Biology • Year 11 • Module 4 • Lesson 12

Abiotic and Biotic Factors Synthesis — Predicting Distribution

Build Band 5–6 extended-response technique — integrating abiotic thresholds, biotic interactions and population dynamics into conditional, evidence-based predictions.

Master · Extended Response

1. Extended response — integrate the three-step framework to predict distribution (Band 5–6)

7 marks   Band 5–6

Q1. Predict and explain how the distribution and abundance of a species are determined by the integration of abiotic, biotic and population dynamic factors. In your response you must:

  • Define carrying capacity and explain why it is not fixed.
  • Apply Shelford’s Law of Tolerance and Liebig’s Law of the Minimum and explain how each limits distribution.
  • Describe how at least one biotic interaction (competition, predation, mutualism, or parasitism) modifies the distribution of a species beyond its abiotic tolerance range.
  • Use at least one named Australian example to illustrate the integrated framework.
  • Reach a conditional prediction: “if [factor] changes, then…”
Plan first: define K (not fixed) → Shelford (physical limits) → Liebig (scarce resource) → one biotic interaction → Australian example → conditional prediction. Use any case study from the lesson; the Murray-Darling fish scenario or the alpine treeline both work well.

2. Stimulus-based extended response — the Millennium Drought and Murray-Darling fish (Band 5–6)

8 marks   Band 5–6

Stimulus. The Millennium Drought (2001–2009) caused pasture cover across the Murray-Darling Basin to drop by 60–70%. During this period, the kangaroo carrying capacity in the region fell by over 60% as food supply collapsed. Simultaneously, a 3°C increase in mean summer water temperature was recorded in the lower Murray River. Murray cod (Maccullochella peelii) — Australia’s largest freshwater fish — are thermal specialists: they spawn only when water temperature drops below 17°C in spring, and survival of juveniles is severely reduced above 28°C. Common carp (Cyprinus carpio), introduced from Europe in the 1960s, tolerate water temperatures from 4°C to 35°C and thrive in turbid, degraded conditions.

Q2. Analyse and evaluate how the conditions described in the stimulus altered the distribution and abundance of Murray cod and common carp. In your answer:

  • Apply Shelford’s Law to explain how rising water temperature affected Murray cod distribution and reproductive success.
  • Use Liebig’s Law to identify the single factor most limiting Murray cod abundance during the drought.
  • Explain how carp benefited from the same abiotic changes that stressed Murray cod, using the concept of interspecific competition.
  • Predict how the Murray cod carrying capacity (K) changed during and after the drought, and identify one management action that could raise K for Murray cod.
  • The kangaroo carrying capacity example is mentioned in the stimulus. Explain what this illustrates about the nature of K as a concept, and apply this principle to Murray cod.
Use the stimulus temperature data directly for Shelford’s Law (spawning threshold 17°C, lethal threshold 28°C). Connect carp’s wider tolerance to their competitive advantage. The kangaroo K example illustrates K is not fixed — apply the same logic to cod.

3. Evaluate this claim (Band 5–6)

6 marks   Band 5–6

“Abiotic factors alone determine where a species can live. If temperature, rainfall and soil type are suitable, a species will be found there. Biotic interactions like competition and predation are secondary — they might reduce numbers but they cannot prevent a species from being present in a habitat.”

Q3. Evaluate this claim. Identify which parts are scientifically defensible, which are incorrect or incomplete, and reformulate the claim into a statement that correctly reflects the multi-factor prediction framework.

The distinction between “fundamental niche” (what abiotic tolerance allows) and “realised niche” (what biotic interactions leave) is the key. Use the European starling vs native hollow-nesters example or the snow gum seedling competition example to show that biotic interactions can prevent species presence.
Answers — Do not peek before attempting

Q1 — Sample Band 6 response (7 marks), annotated

Carrying capacity (K) is the maximum population size that can be sustained by available resources in a habitat at a particular time. K is not fixed — it fluctuates with changes in resource availability, predation pressure, competition intensity and disturbance events. During the Millennium Drought, kangaroo K in the Murray-Darling Basin dropped by over 60% as pasture cover collapsed, demonstrating that K tracks resource supply rather than being a permanent property of the ecosystem. [1 — defines K + explains why not fixed with example]

