Biology • Year 11 • Module 4 • Lesson 12
Abiotic and Biotic Factors Synthesis — Predicting Distribution
Apply the multi-factor prediction framework to real Australian ecosystems — using data, cause-and-effect reasoning, and comparative analysis to predict species distribution and abundance.
1. Interpret population data — mangrove distribution across a salinity gradient
A research team surveyed mangrove cover and species richness at five sites along a Queensland estuary where salinity varies from the river mouth (site 1) to the open sea (site 5). 7 marks
| Site | Salinity (ppt) | Mangrove cover (%) | Species richness (number of species) | Tidal inundation (hours/day) |
|---|---|---|---|---|
| 1 (river) | 3 | 12 | 2 | 1.2 |
| 2 | 18 | 74 | 8 | 3.4 |
| 3 | 28 | 91 | 11 | 5.1 |
| 4 | 38 | 67 | 7 | 6.8 |
| 5 (sea) | 44 | 21 | 3 | 9.2 |
1.1 Describe the relationship between salinity and mangrove cover across the five sites. Identify the optimal salinity range suggested by the data. 2 marks
1.2 Using Shelford’s Law of Tolerance, explain why mangrove cover is low at both Site 1 (salinity 3 ppt) and Site 5 (salinity 44 ppt). 2 marks
1.3 Tidal inundation is also highest at Site 5. Identify one additional abiotic factor (other than salinity) that could further reduce mangrove survival at Site 5, and explain its effect. 1 mark
1.4 Predict what would happen to the mangrove communities at Sites 2–4 if sea level rose by 0.5 m and seawalls were built along the landward edge of the estuary. Justify your prediction using two lesson concepts. 2 marks
2. Trace the cause-and-effect chain — coral bleaching cascade
Below are six events from the Great Barrier Reef bleaching cascade (Case Study 3, Card 2). Causes are provided in the left column; complete the effect boxes on the right. Then write the overall ecosystem outcome. 5 marks
| Cause | Effect (write your answer) |
|---|---|
| Sea surface temperature exceeds 29°C and is sustained for several weeks. | |
| The coral-zooxanthellae mutualism is disrupted by bleaching. | |
| Coral loses its primary energy source and structural integrity weakens. | |
| Herbivorous fish populations are depleted by overfishing on inshore reefs. | |
| Coral cover declines and the three-dimensional reef structure is lost. |
Overall ecosystem outcome (so…):
3. Interpret a graph — snowfall, temperature and snow gum recruitment
The graph below is a modelled projection of snow gum (Eucalyptus pauciflora) seedling recruitment success (%) at the current treeline (1,800 m) under four climate scenarios. 6 marks
Figure 3.1. Modelled snow gum seedling recruitment at the current treeline under four projected climate scenarios. (Stylised projection, adapted from Case Study 2, Lesson 12.)
3.1 Describe the trend in seedling recruitment from Baseline through the +2°C (no fire) scenario. 1 mark
3.2 The +2°C + fire scenario shows a dramatic drop to 8% recruitment despite warming alone producing 61%. Using the lesson’s multi-factor framework, explain why the combination is so damaging. 3 marks
3.3 Predict the long-term effect of the +2°C + fire scenario on the alpine treeline position (up, down, or static). Justify using one population dynamic concept from the lesson. 2 marks
4. Apply to a new scenario — European starlings and native hollow-nesters
A land manager in south-east New South Wales is deciding whether to install nest boxes to support populations of crimson rosellas, a native hollow-dependent bird. European starlings have colonised the area and aggressively compete for the few remaining tree hollows in a heavily cleared landscape. 5 marks
4.1 Using the concept of carrying capacity (K), explain why rosella numbers are currently suppressed below their potential maximum. 2 marks
4.2 Installing nest boxes increases the available hollows. Predict the effect on the rosella population and on the starling-rosella interaction, and explain using the lesson’s framework. 2 marks
4.3 Would installing nest boxes guarantee the long-term persistence of rosellas at this site? Identify one additional factor (abiotic or biotic) that the manager should also consider. 1 mark
Q1.1 — Salinity trend (2 marks)
Mangrove cover increases from Site 1 (12%) to a peak at Site 3 (91%, salinity 28 ppt) then decreases to Site 5 (21%) [1]. The data suggest an optimal salinity range of approximately 18–38 ppt, with peak performance near 28 ppt [1].
Q1.2 — Shelford’s Law (2 marks)
Shelford’s Law of Tolerance states that organisms can only survive within a range of a physical factor; performance is highest at the optimum and decreases toward both the minimum and maximum thresholds [1]. At Site 1 (3 ppt), salinity falls below the lower tolerance threshold for many mangrove species, limiting survival. At Site 5 (44 ppt), salinity exceeds the upper tolerance threshold, again reducing survival and species richness [1].
