Abiotic Factors — The Physical and Chemical Environment
Drive through the Snowy Mountains and you will see a sharp line where snow gums stop and alpine herbfields begin. That line is not drawn by competition or predation — it is drawn by temperature, wind and soil. Understanding how non-living factors shape where organisms can survive is the foundation of distribution ecology.
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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 snow gum (Eucalyptus pauciflora) grows up to exactly 1,800 m elevation in the Snowy Mountains but no higher. What non-living factor(s) do you think prevent it from growing above this line? How would you test your prediction?
Q2. If the average temperature in the Snowy Mountains increased by 2 degrees Celsius over the next 50 years, predict what would happen to the distribution of snow gums. Would the treeline move up, down or stay the same? Explain your reasoning.
Key Terms
A non-living physical or chemical component of an ecosystem that influences the survival, distribution and abundance of organisms. Examples: sunlight, temperature, water, salinity, pH, soil.
Abiotic factors in ecosystems: light, temperature, water, soil and gases
A living component of an ecosystem that influences other organisms. Examples: predation, competition, symbiosis, disease, grazing pressure.
The range of values for an abiotic factor within which an organism can survive. Includes an optimal zone, zones of physiological stress, and lethal limits beyond which the organism dies.
The single abiotic factor that is furthest from the organism's optimal range and therefore most strongly limits its growth, reproduction or distribution at a given time and place.
The timing of seasonal biological events (flowering, fruiting, migration, breeding) in response to environmental cues such as temperature and day length.
An organism whose body temperature is regulated primarily by external heat sources (e.g. reptiles, fish, amphibians, invertebrates). Activity levels and metabolic rates are strongly temperature-dependent.
The Major Abiotic Factors
Every organism on Earth exists within a specific range of physical and chemical conditions. Push those conditions too far in any direction — too hot, too salty, too acidic — and the organism cannot survive. These non-living influences are called abiotic factors, and they are the primary determinants of where species can live.
☼ Sunlight
Intensity and photoperiod (day length) determine photosynthesis rate, plant distribution and plant phenology. In Australia, eucalypts in shaded rainforest understories grow slower and taller than those in open woodland. Seasonal changes in photoperiod trigger flowering in many native plants.
🌡 Temperature
Affects enzyme activity, metabolic rates, and geographic ranges. Ectotherms like skinks and snakes are inactive on cold mornings. Seasonal and daily variation creates distinct ecological niches. Connects to Module 1: enzymes have optimal temperatures; denaturation occurs at extremes.
💧 Water
Availability (rainfall, humidity) is the primary driver of biome distribution. Australia has tropical rainforests in the north (2,000+ mm/year), temperate forests in the east (1,000 mm/year), and semi-arid shrubland in the interior (<250 mm/year). Water quality (dissolved O2, turbidity) affects aquatic organisms.
🧊 Salinity
Determines distribution between freshwater, estuarine and marine organisms. Osmotic stress damages cells when internal and external salt concentrations differ. Mangroves and saltbushes have adaptations to tolerate high salinity; most freshwater fish cannot survive in seawater.
pH
Soil pH affects mineral availability (acidic soils lock up phosphorus) and microbial decomposition rate. Water pH affects aquatic organisms — acid rain lowers lake pH and kills fish; ocean acidification (pH falling from 8.1 to 8.0) dissolves calcium carbonate shells of molluscs and corals.
🌎 Atmospheric gases and soil
CO2 for photosynthesis; O2 for aerobic respiration. Dissolved O2 in water is critical for fish and aquatic invertebrates — warm water holds less O2 than cold water. Soil texture, mineral content and organic matter affect plant establishment and invertebrate communities.
Tolerance Ranges and the Zone of Physiological Stress
No organism can survive under all possible environmental conditions. Every species has a tolerance range for each abiotic factor — a span from the minimum tolerable value to the maximum tolerable value.
Within this range, there is an optimal zone where the organism thrives: growth is fastest, reproduction is most successful, and survival is highest. As conditions move away from the optimum toward the extremes, the organism enters zones of physiological stress: it can survive but growth and reproduction are reduced. Beyond the tolerance limits lie lethal conditions where the organism cannot survive.
