In 2019, fish kills swept through the Murray-Darling River system. Over a million dead fish floated to the surface near Menindee. The immediate cause was a blue-green algal bloom that deoxygenated the water. But the underlying causes were hotter temperatures, reduced flows, and rising salinity — abiotic changes that cascaded through the food web. This lesson integrates everything you have learned in IQ2 to predict how multiple factors together determine where organisms live, thrive, and die.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Before you read, commit to a prediction. You will revisit these at the end.
Q1. The Murray-Darling River system is predicted to warm by 3°C and experience increased salt levels over the next 50 years. Predict which native species would be most at risk of local extinction and which introduced species might thrive. Justify your prediction using at least three factors from IQ2.
Q2. Many students believe that carrying capacity is a fixed number for any given ecosystem. Explain why this belief is incorrect, using a real example to support your answer.
No organism is limited by a single factor. Distribution and abundance are the outcome of abiotic thresholds, biotic interactions, and historical constraints acting simultaneously. To predict where a species will be found — or lost — you must integrate all three.
Identify the physical and chemical limits: temperature, rainfall, salinity, pH, soil type, light. Use Shelford’s Law of Tolerance — organisms survive only within an optimal range. If climate shifts beyond this range, the species must adapt, move, or die.
Map the web of relationships: competition (intra- and interspecific), predation, mutualism, parasitism. Use Liebig’s Law of the Minimum — the scarcest resource or strongest interaction limits the population. A species within its abiotic tolerance may still be excluded by a superior competitor.
Model how the population responds: exponential vs logistic growth, carrying capacity fluctuations, density-dependent feedback. Remember that K is not fixed — it changes with resource availability, predation pressure, and disturbance frequency.
When you encounter a prediction question, use this sentence structure:
“If [abiotic factor] changes to [new state], then [species] will [response: adapt/move/die] because [tolerance limit / competitive interaction / population dynamic]. This will cause [downstream effect on another species or ecosystem process].”
Each case study below requires you to combine abiotic thresholds, biotic interactions, and population dynamics to explain and predict distribution patterns.
Current pattern: Mangroves occupy a narrow band along tropical and subtropical coastlines, between mean sea level and the highest astronomical tide.
Abiotic factors: Salinity gradient (tolerate 0–90 ppt, optimal 20–40 ppt), tidal inundation frequency (need regular flushing but not permanent submersion), sediment type (fine muds for root anchorage, coarse sands exclude them).
Biotic factors: Competition with salt marsh at the landward edge (salt marsh wins where tidal flushing is too infrequent); predation on propagules by crabs limits recruitment.
Prediction: If sea level rises 0.5 m, mangroves will shift landward where topography allows. Where seawalls or steep banks prevent landward migration, mangroves will be squeezed against the shore and decline. Salt marsh will be outcompeted at the new tidal limit.
Current pattern: In the Australian Alps, snow gums (Eucalyptus pauciflora) reach their upper limit around 1,800 m. Above this, only grasses, herbs, and low shrubs survive.
Abiotic factors: Temperature (mean growing season temperature below ~6°C prevents cambial growth), wind exposure (desiccates leaves and breaks branches), soil depth (thin soils at ridge tops limit root development and water storage), frost heaving (dislodges seedlings).
Biotic factors: Competition from grasses and shrubs that establish faster after snowmelt, shading out tree seedlings. Snow gums require several snow-free months to photosynthesise enough to survive winter.
Prediction: A +2°C warming would shift the treeline upward by approximately 150–200 m. However, if warming also brings more frequent fire, the treeline may not advance because adult trees are killed and seedlings cannot establish in the post-fire window before the next fire.
Current pattern: Since 2016, mass bleaching events have affected over 50% of Great Barrier Reef corals. Coral cover on inshore reefs has declined most severely.
Abiotic factors: Elevated sea surface temperature (>29°C sustained for weeks) disrupts the coral-zooxanthellae mutualism. Acidification reduces calcification rates. Sediment runoff from agricultural land smothers corals after cyclones.
Biotic factors: Bleached corals lose their primary energy source and become vulnerable to disease and overgrowth by macroalgae. Herbivorous fish (parrotfish, surgeonfish) that control algae are depleted by overfishing on some reefs, accelerating the shift from coral-dominated to algae-dominated states.
Prediction: Under continued warming, coral cover will decline further, becoming restricted to deeper, cooler refuges and southern latitudes. Macroalgae and soft corals will dominate shallow inshore reefs. The biodiversity of reef-associated fish will decline as structural complexity is lost.
Current pattern: The European starling, introduced to Australia in the 1850s, competes aggressively with native hollow-nesting birds for tree hollows.