Q1.3 — Additional abiotic factor at Site 5 (1 mark)
Tidal inundation (9.2 hours/day) at Site 5 is highest in the dataset. Prolonged permanent submersion prevents gas exchange through pneumatophores (specialised roots), reducing oxygen supply to roots and further excluding most mangrove species [1]. Accept also: wave action / physical disturbance reducing seedling establishment.
Q1.4 — Coastal squeeze prediction (2 marks)
Mangroves at Sites 2–4 would decline because sea level rise would push the tidal inundation regime beyond their upper tolerance, while the seawalls prevent the normal landward shift in response to rising seas [1]. This “coastal squeeze” (abiotic change + physical barrier) means the mangroves are caught between two constraints; their effective habitat area shrinks, reducing carrying capacity and population size [1]. Accept: references to salinity change as sea level alters the tidal gradient, or reference to the elimination of the salt marsh that would normally be outcompeted at the new tidal limit.
Q2 — Cause-and-effect chain (5 marks)
Row 1: Coral zooxanthellae are expelled / the coral-zooxanthellae mutualism breaks down (bleaching occurs). [1]
Row 2: Coral loses its primary energy source (photosynthate from zooxanthellae); coral becomes starved, weakened, and vulnerable to disease and overgrowth. [1]
Row 3: Bleached coral is colonised by macroalgae, which overgrow and smother recovering coral. [1]
Row 4: Without herbivorous fish controlling algal growth, macroalgae expand rapidly across the reef, accelerating the shift from a coral-dominated to an algae-dominated state. [1]
Row 5: Biodiversity of reef-associated fish and invertebrates declines as structural complexity is lost. [1]
Overall outcome: The reef shifts from a coral-dominated, high-biodiversity system to an algae-dominated, low-biodiversity system; the decline is self-reinforcing once herbivore depletion and coral mortality pass critical thresholds.
Q3.1 — Trend description (1 mark)
Seedling recruitment increases steadily from 42% at Baseline to 55% at +1°C and 61% at +2°C (no fire), indicating that warming alone improves recruitment conditions at the current treeline. [1]
Q3.2 — Multi-factor explanation (3 marks)
Warming alone extends the frost-free growing season and raises temperatures above the minimum for snow gum cambial growth, improving seedling survival [1]. However, the multi-factor framework requires integrating biotic and population dynamics as well: increased fire frequency kills adult snow gums (the seed source) before they can set sufficient seed to colonise higher slopes [1]. With adult trees eliminated in frequent fire windows, there are no parents to produce seedlings; even where the temperature is suitable, the population cannot replace itself — carrying capacity for tree recruitment collapses to near zero. This shows Liebig’s Law in action: fire frequency becomes the single most limiting factor, overriding the benefit of warming [1].
Q3.3 — Treeline prediction (2 marks)
The treeline is likely to remain static or retreat downward under the +2°C + fire scenario [1]. The population dynamic concept of K explains why: the effective carrying capacity for snow gum recruitment above the current treeline is near zero because fire kills adult seed sources and prevents seedling establishment. Even if individual trees could physiologically survive at higher altitude, the population cannot grow to colonise those areas because reproduction is insufficient to sustain a viable population — the population is below the threshold needed for replacement [1].
Q4.1 — K and rosella suppression (2 marks)
Carrying capacity is the maximum population size the environment can sustain given available resources. In this modified landscape, tree hollows are the limiting resource for rosellas [1]. Starlings monopolise most available hollows through aggressive interspecific competition, so the effective K for rosellas is set by the number of hollows not occupied by starlings — far below the number that could be supported if hollows were not the limiting factor [1].
Q4.2 — Nest box prediction (2 marks)
Adding nest boxes increases the availability of the limiting resource (hollows), which raises the effective K for rosellas [1]. With more hollows available than starlings can monopolise, interspecific competition intensity decreases; rosellas can access more nesting sites, reproduce more successfully, and increase in abundance toward the new, higher K [1].
Q4.3 — Additional factor (1 mark)
Accept any one of: (a) ongoing foraging habitat — if the surrounding cleared landscape lacks sufficient food sources (seeds, fruits), even with hollows available rosellas cannot sustain a population (Liebig’s Law: food becomes the new limiting factor); (b) predation pressure from introduced predators (cats, foxes); (c) abiotic: temperature or drought reducing food availability. The point is that nest boxes address only one limiting factor; the multi-factor framework requires all critical limits to be considered. [1]