Improving Energy Transfer in Agriculture Revision Notes | A Level AQA Biology

1. Why Energy Is Lost at Every Step

Picture a wheat field. The sun pours energy into those plants, yet by the time that energy reaches a human eating bread, the vast majority has vanished. Agriculture is essentially the science of plugging those leaks — and understanding where energy goes is the first step to keeping more of it in the human food chain.

At every trophic level, energy is lost through respiration, excretion, and the parts of organisms that are not consumed. On average, only around 10% of the energy stored at one trophic level passes to the next. That figure is not fixed — farming practices can push it significantly higher, and that is exactly what this lesson is about.

Net productivity is the figure that actually matters to a farmer: it represents the energy available to the next trophic level — or to harvest. Gross productivity tells you how much energy an organism takes in; net productivity tells you how much it keeps. The difference is respiration, and reducing that difference is one of the central goals of efficient agriculture.

2. Simplifying Food Webs to Reduce Energy Losses

A natural ecosystem contains many competing food chains. Energy flows not only towards humans but also into insects, birds, fungi, bacteria, and countless other organisms that have no agricultural value. Every time energy diverts into one of these non-human pathways, it is lost from the human food chain permanently.

Simplifying the food web means removing or suppressing those competing pathways. In practice, this involves using pesticides to kill herbivorous insects that would otherwise consume crop biomass, using herbicides to eliminate weeds that compete with crops for light and mineral ions, and using fungicides to prevent fungal pathogens from infecting and destroying crop biomass before harvest, reducing energy lost to fungal growth.

The logic is straightforward: if a pest eats 20% of a wheat crop, that 20% of fixed energy never reaches the human consumer. Removing the pest keeps that energy in the human food chain. The fewer trophic levels energy passes through before reaching humans, the more energy is retained — which is also why eating plant material directly is more energy-efficient than eating animals that have themselves eaten plants.

• Pesticides: reduce energy diverted to herbivorous pest populations
• Herbicides: eliminate weeds competing with crops for resources, keeping more photosynthate in the crop
• Fungicides: prevent fungal decomposition of crop biomass before harvest
• Monocultures: growing a single crop species maximises the proportion of primary productivity directed towards human food

Reducing the number of trophic levels between the producer and the human consumer is another powerful strategy. A food chain of grass → human involves only one energy transfer; grass → cow → human involves two. Because each transfer loses roughly 90% of the energy, eating one trophic level lower can increase the energy available to humans by an order of magnitude. This is why plant-based diets are inherently more energy-efficient at a population level.

3. Reducing Respiratory Losses Within the Human Food Chain

Even within the food chains that do lead to humans, energy is lost through the respiration of the organisms being farmed. A cow kept outdoors in a cold climate uses a large proportion of its assimilated energy just to maintain its body temperature — energy that could otherwise have been stored as muscle (meat) or milk. Reducing these respiratory losses is the second major strategy for improving energy transfer efficiency.

Intensive livestock farming addresses this directly. By housing animals indoors, farmers reduce the energy animals expend on thermoregulation. By restricting movement — keeping animals in confined spaces — farmers reduce the energy animals expend on locomotion. Both strategies mean a greater proportion of assimilated energy is channelled into growth, increasing net productivity and therefore the yield available to humans.

• Housing animals indoors: reduces heat loss to the environment, lowering thermoregulatory energy expenditure
• Restricting movement: reduces energy used in locomotion, directing more assimilated energy into growth
• Selective breeding: produces animals with higher growth rates or feed-conversion ratios, improving net productivity per unit of feed
• Optimising diet: providing nutritionally complete feed reduces the energy wasted on digesting low-quality material

It is worth being precise here: these practices increase net productivity by reducing the respiratory component of the net productivity equation. The gross productivity (total energy assimilated from food) may remain similar, but because less is lost to respiration, more is stored — and that stored biomass is what the farmer harvests.

4. Calculating Net Productivity from Data

Exam questions on this topic almost always ask you to calculate net productivity, energy transfer efficiency, or to compare the productivity of producers and consumers. The formula is always the same, but the data can be presented in different ways — so let's work through the method carefully.

Worked example: Calculating Net Productivity and Energy Transfer Efficiency

In a grassland ecosystem, the gross productivity of the grass is 87,000 kJ m⁻² yr⁻¹. The grass uses 37,000 kJ m⁻² yr⁻¹ in respiration. The net productivity of the cattle grazing on the grass is 4,500 kJ m⁻² yr⁻¹. Calculate (a) the net productivity of the grass and (b) the efficiency of energy transfer from grass to cattle.

1. $$\text{Net productivity} = \text{Gross productivity} - \text{Respiratory losses}$$ — Always state the formula first — this earns the method mark [M1] in an exam.
2. $$\text{Net productivity of grass} = 87{,}000 - 37{,}000$$ — Substitute gross productivity = 87,000 kJ m⁻² yr⁻¹ and respiratory losses = 37,000 kJ m⁻² yr⁻¹. [M1]
3. $$\text{Net productivity of grass} = 50{,}000 \text{ kJ m}^{-2} \text{ yr}^{-1}$$ — 87,000 − 37,000 = 50,000. This is the energy stored in grass biomass and available to the next trophic level. [A1]
4. $$\text{Efficiency (\%)} = \frac{\text{Net productivity of cattle}}{\text{Net productivity of grass}} \times 100$$ — Now move to part (b). State the efficiency formula clearly. [M1]
5. $$\text{Efficiency} = \frac{4{,}500}{50{,}000} \times 100$$ — Net productivity of cattle = 4,500 kJ m⁻² yr⁻¹; net productivity of grass = 50,000 kJ m⁻² yr⁻¹. [M1]
6. $$\text{Efficiency} = 0.09 \times 100 = 9\%$$ — 4,500 ÷ 50,000 = 0.09; multiplied by 100 gives 9%. This means only 9% of the energy stored in grass biomass is transferred to cattle biomass. [A1]

Answer: (a) Net productivity of grass = 50,000 kJ m⁻² yr⁻¹. (b) Energy transfer efficiency from grass to cattle = 9%.

5. Evaluating Farming Practices That Improve Energy Transfer

Now that the underlying biology is clear, it is worth stepping back to evaluate these practices as a whole. AQA expects you to appreciate not only how these methods work but also their limitations and trade-offs — this is the kind of higher-order thinking that separates a good answer from a top-band answer.

Notice that every practice has a biological justification rooted in the net productivity equation. Pesticides and herbicides act on the food web, reducing the number of competing energy pathways. Indoor housing and movement restriction act on the respiratory losses term, keeping more assimilated energy in the biomass available for harvest. Both routes lead to the same outcome: more energy reaching the human consumer per unit of land.

When evaluating these practices in an exam, structure your response around the biological mechanism first, then address the limitation. A common error is to discuss ethics without linking back to the biology — examiners want to see that you understand why the practice works before you assess whether it should be used.

6. Exam Technique and Common Mistakes

Questions on this topic tend to fall into three categories: calculation questions, explain/evaluate questions, and data-interpretation questions. Each requires a slightly different approach, and knowing the distinction before you open the paper saves valuable time.

Every concept in this lesson connects back to the same underlying principle: energy is finite, and every transfer loses some of it. Agriculture is the art of minimising those losses — whether by shortening the food chain, removing competing organisms, or reducing the respiratory demands of farmed animals. Master that principle, and every exam question on this topic becomes a variation on the same theme.