Ecosystems, energy flow and food webs, the carbon and nitrogen cycles, succession, and the principles of conservation and managing human impact.

What this dot point is asking

CCEA wants you to describe an ecosystem and energy flow through food webs, explain the carbon and nitrogen cycles, describe succession, and outline the principles of conservation and managing human impact.

Energy flow

Only about 10 percent of the energy at one trophic level passes to the next, because energy is lost in respiration (as heat), in movement, in excretion, and in undigested material that passes to decomposers. This is why food chains rarely have more than four or five links and why pyramids of energy always narrow towards the top. Gross primary production (GPP) is the total energy fixed by producers; net primary production (NPP) is what remains after the producers' own respiration, and is the energy available to the next level.

Nutrient cycles

Nitrogen gas is unreactive, so only nitrogen fixation (by bacteria such as Rhizobium in legume root nodules, or industrially by the Haber process) makes it available. Decomposers (saprobionts) carry out ammonification, breaking down protein in dead organisms and waste to release ammonium. Farmers add nitrate fertilisers and grow legumes to keep soil nitrogen high, but excess nitrate washed into rivers causes eutrophication.

Succession and conservation

Succession is the directional change in a community over time: pioneer species (such as lichens on bare rock) colonise, change conditions (forming soil, retaining water), and are replaced by later communities until a stable climax community forms. Conservation is the active management of ecosystems to maintain biodiversity while allowing sustainable use, for example through protected areas, controlling pollution, reintroducing species, and managing habitats such as peatlands, sand dunes and woodlands. Deflected succession (a plagioclimax), such as grazed grassland or managed heather moorland, is maintained by human activity rather than reaching the natural climax.

Examples in context

Example 1. Eutrophication of a lough. When nitrate and phosphate fertiliser drains into a Northern Ireland lough, algae grow rapidly (an algal bloom) and block light. Plants below die, decomposers multiply and use up the dissolved oxygen in respiration, so fish and invertebrates suffocate. This is a worked illustration of how a nutrient cycle, disrupted by human activity, can collapse an ecosystem, and why farmers are encouraged to manage fertiliser use near waterways.

Example 2. Peatland restoration as carbon storage. Northern Ireland's blanket bogs hold huge amounts of carbon locked in waterlogged peat where decomposition is slow. When bogs are drained for farming or fuel, the peat oxidises and releases carbon dioxide, adding to the carbon cycle's atmospheric pool. Conservation projects re-wet the bogs to halt decomposition and keep the carbon stored. This links the carbon cycle directly to a conservation decision and to climate policy.

Try this

Q1. Explain why food chains rarely have more than four or five trophic levels. [2 marks]

• Cue. Only about 10 percent of energy passes to each level, so there is too little energy to support more levels.

Q2. State the role of nitrogen-fixing bacteria in the nitrogen cycle. [1 mark]

• Cue. They convert nitrogen gas into ammonia or ammonium (nitrogen-containing compounds).

Q3. A producer fixes $40\,000\ \text{kJ m}^{-2}\ \text{yr}^{-1}$. Estimate the energy available to a secondary consumer two levels up, assuming 10 percent transfer at each step. [2 marks]

• Cue. $40\,000 \times 0.1 = 4000$ to the primary consumer, then $4000 \times 0.1 = 400\ \text{kJ m}^{-2}\ \text{yr}^{-1}$ to the secondary consumer.