Biology • Year 11 • Module 4 • Lesson 6
Abiotic Factors: The Physical and Chemical Environment
Apply tolerance ranges, limiting factors and abiotic interactions to real Australian ecosystem data and novel scenarios.
1. Interpret salinity data from the Murray-Darling Basin
A researcher monitored electrical conductivity (a measure of salinity) and the population size of Murray cod (Maccullochella peelii) and common carp (Cyprinus carpio) at a river monitoring station during a four-year drought. Results are shown below. Normal river conductivity is below 800 µS/cm; Murray cod begin to show physiological stress above 1,500 µS/cm. 8 marks
| Year | Conductivity (µS/cm) | Murray cod (individuals / 100 m) | Common carp (individuals / 100 m) | Notes |
|---|---|---|---|---|
| 1 | 780 | 12 | 18 | Baseline — normal flow |
| 2 | 1,100 | 9 | 22 | Drought begins; reduced flow |
| 3 | 1,680 | 4 | 25 | Drought continues; low water volume |
| 4 | 2,240 | 1 | 19 | Severe drought; very low flow |
1.1 Describe the trend in conductivity from Year 1 to Year 4 and explain why conductivity increases as river flow decreases. Connect your explanation to your Module 1 knowledge of solute concentration. 3 marks
1.2 Using the concept of a tolerance range, explain why Murray cod decline more sharply than carp between Year 2 and Year 4. 3 marks
1.3 Identify the limiting factor for Murray cod in Year 3 and predict what would happen to Murray cod abundance if normal river flow returned in Year 5. 2 marks
2. Interpret a graph — temperature and photosynthesis in an alpine plant
The graph below shows the photosynthesis rate of the alpine snow daisy (Celmisia costiniana), a plant endemic to the Snowy Mountains, measured across a range of temperatures. 6 marks
Stylised photosynthesis-temperature curve for an alpine endemic plant — illustrative.
2.1 Identify the optimal temperature for photosynthesis in the alpine snow daisy and the temperature at which net photosynthesis reaches zero (compensation point). 2 marks
2.2 The lesson states that ectotherms have temperature-dependent metabolic rates. Explain, using the graph, why a sustained temperature increase of 10 °C above the current Snowy Mountains summer average (15 °C) would threaten the alpine snow daisy, even though 25 °C is well within the survivable range for most Australian plants. 2 marks
2.3 Using the shape of the curve, identify which part of the tolerance range model the red-shaded region above 28 °C represents. Justify your answer. 2 marks
3. Diagram critique — what is wrong with this student's answer?
A student wrote the following response to the question: "Explain why ocean acidification (pH falling from 8.1 to 7.9) threatens coral reefs." There are three biological errors in the response. Identify each error and write the correction. 6 marks (2 per error: 1 identify, 1 correct)
"When the pH drops from 8.1 to 7.9, the water becomes acidic. This denatures the enzymes in coral polyps so they cannot grow. It also increases the solubility of calcium carbonate, which means corals have more building material available for their skeletons. Additionally, the lower pH increases the metabolic rate of coral-associated zooxanthellae, causing them to photosynthesise faster and warm the water, which bleaches the coral."
3.1 Error 1: What is wrong?
Correction:
3.2 Error 2: What is wrong?
Correction:
3.3 Error 3: What is wrong?
Correction:
4. Apply to a novel scenario — draining a coastal wetland
A developer proposes to drain a coastal wetland near Sydney to build a housing estate. Before drainage, the wetland supports mangroves, saltmarsh plants, migratory shorebirds, mudskippers and estuarine crocodilians (in tropical versions). After drainage, the soil is exposed and dries out over six months. 5 marks
4.1 Identify three abiotic factors that would change after drainage and state the direction of change (increase / decrease) for each. 3 marks
4.2 Mangroves require tidal inundation (water covering roots) for at least 6 hours per day. Using the lesson's limiting factor concept, explain whether drainage or increased salinity is more likely to be the primary limiting factor for mangrove survival after the wetland is drained. 2 marks
Q1.1 — Conductivity trend and explanation (3 marks)
Conductivity increases steadily from 780 to 2,240 µS/cm as drought reduces river flow [1 mark for trend]. As water evaporates, the same quantity of dissolved salts is concentrated into a smaller volume of water — the solute concentration per litre rises (connecting to Module 1: concentration = moles of solute / volume of solvent) [1 mark for mechanism]. Reduced river inflow means less dilution of accumulated salts from surrounding soils [1 mark for elaboration or alternative valid mechanism].
