and why insulating livestock barns reduces their respiratory losses significantly.

Summary

Key Equations

EquationWhat it means

NPP = GPP − REnergy available to next trophic level after plant respiration

N = I − (F + R)Energy stored as new consumer biomass (growth + reproduction)

Efficiency = (output ÷ input) × 100Percentage of energy transferred between trophic levels (~10%)

Why ~10% efficiency

Respiratory heat loss, uneaten biomass, faecal losses, urinary losses — all prevent full transfer of energy between trophic levels.

Farming strategies

Simplify food webs (pesticides, herbicides) AND reduce respiratory losses (indoor housing, restricted movement, selective breeding).

Exam Tip

Every calculation question: write the formula → substitute with units → calculate → state answer with units. Explain inefficiency with mechanism, not just "energy is lost." Describe farming strategies with the biological reason why they work.