Energy Transfer Through Ecosystems

Energy flows through ecosystems in a single direction — from the sun, through producers, to consumers, and eventually lost as heat. Unlike nutrients, energy is not recycled; it must be continuously input from solar radiation. Understanding energy transfer is essential for A-Level Biology, linking the biochemistry of photosynthesis and respiration to ecology, productivity, and human food production.

Key Definition: Energy transfer in an ecosystem describes the flow of energy from one trophic level to the next, with losses at each stage mainly through respiration, excretion, and egestion.

---

Trophic Levels

Organisms in an ecosystem can be classified by their trophic level — their feeding position in a food chain:

Trophic Level	Organisms	Energy Source
T1: Producers	Green plants, algae, photosynthetic bacteria	Sunlight (photosynthesis)
T2: Primary consumers	Herbivores	Producers
T3: Secondary consumers	Carnivores that eat herbivores	Primary consumers
T4: Tertiary consumers	Top carnivores	Secondary consumers
Decomposers	Bacteria, fungi	Dead organic matter at all levels

Energy is transferred along food chains and food webs, but only a small proportion of the energy at one trophic level is transferred to the next.

---

Gross Primary Productivity (GPP)

Key Definition: Gross primary productivity (GPP) is the total rate of energy fixation (photosynthesis) by producers in an ecosystem. It represents the total amount of chemical energy converted from light energy by photosynthesis per unit area per unit time.

• GPP includes all the glucose (and other organic molecules) produced by photosynthesis.
• Units: kJ m⁻² year⁻¹ (energy per unit area per unit time).

---

Net Primary Productivity (NPP)

Not all the energy fixed by producers is available to primary consumers. Producers use some of the energy they fix for their own respiration (R) — to maintain their cells, grow, reproduce, and carry out active transport.

Key Definition: Net primary productivity (NPP) is the rate of energy storage by producers after respiratory losses have been subtracted. It represents the energy available to primary consumers (and decomposers).

Formula

NPP = GPP − R

Where:

• NPP = net primary productivity.
• GPP = gross primary productivity.
• R = respiratory losses (energy used by the plant for its own metabolism).

Example

If a field of wheat has a GPP of 20,000 kJ m⁻² year⁻¹ and the plants use 12,000 kJ m⁻² year⁻¹ for respiration:

NPP = 20,000 − 12,000 = 8,000 kJ m⁻² year⁻¹

This means 8,000 kJ m⁻² year⁻¹ is available as biomass that can be consumed by herbivores (or harvested by humans).

---

Energy Transfer Between Trophic Levels

When a primary consumer (herbivore) eats a producer, not all the energy in the producer is transferred to the consumer. Energy is lost at each transfer:

Sources of Energy Loss

1. Not all biomass is consumed — parts of plants (e.g., roots, woody stems) may not be eaten. Parts of prey animals (e.g., bones, fur) may not be consumed.
2. Egestion (faeces) — not all consumed food is digested and absorbed. Indigestible material (e.g., cellulose in plant cell walls) passes through the gut and is egested as faeces. This energy is available to decomposers.
3. Excretion — metabolic waste products (e.g., urea from deamination of excess amino acids) contain chemical energy that is lost from the organism.
4. Respiration — the largest loss. Organisms use a significant proportion of their assimilated energy for cellular respiration (to produce ATP for movement, active transport, maintaining body temperature in endotherms, growth, and repair). The energy used in respiration is ultimately lost as heat to the environment.

---

Energy Transfer Efficiency

The efficiency of energy transfer between trophic levels can be calculated:

Efficiency (%) = (Energy transferred to next trophic level ÷ Energy available at current trophic level) × 100

Typical Values

• Producers → Primary consumers: approximately 10% (range: 5-20%).
• Primary consumers → Secondary consumers: approximately 10-20%.
• The "10% rule" is a rough guideline — actual values vary depending on the ecosystem and organisms involved.

Why Efficiency is Low

• Large respiratory losses at each level, especially in endotherms (which use energy to maintain body temperature).
• Indigestible material is egested.
• Not all organisms at a trophic level are consumed.

Why Food Chains are Short

• Because only ~10% of energy is transferred between each trophic level, by the fourth or fifth level, there is insufficient energy to support a viable population.
• This limits most food chains to 4-5 trophic levels.

---

Ecological Pyramids