Biology • Year 11 • Module 4 • Lesson 11
Comparing Ecosystems: Abiotic and Biotic Differences
Apply abiotic-to-biotic reasoning to real data, Australian ecosystems, and ecosystem-change scenarios — exam-style short answer and data interpretation.
1. Interpret ecosystem comparison data
A field ecologist recorded key abiotic and biotic measurements for four Australian ecosystems. Study the data table and answer the questions below. 8 marks
| Ecosystem | Annual rainfall (mm) | Mean temp. (°C) | Species richness (plants/ha) | Dominant competition type |
|---|---|---|---|---|
| Daintree Rainforest (Qld) | 2,400 | 26 | 280 | Interspecific for light |
| Murray-Darling Woodland | 420 | 19 | 65 | Intraspecific for water |
| Simpson Desert | 120 | 28 | 18 | Intraspecific for water |
| Mulga Scrubland (SA) | 260 | 22 | 31 | Intraspecific for water |
1.1 Describe the relationship between annual rainfall and species richness shown in the data. 2 marks
1.2 Use lesson content to explain why interspecific competition for light is the dominant type in the Daintree Rainforest but not in the three drier ecosystems. 3 marks
1.3 The Simpson Desert has a higher mean temperature than the Daintree Rainforest but far fewer species. Suggest why temperature alone does not explain the difference in species richness. 3 marks
2. Interpret graph — nutrient levels, productivity, and biodiversity
The graph below is a stylised model showing how species richness changes as nutrient availability increases across aquatic ecosystems. Study it and answer the questions. 6 marks
Stylised model illustrating the intermediate disturbance / nutrient hypothesis. Adapted from Rosenzweig (1995), Species Diversity in Space and Time.
2.1 Describe the overall shape of the relationship between nutrient availability and species richness shown in the graph. 2 marks
2.2 Using lesson content, explain why species richness decreases when nutrient availability is very high (the right-hand side of the graph). 2 marks
2.3 The coral reef is placed at the nutrient-balanced peak. Use your knowledge of zooxanthellae to explain how coral reefs maintain high biodiversity despite having low dissolved nutrient concentrations in the surrounding water. 2 marks
3. Trace cause-and-effect: eutrophication of a freshwater lake
Agricultural runoff increases the nutrient load in a previously oligotrophic (nutrient-poor) Tasmanian lake. Each box below states a cause; write the effect it produces in the ecosystem on the right. 5 marks
| Cause | → | Effect (write your answer) |
|---|---|---|
| Nutrient input (nitrogen & phosphorus) into the lake increases | → | |
| Dense algal bloom forms and covers the lake surface | → | |
| Algae in the bloom die in large numbers | → | |
| Oxygen in the water is depleted by decomposer activity | → | |
| Overall outcome for the lake’s biodiversity: | → |
4. Predict and justify — coral bleaching scenario
A sustained marine heatwave raises sea surface temperatures on the Great Barrier Reef by 2 °C above the long-term mean for three consecutive summers. 5 marks
4.1 Predict what will happen to the zooxanthellae living in coral tissue when water temperature rises significantly above normal. Justify your prediction using lesson content. 2 marks
4.2 Explain how the loss of zooxanthellae from coral tissue affects the biodiversity of the entire reef ecosystem, not just the coral polyps. 3 marks
Q1.1 — Rainfall vs species richness trend (2 marks)
There is a positive relationship between annual rainfall and species richness: as rainfall increases from 120 mm (Simpson Desert, 18 species/ha) to 2,400 mm (Daintree, 280 species/ha), species richness increases substantially. The Daintree has more than 15 times the species richness of the Simpson Desert, despite having a similar mean temperature.
Marking criteria: 1 mark for correctly describing a positive relationship (more rainfall = more species); 1 mark for supporting the claim with data from the table (quote at least two values).
Q1.2 — Why interspecific competition for light dominates in rainforests (3 marks)
In the Daintree, high rainfall (2,400 mm/yr) and year-round warmth (26 °C) mean water is not the limiting resource [1]. Multiple species of plants can grow densely, and the main constraint on growth becomes sunlight — taller plants in the canopy shade shorter species below, driving interspecific competition for light and producing vertical stratification [1]. In drier ecosystems, rainfall is so low and unpredictable that water becomes the critical limiting resource; all plant species share this same constraint, so competition is primarily intraspecific (between individuals of the same species competing for the same water source) rather than between species for different layers [1].
