How do energy and nutrients move through a rainforest, and why is the system so productive yet so fragile?
Explain energy flow through trophic levels and the nutrient cycle in tropical rainforests, and why the system is vulnerable to disturbance
A focused answer to the H2 Geography outcome on rainforest energy and nutrients. Trophic levels and energy loss, the Gersmehl nutrient stores and transfers, the closed rapid cycle, and why clearance breaks it.
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What this dot point is asking
SEAB wants you to explain how energy flows through the trophic levels of a rainforest and how nutrients cycle between stores, and to use this to explain why the lush forest is paradoxically fragile. The central insight is that the rainforest's productivity is sustained by a rapid, tightly closed nutrient cycle in which the biomass holds almost everything, so removing the trees removes the store and the cycle collapses.
The answer
Energy flow through trophic levels
Energy enters as sunlight and moves along food chains:
- Producers (green plants) fix solar energy by photosynthesis. The total fixed is gross primary productivity; after the plant's own respiration, the rest is net primary productivity (NPP), the energy available to consumers. Rainforest NPP is among the highest of any ecosystem.
- Primary consumers (herbivores) eat plants; secondary and higher consumers (carnivores) eat other animals; decomposers break down dead matter.
At each transfer only about 10 per cent of energy passes on. The rest is lost as heat in respiration, in movement, and in material not eaten or not digested (egestion). Because energy declines up the chain, there are usually only four or five trophic levels and few top carnivores.
The nutrient cycle: stores and transfers
The nutrient cycle is best described with three stores and the transfers between them (the Gersmehl model):
- Stores: biomass (living matter), litter (dead matter on the surface), and soil.
- Transfers: litterfall (biomass to litter), decomposition (litter to soil), uptake (soil to biomass).
- Inputs: weathering of rock and dissolved nutrients in rainfall.
- Outputs: leaching from the soil and runoff from the litter.
Why biomass dominates the store
In the rainforest the biomass is the largest store, not the soil. The hot, humid climate drives extremely rapid decomposition, so litter does not accumulate; shallow-rooted plants take up released nutrients almost immediately, before the heavy rainfall can leach them away. The result is a rapid, nearly closed cycle with a small soil store.
Why the system is fragile
Because most nutrients are locked in the living biomass and the soil is poor, removing the vegetation removes the main store. Without litterfall the cycle stalls; exposed soil is leached and eroded by heavy rain. So a forest that looks lush sits on thin, infertile soil, which is why clearance leads quickly to nutrient loss and degraded land.
Examples in context
Example 1. Shifting cultivation in Southeast Asia. Traditional swidden (slash-and-burn) farming in Borneo and the uplands of Southeast Asia works with the nutrient cycle: burning releases nutrients from the biomass into the soil for a few productive years, after which fertility falls and the plot is abandoned to regrow. It demonstrates that the fertility lay in the biomass, and that the soil alone cannot sustain cropping for long.
Example 2. Nutrient-poor soils of the Amazon. Across much of the Amazon basin the soils are deeply weathered, leached and infertile (oxisols), despite supporting the world's largest rainforest. The contrast between exuberant vegetation and poor soil is the clearest large-scale proof that the nutrient store is the biomass, and that deforestation removes it.
Try this
Q1. Name the three nutrient stores in the Gersmehl model and the transfer between biomass and litter. [3 marks]
- Cue. Stores: biomass, litter and soil. The transfer from biomass to litter is litterfall (the fall of dead leaves and branches to the surface).
Q2. Explain why only about four or five trophic levels exist in a rainforest. [2 marks]
- Cue. Roughly 90 per cent of energy is lost at each transfer (to respiration, movement and egestion), so the energy available falls sharply up the chain until too little remains to support a further level.
Q3. Explain why the soil store of nutrients is small in a tropical rainforest. [3 marks]
- Cue. Rapid decomposition releases nutrients quickly, but shallow-rooted plants take them up almost immediately and heavy rainfall leaches the rest away, so few nutrients remain in the soil at any time.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksUsing the idea of nutrient stores and transfers, explain how the nutrient cycle operates in a tropical rainforest and why most nutrients are held in the biomass.Show worked answer →
Argument: the rainforest runs a rapid, almost closed nutrient cycle in which the biomass, not the soil, is the dominant store, because warmth and moisture speed decomposition and uptake while heavy rain leaches the soil.
Stores and transfers (the Gersmehl model): nutrients are held in three stores, biomass (living matter), litter (dead organic matter on the surface) and soil. Transfers move nutrients between them: litterfall moves nutrients from biomass to litter; decomposition moves them from litter to soil; and plant uptake returns them from soil to biomass. Inputs come from weathering and from rainfall (dissolved nutrients); outputs are leaching from the soil and runoff from the litter.
Why biomass dominates: the hot, humid climate gives extremely rapid decomposition, so litter does not accumulate, and rapid uptake by shallow-rooted plants captures nutrients almost as soon as they are released, before heavy rainfall can leach them away. The soil store is therefore small; most of the system's nutrients are locked in the living biomass.
Markers reward the three stores and the named transfers, the climate-driven speed of decomposition and uptake, leaching as the loss, and the conclusion that biomass is the largest store.
Original10 marksExplain energy flow through a tropical rainforest and why energy is lost at each trophic level.Show worked answer →
Argument: energy enters as sunlight, is fixed by producers and passes along food chains, but most is lost at each transfer, so the number of trophic levels is limited.
Process to explain: green plants (producers) fix solar energy by photosynthesis into the gross primary productivity; some is used in respiration, leaving net primary productivity as the energy available to consumers. Herbivores (primary consumers) eat plants; carnivores (secondary and higher consumers) eat herbivores; decomposers break down dead matter. At each transfer only about 10 per cent of energy passes to the next level.
Why energy is lost: at each step energy is lost as heat in respiration, in movement and life processes, and in material that is not eaten or not digested (egestion). Because only a fraction passes on, biomass and energy decline up the chain, which is why there are usually only four or five trophic levels and why top carnivores are few.
Markers reward producers fixing solar energy, the gross-to-net distinction, the roughly 10 per cent transfer, and respiration and egestion as the losses.
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