Shelford’s Law of Tolerance sets the fundamental abiotic limits: a species can only survive within a range of each physical factor (temperature, salinity, pH, rainfall) and performs best at the optimum. If climate shifts beyond this range, the species must adapt, migrate or die. Liebig’s Law of the Minimum identifies which limiting factor actually controls population size: even when most resources are adequate, the scarcest single resource (the “minimum”) caps population growth. A species can survive within its abiotic tolerance but still fail to reach its theoretical maximum abundance because one resource is critically limiting. [1 — Shelford’s Law applied; 1 — Liebig’s Law applied and distinguished from Shelford]

Biotic interactions further constrain distribution beyond abiotic thresholds. Even a species well within its abiotic tolerance can be competitively excluded from a habitat by a superior competitor. For example, European starlings introduced to Australia outcompete native crimson rosellas and other hollow-nesting birds for nest hollows in open woodland. Rosellas remain within their abiotic tolerance for temperature and rainfall across much of south-east Australia, but their actual distribution and abundance are suppressed below potential K because starlings monopolise the limiting resource (hollows) through interspecific competition. The rosella’s realised niche is smaller than its fundamental niche because of this biotic constraint. [1 — biotic interaction named and applied; 1 — Australian example with mechanism]

Applying the framework as a conditional prediction: if nest hollow availability is increased (through nest box installation or retention of large old trees), then the effective K for rosellas rises, interspecific competition intensity decreases, and rosella populations will increase toward the new, higher K. Conversely, if land clearing continues to reduce hollow availability, K for rosellas will decline further regardless of abiotic conditions remaining suitable. [1 — conditional prediction using if/then structure]

In summary, species distribution and abundance are not determined by abiotic tolerance alone but by the integration of all three steps: abiotic thresholds (Shelford), the limiting factor (Liebig), and the web of biotic interactions — each acting simultaneously to produce the realised niche. [1 — integration stated explicitly]

Marking criteria:

  • 1 mark — Defines K correctly and explains why it is not fixed (with a supporting example or mechanism).
  • 1 mark — Correctly applies Shelford’s Law (tolerance range; beyond limits = cannot survive).
  • 1 mark — Correctly applies Liebig’s Law (scarcest resource limits population; distinct from Shelford).
  • 1 mark — Describes how one named biotic interaction further limits distribution within the abiotic tolerance range.
  • 1 mark — Uses at least one named Australian example with a specific mechanism (not just a species name).
  • 1 mark — Produces a conditional prediction in if/then structure that integrates at least two factors.
  • 1 mark — Explicitly states that distribution is determined by integration of all three factor types (not any one alone).

Q2 — Sample Band 6 response (8 marks), annotated

Shelford’s Law of Tolerance: Murray cod spawning requires water temperature below 17°C in spring. A 3°C warming in the lower Murray would push late-spring temperatures beyond the spawning trigger point, reducing or eliminating reproductive events in affected reaches. Summer temperatures above 28°C would directly reduce juvenile survival through thermal stress. Both effects reduce recruitment and lower the population growth rate below replacement level. [1 — Shelford applied specifically to cod thresholds from the stimulus]

Liebig’s Law of the Minimum: during the drought, multiple factors simultaneously stressed cod (temperature, reduced flow, reduced habitat). Liebig’s Law tells us that the single most limiting of these — thermal stress at spawning temperatures — set the ceiling on population recovery. Even if flows partially recovered, cod recruitment remained suppressed while water temperatures stayed elevated. Thermal threshold is the minimum factor here. [1 — Liebig applied with identification of the single most limiting factor]

Interspecific competition: carp tolerate 4–35°C and thrive in turbid, degraded conditions. As water temperatures rose and cod populations fell, carp occupied the ecological space vacated by declining cod, competing for food resources (benthic invertebrates, small fish) and spawning habitat. With cod numbers suppressed below K, carp effectively expanded their realised niche. This is interspecific competition with an asymmetric outcome: environmental change benefited the generalist (carp) at the expense of the specialist (cod). [1 — interspecific competition mechanism; carp advantage via broader tolerance; 1 — correctly uses carp’s thermal data from stimulus]

Changes in K: Murray cod K fell during the drought as thermal habitat (cool, well-oxygenated reaches) contracted and spawning success approached zero in the warmest years. After the drought broke and temperatures partially moderated, K partially recovered — demonstrating that K tracked environmental conditions rather than being fixed. A management action to raise K would include cold-water releases from deep-draw dams, which restore the thermal refuges cod require for spawning and juvenile survival. [1 — K change described during and after drought; 1 — management action with mechanism]

