Feeding Relationships

A trophic level is a position in a food chain, defined by how the organism gets its energy.

Trophic level	Name	Description	Example
1	Producer	Autotrophs — make their own food by photosynthesis	Grass, oak tree, phytoplankton
2	Primary consumer	Herbivore — eats producers	Rabbit, cow, caterpillar, zooplankton
3	Secondary consumer	Carnivore — eats primary consumers	Frog (eats insects), small fish
4	Tertiary consumer	Carnivore — eats secondary consumers	Hawk (eats frog), shark
Side chain	Decomposer	Bacteria + fungi — break down dead matter	Mushrooms, soil bacteria

Producers form the BASE of every food chain. Energy enters the chain only through producers, who CAPTURE light energy from the Sun and FIX it into glucose during photosynthesis.

Food chains use ARROWS to show ENERGY FLOW:

grass → grasshopper → frog → snake → hawk

The arrows point FROM the prey TO the predator (because energy/atoms flow this way).

Food webs. Most real ecosystems are not simple chains — each organism has multiple prey and multiple predators. A food web is a network of overlapping food chains.

Example:

• grass → rabbit → fox
• grass → mouse → fox
• grass → mouse → owl

If one species is removed (e.g. rabbits die from disease), the effects ripple through the web — foxes lose food and may decline; mice may benefit from less competition; grass may grow more abundantly. Food-web questions test students' ability to TRACE these effects.

Decomposers. Bacteria and fungi that break down DEAD organisms and waste using extracellular enzymes. They recycle nutrients (carbon, nitrogen) back into the ecosystem. Some decomposers (e.g. earthworms, woodlice) are eaten by larger animals → forming a parallel detritus food chain.

Key idea: energy is NOT recycled — only atoms are.

• Energy flows through the ecosystem in ONE DIRECTION: sunlight → producer → consumer → consumer → heat (lost to space).
• Atoms / nutrients (C, N, P, etc.) are RECYCLED — passed between organisms and the environment in cyclical processes (carbon cycle, nitrogen cycle).

Why energy is non-cyclical:

• Each step in a food chain CONVERTS some chemical energy to HEAT (mostly via respiration).
• Heat is lost to the environment and ultimately to space.
• Heat cannot be 'used' by organisms to grow — they can't re-capture it.
• So the only way to keep the ecosystem running is for the Sun to KEEP supplying fresh energy via photosynthesis.

Why atoms cycle:

• Atoms (C, N) cannot be created or destroyed — they only change form.
• Carbon atoms in CO₂ → glucose (photosynthesis) → animal proteins → CO₂ (respiration / decomposition) → back to atmosphere.
• Nitrogen atoms in N₂ → ammonia (fixation) → nitrate → plant proteins → animal proteins → urea → ammonia (decomposition) → back to N₂ (denitrification).

Implication. An ecosystem cut off from sunlight (e.g. an underground bunker) would run down quickly — decomposers would respire all the chemical energy and convert it to heat. Only ecosystems near hydrothermal vents survive without sunlight, by relying on chemosynthesis instead.

Only about 10 % of the energy in one trophic level is passed up to the next. The rest (~90 %) is lost for FIVE main reasons:

1. Respiration. Each organism RESPIRES the food it eats to release energy as ATP, which it uses for movement, growth, and (in endotherms) body temperature. This energy is eventually lost as HEAT to the environment. Endotherms (mammals, birds) lose A LOT to maintain body temperature.

2. Egestion — faeces. Not all food is digested. Indigestible material (cellulose in plant cell walls for carnivores; bones, fur, feathers) passes through the gut and is EGESTED as faeces. The energy in faeces goes to decomposers, not up the chain.

3. Excretion — urine. Excess amino acids are broken down (deamination in liver) → urea, which is excreted in urine. The chemical energy in urea is lost from the food chain.

4. Heat lost from respiration. Mammals and birds (endotherms) generate body heat from respiration → this heat radiates away → not available to the predator. Cold-blooded animals (reptiles, fish) lose less, so are slightly more energy-efficient.

5. Parts not eaten. The predator doesn't usually eat ALL of the prey — bones, skin, horns, leaves on a tree the herbivore doesn't reach. This energy goes to decomposers.

The 10 % rule of thumb. A consumer typically gains about 10 % of the energy available in its food. The actual number varies:

• Producers capture only ~1-2 % of incident sunlight (most reflected, lost as heat, in unused wavelengths).
• Herbivores: ~5-10 % efficient.
• Carnivores: ~10-20 % efficient (meat is more digestible than plant cellulose).

Implication. A food chain rarely has more than 4-5 trophic levels — at each level, energy reduces ~10×, so by level 5 only 0.01 % of producer energy remains. There simply isn't enough energy for a level-6 super-carnivore.

Edexcel may give you data and ask you to calculate the % efficiency of energy transfer between two trophic levels.

The formula:

$\%\text{ efficiency} = \dfrac{\text{energy transferred to next level}}{\text{total energy in current level}} \times 100\%$

Worked example. Producer captures 10 000 kJ m⁻² yr⁻¹ of energy. Primary consumer gains 800 kJ m⁻² yr⁻¹.

