Lesson 13: Energy Transfers in Ecosystems

Energy flows through ecosystems in one direction — from the Sun through producers to consumers, dissipating as heat at every step — while matter cycles endlessly. Understanding energy transfer explains why food chains are short, why farming works the way it does, and how human activity reshapes the biosphere's energy budget. This lesson covers trophic levels and food webs, pyramids of number/biomass/energy, calculating transfer efficiency, the losses at each stage, decomposition and detritus food chains, and the energetics of agriculture — intensive farming, pest control and sustainability.

Trophic levels and food webs

Trophic levels: producers (photosynthetic — plants, algae, some bacteria), primary consumers (herbivores), secondary consumers (carnivores eating herbivores), tertiary consumers (top carnivores), and decomposers/detritivores (breaking down dead matter and faeces). Food chains show single pathways; food webs show the interconnected reality — most organisms eat (and are eaten by) several species. Arrows point in the direction of energy flow (from food to feeder) — a common diagram error to avoid.

Decomposers (bacteria, fungi — saprobionts) and detritivores (earthworms, woodlice) process dead organic matter, releasing nutrients for producers (Lesson 14) and respiring much of the energy as heat. The detritus food chain (dead leaves → earthworms → blackbirds) often carries more energy than the grazing chain in woodlands. Exam questions ask you to construct webs from species lists and to predict the effects of removing a species — trace the consequences through all connected pathways, noting that alternative food sources buffer the impact.

The ecological niche encompasses a species' role: its food, habitat, behaviour and interactions. Competition (interspecific and intraspecific) for limited resources shapes niches; the competitive-exclusion principle states that two species cannot occupy the identical niche indefinitely. Niche questions ask "explain" — link the resource, the competitors and the outcome (exclusion, partitioning, or coexistence via slightly different niches).

Pyramids of number, biomass and energy

Pyramids of number show individuals per trophic level — but can invert (one oak → thousands of insects → few birds), because size differences distort the picture. Pyramids of biomass (dry mass per unit area, g m⁻²) are better but still distort: a small standing crop of fast-reproducing phytoplankton can support a larger zooplankton biomass (an inverted pyramid in marine systems). Pyramids of energy (kJ m⁻² year⁻¹) never invert — energy is always lost between levels, so each bar is narrower than the one below.

Measuring biomass: sample, dry to constant mass (removing variable water content), weigh — destructive but accurate. Measuring energy: burn samples in a bomb calorimeter. Productivity is the rate of biomass/energy production: gross primary productivity (GPP — total photosynthesis), net primary productivity (NPP = GPP − respiratory losses — the energy available to herbivores); for consumers, net production = energy ingested − (energy lost in faeces + respiration + excretion). Learn these definitions precisely — "distinguish between GPP and NPP" is a standard 2-marker.

Exam technique: when given data, choose the pyramid type that answers the question — energy pyramids for transfer efficiency, biomass for standing crop, numbers for population structure. Explain inversions explicitly ("the pyramid of number inverts because one large producer supports many small consumers") rather than calling them errors.

Energy losses and transfer efficiency

Only ~10% of energy transfers between trophic levels (the "10% rule" — actually ~5–20% depending on the system). Losses: not all of one level is eaten (roots, bones, thorns uneaten); not all eaten is digested (egested as faeces — cellulose-rich diets are poorly digested); digested energy is largely respired (movement, heat, basal metabolism — endotherms lose far more than ectotherms); some is excreted (urea, urine). Net production efficiency = (net production ÷ energy ingested) × 100.

Calculations: given 20,000 kJ m⁻² year⁻¹ in producers and 1,800 in primary consumers, efficiency = 1,800 ÷ 20,000 × 100 = 9%. Food chains are short (rarely >5 levels) because energy dwindles exponentially: 100% → 10% → 1% → 0.1% — insufficient to support another viable population. Top predators are therefore rare and vulnerable (biomagnification concentrates toxins in them — DDT, mercury). "Explain why" questions need the loss mechanisms listed with the compounding effect across levels.

Endotherms vs ectotherms: mammals and birds respire roughly 90% or more of their assimilated energy maintaining body temperature, so their production efficiency is only ~1–3%; ectotherms (fish, insects) convert ~10–20% (some insects far more). This is why aquaculture of fish and shellfish is energetically efficient and beef production is not — a single physiological fact with huge agricultural consequences. Quantify where data allow.

Decomposition and nutrient release

Decomposition: detritivores fragment litter (increasing surface area), saprobiontic bacteria and fungi secrete extracellular enzymes digesting it externally, absorb the soluble products, and respire — releasing CO₂, water and mineral ions (nitrates, phosphates) for producers. Temperature, moisture and oxygen control the rate: warm, moist, aerobic conditions maximise decomposer activity; cold, dry or waterlogged conditions (peat bogs) preserve organic matter for millennia.

Investigations measure decomposition via mass loss of leaf litter in bags, CO₂ evolution, or temperature rise in compost. Variables: leaf species (lignin content resists decay), temperature, moisture. The data link to nutrient cycling (Lesson 14): faster decomposition → faster nutrient release → higher productivity — explaining why many tropical soils are poor (nutrients held in the biomass and leached rapidly once released) despite lush growth.

Composting applies the science: turning aerates the heap (aerobic decomposers work faster than anaerobes), the C:N ratio is balanced (~25:1 — too much carbon slows decay; too much nitrogen loses ammonia), and moisture is maintained. The heat generated (up to 60–70 °C) kills pathogens and weed seeds. "Advise a gardener" questions apply the decomposition factors practically.