The kangaroo K example illustrates the core principle that K is a dynamic property of the environment, not an intrinsic property of the species. Pasture decline during the drought reduced kangaroo K by over 60%; when rains returned, K recovered. The same logic applies to Murray cod: their K is determined by available thermal habitat, spawning conditions and food supply. When those resources contract (drought, warming), K falls; when they recover, K rises. K is always “carrying capacity given current conditions”, not a permanent ecological constant. [1 — kangaroo K example used to correctly generalise the non-fixed nature of K and applied to cod]

Marking criteria:

  • 1 mark — Applies Shelford’s Law using the specific cod temperature thresholds from the stimulus (spawning <17°C; juvenile mortality >28°C) to explain reduced distribution / reproductive success.
  • 1 mark — Applies Liebig’s Law by identifying temperature as the single most limiting factor for cod during the drought.
  • 1 mark — Explains carp’s competitive advantage using their broader thermal tolerance from the stimulus data.
  • 1 mark — Frames carp’s expansion as interspecific competition for the space / resources vacated by declining cod (asymmetric outcome due to tolerance difference).
  • 1 mark — Correctly describes K as falling during the drought and potentially recovering afterwards, tied to specific environmental conditions.
  • 1 mark — Identifies one specific, mechanistically justified management action to raise cod K (e.g. cold-water releases, flow supplementation, re-snagging for spawning habitat).
  • 1 mark — Uses the kangaroo K example from the stimulus to state and correctly apply the principle that K is dynamic and tracks resource availability.
  • 1 mark — Integrates all threads into a coherent account that links abiotic change → Shelford thresholds exceeded → population suppressed below K → competitive release for carp — reaching a justified prediction about cod abundance.

Q3 — Sample Band 6 response (6 marks)

The claim is partly correct but substantially incomplete and misleading in its conclusion. [1 — judgement]

What is defensible: Abiotic factors do establish the fundamental niche — the physical and chemical range within which a species can physiologically survive. Shelford’s Law confirms that outside the tolerance range, no population can persist regardless of biological conditions. Temperature, rainfall and soil type are real and important determinants of distribution. [1 — concedes the correct element]

What is wrong: The claim that biotic interactions “cannot prevent a species from being present” directly contradicts both Gause’s principle (competitive exclusion) and the empirical record. Snow gum seedlings survive within the abiotic tolerance range for temperature at 1,800 m but are shaded out by faster-establishing grasses and shrubs — biotic competition prevents establishment even where physical conditions are suitable. European starlings exclude rosellas from nest hollows in habitats well within the rosella’s abiotic tolerance. In both cases, biotic interactions reduce the realised niche to a subset of the fundamental niche, and can prevent a species from being present in an area where it could physiologically survive. [1 — refutes the claim using a specific named example; 1 — correctly uses fundamental vs realised niche logic]

The claim also implies that competition and predation only reduce numbers but do not affect presence — this ignores cases where populations are driven below the minimum viable population size. A species can be entirely absent from a habitat because biotic pressure has reduced numbers to extinction at that location (local exclusion). [1 — local exclusion / extinction as the extreme case]

Defensible reformulation: “Species distribution is determined by the integration of abiotic factors (which set the fundamental niche through tolerance limits), biotic interactions (which compress the realised niche within those limits through competition, predation and symbiosis), and population dynamics (which determine whether a population can persist given both constraints). A species will only be found where its abiotic tolerance is met and its biotic interactions permit establishment and reproduction. Neither set of factors can be considered secondary.” [1 — defensible reformulation integrating all three elements]

Marking criteria:

  • 1 mark — States an overall evaluative judgement (e.g. “partly correct but incomplete/misleading”).
  • 1 mark — Correctly identifies the defensible element: abiotic factors do set the fundamental niche / tolerance limits are real.
  • 1 mark — Refutes the claim using a specific named biotic example showing a species absent from a habitat within its abiotic tolerance due to biotic exclusion.
  • 1 mark — Correctly applies the fundamental niche vs realised niche logic to explain the distinction between “where a species can survive abiotically” and “where it is actually found”.
  • 1 mark — Notes the extreme case: biotic interactions can lead to local exclusion / absence (not just reduced numbers).
  • 1 mark — Reformulates the claim into a biologically accurate statement integrating abiotic tolerance, biotic interactions, and the distinction between fundamental and realised niche.

HSCScience • Biology • Year 11 • Module 4 • Lesson 12 • Worksheet 3 (Master)