Biology • Year 11 • Module 4 • Lesson 2
Autotrophs, Heterotrophs and Saprotrophs
Apply feeding-strategy concepts to ecosystem data, an Australian case study, and a diagram interpretation.
1. Interpret decomposer removal data
A research team set up four experimental forest plots. In each plot they gradually removed increasing proportions of fungal and bacterial decomposers over three years, then measured the total organic matter accumulation (kg/m²) and the available inorganic nitrogen in the soil (mg/kg). 8 marks
| Plot | Decomposers removed (%) | Organic matter accumulated (kg/m²) | Soil inorganic nitrogen (mg/kg) | Producer growth (relative) |
|---|---|---|---|---|
| A (control) | 0 | 0.8 | 42 | 100% |
| B | 25 | 1.4 | 34 | 88% |
| C | 50 | 2.9 | 21 | 64% |
| D | 75 | 5.6 | 9 | 31% |
1.1 Describe the trend in organic matter accumulation as the percentage of decomposers removed increases. Quote data to support your description. 2 marks
1.2 Using lesson content, explain why soil inorganic nitrogen decreases as more decomposers are removed. 3 marks
1.3 Account for the decrease in producer growth (Plot A → Plot D) using the data in the table. 3 marks
2. Apply to a real ecosystem — Great Barrier Reef
The lesson describes how coral polyps live in symbiosis with photosynthetic dinoflagellates (zooxanthellae). Zooxanthellae provide up to 90% of the coral’s energy needs through photosynthesis. When water temperatures rise, coral expels the zooxanthellae — a process called coral bleaching. 7 marks
2.1 Classify coral polyps by their feeding strategy. Justify your answer using evidence from the scenario. 2 marks
2.2 Classify zooxanthellae by their feeding strategy and explain their role in the reef food web. 2 marks
2.3 Explain, using your understanding of autotrophs and energy flow, why bleached coral often dies if the bleaching event is prolonged. 3 marks
3. Interpret the nutrient cycle diagram
The figure below is a simplified nutrient cycle for a dry sclerophyll forest. Use it to answer the questions. 6 marks
Diagram: simplified nutrient cycle for a dry sclerophyll forest. Arrows X and Y are unlabelled deliberately.
3.1 Name the process represented by arrow X (from Producers to Consumers). 1 mark
3.2 Describe what arrow Y represents and identify what is passing along it. 2 marks
3.3 A student claims that if the Decomposers box were removed from the diagram, the cycle could continue indefinitely as long as sunlight was available. Using the diagram, explain why this claim is incorrect. 3 marks
4. Apply to a new scenario — bushfire aftermath
A bushfire burns through a dry sclerophyll forest in eastern Australia, killing most eucalypts, acacias and grass trees. Termites, dingoes and wedge-tailed eagles survive in low numbers on the margins. Soil bacteria and fungi in the affected zone are also greatly reduced. 5 marks
4.1 Identify which feeding-strategy group was most severely disrupted by the fire and explain the immediate consequence for nutrient cycling. 2 marks
4.2 Predict the effect on termite populations over the next two years if dead wood from burned trees is plentiful but soil fungi and bacteria remain scarce. Justify your prediction using lesson content. 2 marks
4.3 A conservationist argues that reintroducing decomposers is more critical to forest recovery than replanting trees. Briefly evaluate this claim. 1 mark
Q1.1 — Trend in organic matter accumulation
As the percentage of decomposers removed increases from 0% to 75%, organic matter accumulation increases markedly from 0.8 kg/m² to 5.6 kg/m² — a 7-fold increase. [1 mark for identifying the positive correlation; 1 mark for quoting supporting data values]
Q1.2 — Why soil inorganic nitrogen decreases (3 marks)
Decomposers (saprotrophs) externally digest dead organic matter and release inorganic nutrients, including nitrogen, into the soil [1]. When decomposers are removed, dead organic material accumulates (as shown by the organic matter column) and nitrogen remains locked within organic compounds [1]. Because nitrogen is no longer being mineralised and released, the pool of soil inorganic nitrogen available to producers falls [1].