Q1.2 — Tolerance range explanation (3 marks)
Murray cod have a narrower tolerance range for salinity than common carp [1 mark]. By Year 3, conductivity (1,680 µS/cm) has exceeded the stated stress threshold (1,500 µS/cm) for Murray cod, pushing them into their physiological stress zone and then toward their lethal limit; cod must expend increasing energy on osmoregulation, reducing energy available for growth and reproduction [1 mark]. Common carp are a hardy introduced species with a broader salinity tolerance range; they remain in their optimal or low-stress zone at conductivity levels that are lethal for cod, so their population decline only begins at extreme conductivity in Year 4 [1 mark].
Q1.3 — Limiting factor + prediction (2 marks)
In Year 3, salinity (high conductivity) is the limiting factor for Murray cod, as it is the abiotic factor furthest outside the cod's tolerance range [1 mark]. If normal flow returned in Year 5, conductivity would drop toward baseline, relieving the osmotic stress; cod populations would likely recover as surviving adults breed and juvenile fish are able to establish in the now-favourable conditions, assuming the population has not fallen below a viable breeding size [1 mark].
Q2.1 — Optimal temperature and compensation point (2 marks)
Optimal temperature: approximately 14 °C (peak of curve, ~22 units net photosynthesis) [1 mark]. Compensation point (net photosynthesis = 0): approximately 28 °C [1 mark]. Accept ±2 °C for both values.
Q2.2 — Why +10 °C threatens the snow daisy (2 marks)
At 15 °C (current summer average) the snow daisy is operating near its optimal zone (near-peak photosynthesis). A sustained +10 °C increase would push temperatures to 25 °C, which the graph shows is well past the compensation point — net photosynthesis is negative at this temperature, meaning the plant is respiring more carbon than it fixes [1 mark]. Over time, the plant would be in a carbon deficit, unable to sustain growth and reproduction; while 25 °C might not be immediately lethal, the plant would enter a zone of physiological stress and die if the condition persists across the growing season [1 mark].
Q2.3 — Red-shaded region (2 marks)
The red-shaded region represents the zone of physiological stress (high) / approaching the lethal high limit of the tolerance range [1 mark]. At temperatures above the compensation point (>28 °C), net photosynthesis is negative — the plant respires more than it photosynthesises, losing organic molecules. If prolonged, this leads to tissue damage and death, consistent with the lethal upper limit of the tolerance range model [1 mark].
Q3 — Diagram critique (6 marks)
3.1 Error 1 — "the water becomes acidic": A pH of 7.9 is still alkaline (above 7.0), not acidic. Correction: seawater at pH 7.9 is less alkaline than at 8.1 but remains alkaline; the correct term is ocean acidification refers to a reduction in pH (increase in acidity), not a shift to acidic conditions. [1 + 1]
3.2 Error 2 — "increased solubility of calcium carbonate gives corals more building material": This is the opposite of the real mechanism. Correction: lower pH reduces the concentration of carbonate ions (CO32−) in seawater — H+ ions react with carbonate to form bicarbonate (HCO3−), so corals have less carbonate available to precipitate calcium carbonate shells and skeletons. Shells actually dissolve more readily under acidic conditions. [1 + 1]
3.3 Error 3 — "lower pH increases metabolic rate of zooxanthellae, warming the water and causing bleaching": Coral bleaching is caused by elevated sea surface temperature (typically above 29–30 °C for extended periods), not by pH-driven warming. Correction: ocean acidification causes bleaching indirectly by stressing coral polyps, but the direct cause of bleaching is thermal stress from warming water; additionally, zooxanthellae photosynthesis does not warm seawater measurably. [1 + 1]
Q4.1 — Three abiotic changes after drainage (3 marks)
(1) Water availability / tidal inundation — decreases: roots no longer submerged. (2) Salinity of soil — increases: evaporation concentrates salt in drying soil. (3) Soil oxygen concentration — increases initially (oxidation of peat), then pH drops as sulfides oxidise to sulfuric acid. Accept also: temperature increases (loss of water moderating microclimate); dissolved O2 in remaining water decreases. [1 mark per valid factor + direction, max 3]
Q4.2 — Primary limiting factor for mangroves (2 marks)
The primary limiting factor after drainage is absence of tidal inundation (water availability). Applying Liebig's Law of the Minimum, mangroves require tidal inundation for at least 6 hours per day as a non-negotiable physiological requirement; without it, root aeration (via pneumatophores) fails and the tree cannot survive regardless of salinity. Since tidal inundation drops to zero after drainage, this factor is furthest from the optimal range and therefore the limiting factor — increasing salinity tolerance would not help because the water requirement cannot be met at all [1 mark for identifying water/inundation as limiting factor + 1 mark for linking to Liebig's Law with reasoning].