Q1.3 — Why temperature alone does not explain species richness (3 marks)
Temperature is only one of several key abiotic factors. While the Simpson Desert is warm (28 °C), it receives very little rainfall (120 mm/yr) — water is the critical limiting factor and most species cannot tolerate this level of aridity [1]. The Daintree has a slightly lower temperature (26 °C) but receives 20 times more rainfall, meaning water stress is not a barrier to survival for many plant species [1]. Furthermore, the lesson identifies rainfall, temperature and seasonality together as the three dominant abiotic drivers of terrestrial biodiversity — stable, high-rainfall conditions support niche specialisation and mutualism, which further increases species richness beyond what temperature alone would predict [1].
Q2.1 — Graph shape description (2 marks)
The graph shows a hump-shaped (unimodal) relationship. Species richness is low when nutrients are very low (nutrient-limited, e.g. open ocean), rises to a peak at intermediate nutrient levels (nutrient-balanced, e.g. coral reef), and then falls again when nutrients are very high (nutrient-excess, e.g. eutrophic lake). Neither extreme supports high biodiversity.
Marking criteria: 1 mark for describing the rising and falling (hump-shaped) pattern; 1 mark for correctly identifying the two ends (low and high nutrients) as having low species richness with a peak in the middle.
Q2.2 — Why very high nutrients reduce species richness (2 marks)
When nutrients are very high (eutrophication), a small number of highly competitive species (typically fast-growing algae or cyanobacteria) can dominate by consuming resources faster than other species, outcompeting slower-growing specialists and reducing biodiversity [1]. In a lake, dense algal blooms block light to submerged aquatic plants, leading to their death; when the algae die, decomposers consume dissolved oxygen, creating hypoxic conditions that kill fish and invertebrates, causing a net loss of species [1].
Q2.3 — How reefs maintain biodiversity in nutrient-poor water (2 marks)
Coral polyps form a mutualistic symbiosis with zooxanthellae. The zooxanthellae photosynthesise using sunlight and supply up to 90% of the coral’s energy, while the coral provides shelter and CO₂ [1]. This creates a tight internal nutrient cycle — nitrogen and phosphorus are recycled within the coral tissue rather than being lost to the surrounding water, allowing the reef to sustain high productivity and support thousands of associated species without relying on an external nutrient supply [1].
Q3 — Eutrophication cause-and-effect chain (5 marks)
Row 1 Effect: Rapid growth of phytoplankton and algae (algal bloom). [1]
Row 2 Effect: Light is blocked from reaching submerged aquatic plants, which die; biodiversity begins to fall. [1]
Row 3 Effect: Decomposers (bacteria) break down the dead algae, consuming large amounts of dissolved oxygen in the process. [1]
Row 4 Effect: Fish, invertebrates, and other aerobic organisms suffocate; a hypoxic “dead zone” forms in deeper water. [1]
Overall outcome: Biodiversity decreases sharply; a once species-rich oligotrophic lake becomes dominated by a few tolerant, competitive species (e.g. cyanobacteria, decomposer bacteria). [1]
Q4.1 — Zooxanthellae and heat stress (2 marks)
When water temperature rises significantly above the long-term mean, zooxanthellae are expelled from or leave the coral tissue (a process called coral bleaching) [1]. Without the zooxanthellae, the coral loses up to 90% of its energy supply; if temperatures remain elevated, the coral starves and dies [1].
Q4.2 — Loss of zooxanthellae and reef biodiversity (3 marks)
Coral polyps are the foundation species of the reef — their calcium carbonate skeletons build the three-dimensional structure that provides micro-habitats for thousands of other species [1]. When corals bleach and die, this physical structure collapses or becomes overgrown by algae, destroying the niches that supported specialised fish, invertebrates, and other reef organisms [1]. Because up to 25% of all marine species depend on coral reef structure for shelter and feeding, the loss of foundation corals causes a cascade of population declines across the entire reef ecosystem, dramatically reducing biodiversity [1].