< 5 degrees C
5-18 degrees C
18-32 degrees C
32-40 degrees C
> 40 degrees C
Temperature tolerance schematic for illustration — actual values vary by species and acclimation.
Shelford's Law of Tolerance states that an organism's distribution is controlled by the environmental factor for which it has the narrowest tolerance. If a plant can tolerate temperatures from 0 to 40 degrees C but requires soil pH between 6.0 and 6.5, then pH — not temperature — is the factor that limits its distribution.
HSC exam tip: When asked to explain tolerance ranges, always include: (1) the optimal zone where the organism thrives, (2) the zones of stress where survival is possible but reduced, and (3) the lethal limits beyond which the organism dies. Simply listing a minimum and maximum is insufficient for Band 5-6.
Limiting Factors — Liebig's Law of the Minimum
A population is rarely limited by just one abiotic factor. However, at any given time and place, there is usually one factor that is most strongly restricting growth, reproduction or distribution. This is called the limiting factor.
Liebig's Law of the Minimum states that growth is dictated not by total resources available, but by the scarcest resource (the limiting factor). A farmer can provide perfect sunlight, temperature and soil pH — but if nitrogen is deficient, crop growth will still be poor. Nitrogen is the limiting factor.
Limiting factors change over time and space:
- Seasonal change: Water is the limiting factor for Australian grasslands in summer; temperature may be limiting in winter.
- Spatial change: Nitrogen limits plant growth in many Australian soils; phosphorus limits growth in others.
- Life stage change: Oxygen is limiting for fish eggs in stagnant water; food becomes limiting for adult fish.
The concept of limiting factors is critical for understanding why organisms live where they do — and why they do not live everywhere they could theoretically tolerate. A species might be able to tolerate the temperature of a region but be excluded because the soil pH is wrong, or because water is unavailable during the breeding season.
Australian Anchor: The Snow Gum Treeline
The snow gum (Eucalyptus pauciflora) is the hardiest eucalypt in Australia. It survives winter temperatures below -10 degrees C, gale-force winds, and heavy snow. Yet it stops growing at approximately 1,800 metres elevation in the Snowy Mountains — a sharp, visible treeline.
What limits the snow gum? Research shows it is not a single factor but a combination:
- Temperature: The growing season above 1,800 m is too short (snow cover persists into November; frost returns in March). The tree cannot photosynthesise long enough to accumulate the carbohydrates needed for growth and reproduction.
- Wind: Wind speeds increase with altitude, causing physical damage (desiccation, broken branches) and increasing transpiration rates beyond what roots can replace.
- Soil: Alpine soils above 1,800 m are thin, poorly developed and subject to frost heave, which damages root systems.
Interestingly, snow gum seedlings can germinate above the treeline during warm summers. But they rarely survive their first winter — the combination of freezing temperatures, wind desiccation and frost heave kills them. The treeline is not a fixed barrier but a dynamic equilibrium where mortality exceeds recruitment.
Climate models predict that a 2 degrees C warming would shift the snow gum treeline upward by approximately 150-200 metres. This has profound implications for alpine ecosystems: herbfields and bogs would be invaded by woodland, altering habitat for endemic species like the mountain pygmy possum and corroboree frog.
Copy Into Your Books
▼Abiotic = non-living physical/chemical factors (sunlight, temperature, water, salinity, pH, soil). Biotic = living interactions (predation, competition, symbiosis, disease).
Lethal low → stress low → optimal → stress high → lethal high. The factor with the narrowest tolerance controls distribution (Shelford's Law).
Liebig's Law of the Minimum: growth is dictated by the scarcest resource, not the total available. The limiting factor changes with season, location and life stage.
Sunlight (intensity, photoperiod), temperature (enzyme activity, metabolic rate), water (availability, quality), salinity (osmotic stress), pH (mineral availability), atmospheric gases (CO2, O2), soil (texture, minerals, organic matter).
Abiotic Factor Analysis — Australian Ecosystems
Predicting Distribution Changes
Test Your Understanding
1. Which of the following is an abiotic factor?
2. A lizard is most active when the air temperature is 28 degrees C. At 15 degrees C it moves slowly and feeds little. Above 38 degrees C it seeks shade and becomes torpid. Which statement best describes its tolerance range for temperature?
