Energy Transfer and Nutrient Cycles

Ecosystems and energy

An ecosystem is all the living organisms (community) and the non-living (abiotic) factors in an area, interacting together. Energy enters as light and flows through food chains/webs; nutrients are recycled.

Energy transfer through trophic levels

• Producers (plants) capture light energy by photosynthesis, forming biomass.
• Energy passes to primary consumers, then secondary/tertiary consumers, and finally decomposers.

Energy transfer between trophic levels is inefficient:

• Only ~1–3% of light energy is captured by producers (gross primary production, GPP); net primary production (NPP) = GPP − respiratory losses.
• Only about 10% of energy passes to the next trophic level. The rest is lost through respiration (heat), not all being eaten, and egestion/excretion.

This is why food chains rarely have more than 4–5 trophic levels — there isn't enough energy to support them.

Measuring productivity

NPP = GPP − respiratory losses

NPP is the energy available for growth and to the next trophic level, measured in kJ m⁻² yr⁻¹.

Farming and efficiency

Farmers increase the energy available to humans by reducing losses: e.g. keeping animals warm and restricting movement (less respiratory loss), and using pesticides/herbicides so more energy goes into the crop/animal rather than pests/weeds.

Nutrient cycles

Unlike energy (which flows through and is lost), nutrients are recycled. Decomposers (saprobionts — bacteria and fungi) break down dead matter, releasing nutrients back to the soil.

• Carbon cycle: CO₂ is fixed by photosynthesis, passed along food chains, and returned by respiration, decomposition and combustion.
• Nitrogen cycle: Nitrogen fixation (bacteria/lightning → ammonium) → nitrification (ammonium → nitrite → nitrate, by nitrifying bacteria) → absorbed by plants → denitrification (nitrate → N₂ gas) returns nitrogen to the air. Mycorrhizae help plants absorb nutrients.

Succession

Over time, communities change in a directional process called succession: pioneer species colonise, change the environment, and are replaced by other species until a stable climax community forms.

Worked example

If producers store 20 000 kJ m⁻² yr⁻¹ and primary consumers receive 2 000, what is the percentage efficiency of transfer?

• (2 000 ÷ 20 000) × 100 = 10% — typical for energy transfer between trophic levels. ✓

Common mistakes

• Saying energy is recycled — energy flows through and is lost (as heat); nutrients are recycled.
• Forgetting NPP = GPP − respiration.
• Confusing nitrogen-cycle steps (fixation, nitrification, denitrification).

Exam tips

• Explain why only ~10% transfers (respiration, not eaten, egested) and its effect on food-chain length.
• Learn NPP = GPP − respiratory losses and be able to calculate transfer efficiency.
• Know the roles of bacteria in the nitrogen cycle and decomposers in recycling.

Key facts to remember

• Energy flows (and is lost as heat); only ~10% passes between trophic levels; NPP = GPP − respiration.
• Farming raises efficiency by cutting energy losses (warmth, restricted movement, pest control).
• Nutrients recycle via decomposers; nitrogen cycle = fixation → nitrification → (plant uptake) → denitrification; communities change by succession to a climax community.