Agriculture — energetics, pests and sustainability

Farming manipulates energy flow for human benefit. Intensive livestock production: animals are kept warm and inactive (reducing respiratory losses), fed high-energy diets, and slaughtered young (before maintenance costs accumulate) — maximising the fraction of feed energy converted to meat. But the trophic inefficiency remains: ~10 kg of grain protein makes ~1 kg of beef protein. Eating producers directly (crops) is energetically ~10× more efficient than eating herbivores — the core argument for plant-rich diets, quantified via transfer efficiencies.

Pest control: chemical pesticides (fast, effective, but resistance evolves — Lesson 7; biomagnification; harm to pollinators), biological control (predators/parasites/pathogens — e.g. ladybirds vs aphids, Bacillus thuringiensis; self-sustaining but slow and specific) and integrated pest management (combining methods with monitoring and economic thresholds). Evaluate each on effectiveness, cost, environmental impact and sustainability — IPM is usually the exam's preferred conclusion.

Fertilisers replace the nutrients exported in harvests: artificial (Haber ammonia — energy-intensive, precise, but leaching causes eutrophication) vs natural (manure — slow-release, improves soil structure, but bulky and variable). Sustainability strategies: crop rotation (legumes fix nitrogen), reduced tillage (protects soil structure), precision farming (targeted inputs) and agroforestry. The evaluation framework: productivity vs environmental cost vs long-term soil health — strong answers recognise that "sustainable" means maintaining yields without degrading the resource base.

Worked examples — transfer calculations

Worked example 1: full transfer-efficiency calculation. A field's producers fix 24,000 kJ m⁻² year⁻¹ (GPP); they respire 14,000, leaving NPP = 10,000. Primary consumers ingest 1,200 (not all producers are eaten), egest 400, respire 600, leaving net production = 1,200 − 400 − 600 = 200. Transfer efficiency (NPP → consumer net production) = 200 ÷ 10,000 × 100 = 2%. Explain the losses at each step: uneaten (roots, stems), egested (indigestible cellulose), respired (movement, heat — herbivore endotherms). Low efficiencies compound: only 2% of NPP becomes herbivore biomass, and a carnivore level would capture ~2% of that.

Worked example 2: why a biomass pyramid can invert. In the English Channel, phytoplankton biomass at any moment (~4 g m⁻²) is less than zooplankton biomass (~6 g m⁻²) — an inverted pyramid of biomass. Explanation: phytoplankton reproduce extremely fast (populations can double daily), so a small standing crop supports a larger zooplankton population through rapid turnover; the pyramid of energy (productivity, a rate) is never inverted. "Explain" answers must distinguish standing crop (biomass at one instant) from productivity (rate of production) — the inversion is a snapshot artefact, not an energy violation.

Worked example 3: the meat-eating calculation. Producing 1 kg of beef protein needs roughly 10 kg of feed protein (at ~10% transfer efficiency); producing 1 kg of wheat protein needs ~1 kg of inputs, because wheat is the producer. Feeding 100 people on beef therefore requires roughly 10× the land of feeding them on wheat. Extend the logic: aquaculture of ectothermic fish (~15% efficiency) outperforms beef on the same arithmetic. This single efficiency fact is why dietary shifts are among the most powerful environmental levers — and why examiners love it in evaluation questions.

Common mistakes

• Arrows pointing the wrong way. In food webs, arrows show energy flow: food → feeder (grass → rabbit, not rabbit → grass). One reversed arrow can void an entire web question.
• "Energy is recycled." It is not — energy flows one way and leaves as heat; nutrients are recycled. Confusing the two is the most heavily penalised conceptual error in ecology.
• GPP/NPP mix-ups. NPP = GPP − respiration, and it is NPP (not GPP) that is available to the next trophic level. Saying herbivores "eat the GPP" double-counts the producers' own respiration.
• Pyramids of energy "sometimes invert." They never do — if your energy pyramid widens upward, you have confused it with biomass (standing crop) or numbers. State the units: energy pyramids are rates (kJ m⁻² year⁻¹).
• Efficiency without a formula. Always write efficiency = (energy at higher level ÷ energy at lower level) × 100 and show the substitution; a bare percentage forfeits the method mark.

Exam Experience

Q1 (data). A grassland's producers have GPP 30,000 kJ m⁻² year⁻¹ and respire 18,000. Herbivore net production is 600. (a) Calculate NPP and the transfer efficiency to herbivore net production. (b) Give two reasons the efficiency is so low. [5 marks] Q2. Explain why pyramids of energy are never inverted, while pyramids of biomass sometimes are. [4 marks]

Mark scheme — Q1: (a) NPP = 30,000 − 18,000 = 12,000 [1]; efficiency = 600 ÷ 12,000 × 100 = 5% [1]. (b) Any two: much producer material is never eaten (roots, woody tissue) [1]; much eaten material is egested undigested [1]; most assimilated energy is respired (movement, thermoregulation in endotherm herbivores) or excreted [1].

Mark scheme — Q2: Energy transfer between levels is always lossy (respiration, egestion, excretion), so the rate of energy production must fall at each level — an energy pyramid cannot widen [1][1]. Biomass is a standing crop at one instant [1]; a fast-turnover producer (phytoplankton doubling daily) can hold less standing biomass than the consumers it continuously feeds [1].