Abiotic factors: Starlings tolerate a wide temperature range and diverse habitats, giving them broader distribution than many specialists. They also prefer hollows in open woodland — the same habitat cleared for agriculture, concentrating competition.
Biotic factors: Interspecific competition for nest hollows — starlings are aggressive cavity squatters that evict rosellas, kingfishers, and parrots. Starlings also outcompete native species in foraging efficiency.
Population dynamics: Starling populations grow logistically with a high carrying capacity in modified landscapes. Native bird populations are suppressed below their potential K because the limiting resource (hollows) is monopolised.
Prediction: In areas with high starling density and few hollows, native hollow-dependent species will decline locally. Conservation efforts must increase hollow availability (nest boxes, retained old trees) to raise the effective carrying capacity for natives.
These three misconceptions appear repeatedly in HSC responses. Learn to recognise and correct them before they cost you marks.
Students often write “the carrying capacity of the ecosystem is 500” as if it were a permanent property of the land. This is wrong. Carrying capacity (K) is the population size that can be sustained given current resources, predators, competitors, and climate. It changes constantly.
K fluctuates with drought (less water = lower K for grazers), predator removal (fewer predators = higher K for prey), habitat destruction, and seasonal resource pulses. During the Millennium Drought (2001–2009), the carrying capacity for kangaroos in the Murray-Darling Basin dropped by over 60% as pasture dried. When rains returned, K recovered. The ecosystem did not change — the resources did.
Gause’s principle states that two species with identical niches cannot coexist indefinitely. Students extrapolate this to mean that the inferior competitor always goes extinct.
Competitive exclusion leads to local exclusion, not necessarily global extinction. The inferior competitor may persist in microhabitats where the superior competitor is absent, or it may evolve niche differentiation (spatial, temporal, or morphological partitioning). Red kangaroos and eastern grey kangaroos coexist across much of eastern Australia because they use different habitats and feed at different heights — competitive exclusion was avoided through resource partitioning.
Students assume parasites are purely destructive and that host populations would be better off without them.
While parasitism harms individual hosts, host populations often adapt over evolutionary time. Coevolution can reduce parasite virulence (a dead host is a dead habitat for the parasite). In some cases, parasites regulate host populations below carrying capacity, preventing overexploitation of resources. The myxoma virus introduced to control rabbits in Australia initially killed 99% of infected rabbits, but both host and parasite evolved — rabbits became more resistant, and the virus evolved lower virulence. The population now persists in a dynamic equilibrium.
Apply the multi-factor framework to predict outcomes in a complex Australian scenario. Show your reasoning at each step.
Climate models predict that by 2050, the Australian Alps will be 2.5°C warmer on average, with 20% less snowfall and a 40% increase in bushfire frequency. Use the multi-factor framework to answer the following:
After completing your answer, check each box. A Band 6 response should satisfy all criteria:
Framework
Multi-factor prediction: Step 1 — abiotic thresholds (Shelford’s Law); Step 2 — biotic interactions (Liebig’s Law, competition, predation, mutualism); Step 3 — population dynamics (K, logistic growth, density-dependent feedback).
Mangroves
Mangrove distribution is limited by salinity, tidal inundation, and sediment type. Sea level rise will shift mangroves landward where topography allows; seawalls prevent migration and cause squeeze.
Alpine treeline
Snow gums reach their limit at ~1,800 m due to temperature, wind, soil depth, and frost heaving. Warming shifts treeline up, but increased fire may prevent advance by killing adult trees and seedlings.
Misconception 1
Carrying capacity is NOT fixed. K changes with resource availability, predation, competition, and disturbance. Example: kangaroo K dropped 60% during the Millennium Drought.
Misconception 2
Competitive exclusion causes local exclusion, not necessarily global extinction. Niche differentiation (resource partitioning) allows coexistence — red and eastern grey kangaroos are the Australian example.
Syllabus link
ACSBL049, ACSBL050, ACSBL060: Predict how distribution and abundance are affected by environmental factors; integrate abiotic, biotic, and population concepts into conditional predictions.
Now that you have completed the lesson, review your initial answers. What did you get right? What surprised you?
Q1. The Murray-Darling River system is predicted to warm by 3°C and experience increased salt levels over the next 50 years. Predict which native species would be most at risk of local extinction and which introduced species might thrive. Justify your prediction using at least three factors from IQ2.
Q2. Many students believe that carrying capacity is a fixed number for any given ecosystem. Explain why this belief is incorrect, using a real example to support your answer.
In this consolidation lesson you integrated all of IQ2: