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How does energy flow through an ecosystem and how do communities change over time?

The structure of ecosystems, the flow of energy through food chains and webs, the recycling of nutrients, and the process of ecological succession.

An Edexcel A-Level Biology B (Salters-Nuffield) answer on ecosystems and succession, covering ecosystem structure, energy flow through food chains and webs, the carbon and nitrogen cycles, and the process of ecological succession.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. Ecosystem structure and energy flow
  3. Nutrient cycles
  4. Ecological succession
  5. Examples in context
  6. Try this

What this dot point is asking

Edexcel wants you to describe the structure of an ecosystem, explain how energy flows through food chains and webs, describe how nutrients are recycled, and explain ecological succession. Energy-transfer efficiency calculations and the stages of succession are the staple exam questions.

Ecosystem structure and energy flow

Energy passes along a food chain from one trophic level to the next.

The efficiency of energy transfer can be calculated as efficiency=energy in trophic levelenergy in previous level×100\text{efficiency} = \frac{\text{energy in trophic level}}{\text{energy in previous level}} \times 100. Because so much is lost at each step, the total biomass and energy available falls sharply up the chain, which is why pyramids of energy are always narrower at the top and why food chains are short.

Nutrient cycles

Unlike energy, nutrients are recycled. In the carbon cycle, carbon dioxide is fixed by photosynthesis into organic molecules and returned to the air by respiration, decomposition and combustion of fuels. In the nitrogen cycle, nitrogen-fixing bacteria convert nitrogen gas to ammonium compounds, nitrifying bacteria oxidise ammonium to nitrites then nitrates (which plants absorb), and denitrifying bacteria return nitrogen gas to the air; decomposers release ammonium from dead organisms and waste (ammonification). Decomposers (bacteria and fungi) are essential because they release the nutrients locked in dead organisms back into the ecosystem.

Ecological succession

At each stage the existing species change the abiotic environment, often making it less hostile, which lets new species outcompete and replace them. Biodiversity usually increases through succession until the climax community.

Examples in context

Example 1. Sand dune succession. On bare sand, pioneer grasses such as marram grass tolerate salt and instability and trap sand, building dunes and adding organic matter as they die. This allows other plants, then shrubs and finally trees to colonise inland dunes, giving a clear gradient of seral stages from beach to woodland. Ecologists study a dune system as a snapshot of succession across space.

Example 2. Deforestation and energy flow. Clearing a forest removes the producers that fix energy for the whole ecosystem, so the energy available to consumers collapses and food webs break down. Replanting starts a secondary succession because soil remains, recovering faster than primary succession on bare rock. This links energy flow, succession and conservation.

Try this

Q1. Explain why only about 10 per cent of energy is passed between trophic levels. [3 marks]

  • Cue. Energy is lost through respiration (as heat), movement and indigestible waste, so little is available to the next level.

Q2. Describe the role of pioneer species in succession. [2 marks]

  • Cue. They colonise bare ground and change the environment (for example forming soil), allowing other species to establish.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20195 marksA producer fixed 87000 kJ m−2 yr−187000 \text{ kJ m}^{-2} \text{ yr}^{-1} of energy. Primary consumers received 9000 kJ m−2 yr−19000 \text{ kJ m}^{-2} \text{ yr}^{-1}. Calculate the percentage efficiency of energy transfer from producers to primary consumers, and explain why so much energy is lost between the trophic levels.
Show worked answer →

A worked efficiency calculation plus explanation.

Efficiency =energy in next levelenergy in previous level×100=900087000×100=10.3%= \frac{\text{energy in next level}}{\text{energy in previous level}} \times 100 = \frac{9000}{87000} \times 100 = 10.3\% (to 3 significant figures). Energy is lost because not all of the producer is eaten or digestible (some passes out in faeces), and the energy that is assimilated is largely used in respiration and released as heat, or lost in movement and excretion, so only about a tenth is available to build the consumer biomass passed to the next level.

Markers reward: correct 10.3%10.3\% (or about 10%10\%); energy lost as heat from respiration; losses in faeces, movement and excretion; only the energy stored in new biomass passes on.

Edexcel 20224 marksDescribe the process of primary succession that occurs on bare rock, from the arrival of pioneer species to the development of a climax community.
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Markers want an ordered sequence with the role of each stage.

Pioneer species such as lichens colonise the bare rock; they are adapted to harsh conditions and begin to break down the rock and trap debris. When pioneers die, they are decomposed, adding organic matter so a thin soil forms. This changed environment allows mosses, then small plants and grasses, to establish, each making conditions less hostile (deeper soil, more water and nutrients) and outcompeting earlier species. Larger plants and shrubs, then trees, colonise in turn (seral stages) until a stable climax community (such as woodland) is reached, where the community is in balance with the environment.

Award marks for: pioneers colonise bare rock; they alter the environment and form soil on death; new species establish and outcompete; succession through seral stages; stable climax community reached.

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