How do energy and nutrients flow through ecosystems, and how do communities change through succession?
Ecosystems as systems; energy flow, trophic levels and food webs; the Gersmehl nutrient cycle; primary and secondary succession; and the climatic climax and plagioclimax.
A focused answer to the AQA A-Level Geography 3.1.6 content on ecosystems change and challenge, covering ecosystems as systems, energy flow and trophic levels, the Gersmehl nutrient cycle, primary and secondary succession, and the climatic climax and plagioclimax.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA section 3.1.6 begins ecosystems with the systems concepts: how energy flows through trophic levels and food webs, how nutrients cycle (the Gersmehl model), and how communities change through succession to a climatic climax or an arrested plagioclimax. These ideas underpin the global biomes and the conservation issues that follow.
Ecosystems as systems
Ecosystems are open systems: energy enters as sunlight and leaves as heat; nutrients are recycled internally with inputs and outputs across the boundary. Like other physical systems, ecosystems tend towards dynamic equilibrium.
Energy flow and trophic levels
Energy enters through producers (green plants) fixing solar energy by photosynthesis, measured as primary productivity. It then passes along a food chain through trophic levels: producers, primary consumers (herbivores), secondary and tertiary consumers (carnivores), with decomposers breaking down dead matter. Real ecosystems form complex food webs.
Nutrient cycling: the Gersmehl model
The Gersmehl model represents nutrient cycling with three stores, drawn as circles whose size shows the store and whose connecting arrows show the flow:
- Biomass: nutrients held in living matter.
- Litter: dead organic matter on the surface.
- Soil: nutrients in the ground, available for uptake.
The transfers are fallout (biomass to litter, as leaves and organisms die), decomposition (litter to soil) and uptake (soil to biomass via roots). Inputs come from rainfall (dissolved nutrients) and weathering of rock; outputs are leaching (loss from soil) and runoff (loss from litter). Different ecosystems have very different cycles: the tropical rainforest holds most nutrients in a huge biomass store and cycles them rapidly in the warm, wet climate, whereas the cold tundra holds nutrients in a large litter store because slow decomposition locks them up.
Succession and the climax
Succession is the directional change in a community over time. Primary succession begins on a bare surface with no soil (a lithosere on bare rock, a hydrosere in water); pioneer species colonise, modify the environment and are replaced by successive seral stages until a stable climatic climax community forms, in balance with the climate. Secondary succession is faster because it begins where soil already exists after a disturbance (fire, clearance). Each stage alters conditions (soil depth, shade, moisture) that enable the next.
Try this
Q1. Name the three nutrient stores in the Gersmehl model. [3 marks]
- Cue. Biomass, litter and soil.
Q2. Explain why food chains rarely have more than four or five trophic levels. [3 marks]
- Cue. About 90 percent of energy is lost at each level, so there is too little left to support many higher consumers.
Q3. Define a plagioclimax with an example. [2 marks]
- Cue. A community held below its climatic climax by an arresting factor; for example moorland maintained by grazing and burning.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 2019 (style)6 marksExplain how nutrients cycle through an ecosystem using the Gersmehl model.Show worked answer →
A 6 mark "explain" question (AO1). The Gersmehl model shows nutrients held in three stores, biomass (living matter), litter (dead organic matter on the surface) and soil, with the circle size proportional to the store and arrow width to the flow.
Nutrients transfer between stores: fallout (biomass to litter as leaves and organisms die), decomposition/release (litter to soil) and uptake (soil to biomass via roots). Inputs come from rainfall (dissolved nutrients) and weathering of rock; outputs are leaching (from soil) and runoff (from litter).
Markers reward naming the three stores, the transfers between them and the inputs/outputs. Top answers contrast ecosystems, for example the tropical rainforest's huge biomass store and rapid cycling versus the tundra's large litter store and slow decomposition.
AQA 2020 (style)9 marksAssess the role of succession in shaping ecosystems over time.Show worked answer →
A 9 mark "assess" question (AO1 plus AO2): reach a judgement. Primary succession begins on bare surfaces (lava, bare rock) with pioneer species that modify the environment, allowing later seral stages to establish until a stable climatic climax community forms. Secondary succession is faster, beginning where soil already exists after disturbance (fire, clearance).
Succession is powerful because each stage changes conditions (soil depth, shade, moisture) that enable the next, driving predictable change towards climax. But arresting factors (grazing, fire, human activity) can halt it at a plagioclimax (such as moorland or heathland), so the endpoint is not always the natural climax.
The judgement: succession is the key process structuring ecosystems over time, but the outcome depends on whether it runs to completion or is arrested by physical or human factors. Reward a calibrated conclusion with examples (lithosere, plagioclimax moorland).
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Sources & how we know this
- AQA A-level Geography (7037) specification — AQA (2016)