• Efficiency = (800 / 10 000) × 100 = 8 %.

Worked example 2 (two transfers).

• Producer: 10 000 kJ; primary: 800 kJ; secondary: 80 kJ.
• Producer → primary efficiency = 800 / 10 000 × 100 = 8 %.
• Primary → secondary efficiency = 80 / 800 × 100 = 10 %.
• Carnivores (secondary consumers) are slightly more efficient than herbivores at converting their food to body mass — meat is more digestible than plant matter.

Marks scheme points to remember:

• Always express as a PERCENTAGE (× 100 + %).
• Show your working — you can get marks for the formula even if the arithmetic is off.
• Round to 1-2 significant figures unless told otherwise.

Bigger picture — energy budget. Some textbooks split the input energy into:

• Gross primary production (GPP) = total energy captured by photosynthesis.
• Net primary production (NPP) = GPP minus respiration. This is the energy actually available to herbivores.
• Similarly, consumer efficiency = secondary production / energy intake.

A pyramid is a stack of horizontal bars, each representing one trophic level. Producer at the BOTTOM. The width of each bar shows a different quantity in each pyramid type.

1. Pyramid of NUMBERS.

• What it shows: the NUMBER of individual organisms at each trophic level.
• Strength: quick — just count.
• Limitation: doesn't account for SIZE. ONE oak tree (huge biomass) supports thousands of small caterpillars → an INVERTED pyramid of numbers (narrow at producer, wide at consumer). Same problem with parasites (a few hosts → many parasites).

Example (inverted):

• Oak tree: 1 organism.
• Caterpillars: 10 000.
• Blue tits: 4.
• Hawk: 1.

2. Pyramid of BIOMASS.

• What it shows: the DRY MASS of organisms at each trophic level (usually g m⁻² or kg ha⁻¹).
• Strength: corrects for the size problem. Usually a TRUE PYRAMID — biomass decreases ~10× per level.
• Limitation: measured at ONE moment in time. In OPEN OCEANS, fast-reproducing phytoplankton (producers) are eaten as fast as they grow → at any instant, more zooplankton biomass than phytoplankton biomass → INVERTED pyramid. Also requires killing organisms to measure dry mass accurately.

3. Pyramid of ENERGY.

• What it shows: the ENERGY flow through each trophic level over a period (usually kJ m⁻² yr⁻¹).
• Strength: ALWAYS pyramid-shaped — the second law of thermodynamics guarantees energy cannot increase up a chain.
• Limitation: difficult and time-consuming to measure — requires measuring respiration rates, biomass changes and feeding rates over time. Less commonly used in school.

Summary:

Pyramid type	Always pyramid?	Easy to measure?	Best for
Numbers	NO — can invert	Yes	Quick comparisons
Biomass	Usually	Moderate	Standing-stock comparisons
Energy	YES — always	Hard	True energy flow

Edexcel exam tip. When asked which is 'the best' pyramid, the answer is usually pyramid of ENERGY because it ALWAYS shows the correct shape and reflects the actual flow of energy through the ecosystem.

Energy loss at each trophic level has direct implications for how we feed the world.

Eating plants directly:

SUN → PLANT → HUMAN.

• ONE energy transfer.
• Human gets ~10 % of the plant's energy.

Eating meat (e.g. beef):

SUN → PLANT → CATTLE → HUMAN.

• TWO energy transfers.
• Human gets ~10 % of ~10 % = ~1 % of the plant's energy.

The numbers: the same field area produces ~10× more food energy as crops than as beef. A vegetarian diet has a much smaller land, water and energy footprint than a meat-based diet.

Practical examples:

1. Wheat vs beef. 100 ha of wheat can feed ~1000 people for a year. 100 ha of beef (cattle grazing) can feed ~100 people for a year.

2. Animal differences. Cattle (large endotherms, slow reproducers) are particularly inefficient. Chicken and farmed fish are MORE efficient (smaller, faster reproduction, less heat loss). Insects (cricket flour) are ~10× more efficient than cattle and are increasingly being trialled as protein sources.

3. Intensive farming. Indoor cattle / chicken farms reduce energy loss by limiting movement (less respiration), controlling temperature (less heat loss), and feeding processed food (more digestible). This makes meat production more efficient but raises ethical and environmental concerns.

4. Aquaculture. Fish farming is more energy-efficient than catching wild fish — but generates pollution and disease risks.

Implication for global food security. As the world population grows and faces climate change, shifting toward more plant-based diets is increasingly seen as essential — same land area, more food, lower greenhouse-gas emissions.

Counter-arguments.

• Meat provides high-quality protein and certain nutrients (iron, B12, omega-3) in concentrated forms.
• Some land (steep, dry, infertile) is suitable only for grazing — not crops.
• Traditional pastoral cultures depend on livestock for livelihood.
• Vegetable proteins (soy, nuts, pulses) can substitute most nutrients but require careful diet planning.

Conclusion. From an energy-efficiency perspective, plant-based diets are far more efficient. The world cannot feed 10 billion people with current per-capita meat consumption. Reducing meat consumption (especially beef) is one of the most impactful changes for sustainable food production.