Q1.3 — Account for decreased producer growth (3 marks)
As more decomposers are removed (Plot A to D), soil inorganic nitrogen falls from 42 mg/kg to 9 mg/kg [1]. Producers (autotrophs) require inorganic nitrogen from the soil for biosynthesis (e.g. proteins, nucleic acids). With less nitrogen available for uptake, producers cannot grow as effectively [1]. This is confirmed by the parallel decline in producer growth from 100% to 31% as nitrogen availability drops [1].
Q2.1 — Classify coral polyps (2 marks)
Coral polyps are heterotrophs [1]. They obtain organic molecules by consuming other organisms or their products — they receive up to 90% of their energy from the photosynthate produced by their zooxanthellae symbionts, and they also capture zooplankton. They cannot produce their own organic molecules from inorganic inputs [1].
Q2.2 — Classify zooxanthellae (2 marks)
Zooxanthellae are autotrophs (specifically photosynthetic dinoflagellates) [1]. They capture light energy and convert it into organic molecules via photosynthesis, forming the entry point for energy in the coral symbiosis and the wider reef food web — they are the producers upon which the heterotrophic coral and many reef organisms depend [1].
Q2.3 — Why bleached coral often dies (3 marks)
When coral bleaches, it expels its zooxanthellae autotrophs. Without these producers the coral loses up to 90% of its energy supply [1]. As a heterotroph, the coral cannot produce its own organic molecules from inorganic inputs — it depends entirely on consuming autotrophic products and captured prey [1]. If the bleaching is prolonged, the coral’s energy demand cannot be met, metabolic processes cannot be sustained, and the coral starves and dies [1].
Q3.1 — Arrow X
Arrow X represents consumption / feeding — heterotrophs eating producers (e.g. kangaroos grazing on grass) and transferring organic matter and energy through the food web. Accept: “energy and matter transfer from producers to consumers”. [1 mark]
Q3.2 — Arrow Y (2 marks)
Arrow Y represents the transfer of dead organic matter and waste from consumers to decomposers [1]. The material passing along it includes dead animal bodies, faeces, shed tissue and other organic waste produced by heterotrophs — the raw material that saprotrophs break down through external digestion [1].
Q3.3 — Why removing decomposers breaks the cycle (3 marks)
Without the Decomposers box, the pathway from dead organic matter back to inorganic nutrients (N, P, K) in the soil is severed [1]. Dead matter from producers and consumers would accumulate but the nutrients within it could not be released in inorganic form for producers to reabsorb [1]. Even with sunlight available, producers would eventually exhaust available inorganic nutrients in the soil and be unable to synthesise new organic molecules, so the cycle would collapse [1].
Q4.1 — Most disrupted feeding group and immediate consequence (2 marks)
The autotrophs (producers) were most severely disrupted as the eucalypts, acacias and grass trees were killed. Additionally, saprotrophs (decomposers) were greatly reduced. The immediate consequence is that dead organic matter from the burned plants will accumulate but cannot be decomposed efficiently, so inorganic nutrients (N, P, K) cannot be released for future plant growth. Accept either group if well-reasoned. [1 mark for identifying the group; 1 mark for explaining the nutrient cycling consequence]
Q4.2 — Predict termite population (2 marks)
Termite populations may initially increase or remain stable because dead wood (their food source) is abundant [1]. However, if soil fungi and bacteria remain scarce, overall ecosystem productivity and nutrient cycling remain compromised. Over time, as dead wood is consumed but decomposer-mediated nutrient release remains low, the long-term productivity of the ecosystem will decline, eventually reducing food availability for all heterotrophs including termites [1]. Accept also: termites are detritivores, so their immediate food supply increases but long-term decline follows nutrient lockup.
Q4.3 — Evaluate the conservationist’s claim (1 mark)
The claim has merit: without functional decomposers, inorganic nutrients cannot be released from dead organic matter for replanted trees to absorb, so tree growth would remain severely limited regardless of replanting effort. Both producers and decomposers are essential — however restoring decomposer populations enables nutrient cycling that supports the long-term survival of replanted trees. [1 mark for a balanced evaluation referencing nutrient cycling]