Covering Lessons 01–05: ecosystem components, autotrophs/heterotrophs/saprotrophs, food chains and webs, trophic efficiency and the 10% rule, and ecological pyramids.
Which of the following correctly describes the role of saprotrophs in an ecosystem?
In a food chain, the arrow points from grass to grasshopper to frog to snake. Which statement about arrow direction is correct?
A barramundi feeds on aquatic insects, small fish and crustaceans. Which statement best describes its trophic position?
Why are food webs generally more resilient to species removal than single food chains?
A rabbit ingests 800 kJ of grass energy. Approximately how much of this energy is likely to become new rabbit biomass available to the next trophic level?
Which of the following is the largest energy loss pathway when a primary consumer eats a producer?
Which statement about pyramids of energy is correct?
In an open ocean ecosystem, the standing crop biomass of zooplankton exceeds that of phytoplankton. What does this indicate about the pyramid of biomass?
A food chain in an Australian grassland contains four trophic levels. If producers contain 50,000 kJ m⁻² yr⁻¹ and trophic efficiency is 10% at each level, how much energy is available to quaternary consumers?
A student claims that decomposers recycle both matter and energy back to the producer level in an ecosystem. Which statement best evaluates this claim?
The following food chain occurs in a Kakadu billabong: phytoplankton → zooplankton → small fish → barramundi → saltwater crocodile.
(a) Identify the trophic level of the barramundi and explain whether it could occupy more than one trophic level. 2 MARKS
(b) If the phytoplankton contain 20,000 kJ m⁻² yr⁻¹ of energy, calculate the energy available to the saltwater crocodile assuming 10% trophic efficiency at each transfer. Show your working. 2 MARKS
(c) Explain why the removal of saltwater crocodiles would have a greater impact on this food chain than the removal of zooplankton. In your answer, refer to trophic cascades and the concept of keystone species. 3 MARKS
(a) Barramundi is a tertiary consumer (T4) in this chain [1 mark]. It could occupy multiple trophic levels if it also eats herbivorous organisms directly (e.g. aquatic insects at T2), making it a secondary consumer in those feeding relationships [1 mark].
(b) T2 = 20,000 x 0.10 = 2,000 kJ. T3 = 2,000 x 0.10 = 200 kJ. T4 = 200 x 0.10 = 20 kJ. T5 (crocodile) = 20 x 0.10 = 2 kJ [1 mark for method, 1 mark for correct answer].
(c) Crocodiles are apex predators (keystone species) whose removal triggers a trophic cascade: mid-sized predators (e.g. large fish, turtles) increase in number, exerting heavier predation pressure on their prey [1 mark]. This reshapes the entire community structure. Zooplankton removal would affect small fish but the ecosystem could partially compensate through alternative prey pathways [1 mark]. Crocodiles have a disproportionately large impact relative to their abundance, fitting the keystone definition [1 mark]. Total: 3 marks.
Compare pyramids of numbers, biomass and energy for the following two ecosystems. In your answer, explain which pyramid type provides the most accurate representation of ecosystem structure and justify your choice.
Ecosystem A: Australian grassland with many small grasses, fewer kangaroos, few dingoes.
Ecosystem B: Australian eucalypt forest with one large ironbark tree supporting thousands of insects, hundreds of spiders and dozens of birds.
5 MARKSEcosystem A (grassland): All three pyramids are upright. Numbers: many grasses > fewer kangaroos > fewest dingoes. Biomass: grass biomass > kangaroo biomass > dingo biomass. Energy: producer energy > primary consumer energy > secondary consumer energy [1.5 marks for all three correct].
Ecosystem B (forest): Numbers pyramid is inverted (1 tree < thousands of insects < hundreds of spiders < dozens of birds) because the producer is large and supports many small consumers [1 mark]. Biomass pyramid is upright (tree weighs tonnes; insects weigh grams) [0.5 marks]. Energy pyramid is upright [0.5 marks].
Most accurate: Pyramid of energy is most accurate because it measures energy flow through each level over time, independent of organism size or lifespan [1 mark]. Numbers pyramids are distorted by body size (one tree inverts the pyramid). Biomass pyramids can be inverted in aquatic systems and measure standing crop, not productivity [0.5 marks]. Total: 5 marks.
Using the Australian grazing case study (54% of Australia used for livestock grazing), evaluate whether reducing beef consumption would be an effective strategy for decreasing land-use pressure and protecting native biodiversity. In your answer, apply the 10% rule and compare the land required for a beef-based diet versus a plant-based diet.
6 MARKSThe 10% rule means cattle (primary consumers) convert only ~3-10% of grass energy into beef [1 mark]. Producing 1 kg of beef requires approximately 13 m² of pasture, while delivering equivalent energy from wheat (eaten directly as a producer) requires only ~0.2 m² — a 50-100 fold difference [1 mark]. This is because bypassing the T1 to T2 transfer avoids the ~90% energy loss [1 mark].
Ecological consequences of grazing: (1) removal of ground cover reduces habitat for ground-nesting birds, reptiles and small mammals; (2) hoof compaction reduces water infiltration and increases erosion; (3) overgrazing can lead to dryland salinity and woody weed invasion [1.5 marks for two valid consequences].
Evaluated conclusion: reducing beef consumption is an effective strategy because it addresses the root cause (trophic inefficiency) while reducing habitat destruction and soil degradation [1 mark]. However, it is not the only strategy needed; sustainable grazing management, protected areas and restoration are also required [0.5 marks]. Total: 6 marks.
Mark checkpoint as complete