3. A wheat farmer provides optimal sunlight, temperature and soil pH. Despite this, wheat growth is poor. Soil testing reveals nitrogen levels are very low. Which principle best explains this observation?
4. Ocean acidification (falling pH) is predicted to harm shell-forming organisms such as oysters and corals. Which mechanism explains this effect?
5. A student argues: "Abiotic factors are more important than biotic factors in determining organism distribution, because if the temperature or pH is wrong, no amount of competition or predation matters — the organism will simply die." Evaluate this argument using specific Australian ecosystem examples.
Short Answer Questions
6. The mountain pygmy possum (Burramys parvus) lives only in alpine rock heaps above 1,500 m in the Snowy Mountains. It requires: (a) temperatures below 10 degrees C for hibernation; (b) reliable winter snow cover for insulation; (c) bogong moths as a primary food source in summer.
(a) Identify which of these three requirements are abiotic factors and which are biotic factors. 2 MARKS
(b) Explain why climate warming that reduces snow cover and shifts moth migration timing would threaten this species, using the concepts of tolerance ranges and limiting factors. 3 MARKS
7. Explain how each of the following abiotic factors affects the distribution of organisms in Australian ecosystems. For each factor, give a specific Australian example and connect the explanation to your knowledge from Module 1 (cell biology). 5 MARKS
(i) Temperature
(ii) Water availability
(iii) Salinity
8. Using the snow gum treeline case study, evaluate whether a 2 degrees C temperature increase would be more likely to shift the treeline upward or to change the species composition of the alpine herbfield below the treeline. In your answer, consider: (a) which abiotic factor is currently most limiting for snow gum recruitment above 1,800 m; (b) how warming would affect this factor and two other abiotic factors; (c) whether other eucalypt species might colonise the alpine zone before snow gums do; and (d) the implications for endemic alpine species such as the mountain pygmy possum. 6 MARKS
Revisit Your Thinking
Return to your Think First responses at the start of this lesson.
- Q1 — snow gum limit: Did you identify temperature, wind and soil as the key factors? Did you recognise that it is the combination of factors (not just one) that creates the treeline? Did you propose a transplant experiment or greenhouse experiment to test your prediction?
- Q2 — warming prediction: Did you predict the treeline would move upward (150-200 m) because temperature is currently limiting? Did you consider that other factors (wind, soil) might become limiting at the new upper boundary?
- Write the distinction between abiotic and biotic factors from memory, with one example of each.
Comprehensive Answers
▼Activity 1 — Abiotic Factor Analysis
(a) Great Barrier Reef: Primary = temperature (coral bleaching occurs above 29-30 degrees C for extended periods) [0.5 marks]. Secondary = light availability / turbidity (sediment runoff reduces light for zooxanthellae photosynthesis) [0.5 marks]. Interaction: warming + turbidity together cause more severe bleaching than either factor alone; turbidity reduces the temperature threshold for bleaching because stressed corals are less resilient [0.5 marks].
(b) Semi-arid shrubland: Primary = water availability (rainfall <250 mm/year limits plant growth and determines biome boundaries) [0.5 marks]. Secondary = temperature / soil nutrient content (high temperatures increase evaporation; low soil organic matter reduces water retention) [0.5 marks]. Interaction: high temperature + low rainfall = extreme water stress; only drought-adapted species (saltbush, bluebush) survive [0.5 marks].
(c) Alpine zone: Primary = temperature (growing season too short above 1,800 m; winter temperatures lethal for most plants) [0.5 marks]. Secondary = wind (causes physical damage and desiccation) or soil (thin, poorly developed, frost heave) [0.5 marks]. Interaction: wind + low temperature = wind chill, increasing desiccation and mechanical damage beyond what either factor alone would cause [0.5 marks].
(d) Coastal wetland: Primary = salinity (determines which plant species can establish; mangroves tolerate high salinity; reeds tolerate low salinity) [0.5 marks]. Secondary = water level / tidal inundation frequency (affects oxygen availability to roots and seed germination) [0.5 marks]. Interaction: salinity + inundation together create distinct zonation patterns (mangroves at high salinity/high inundation; saltmarsh at intermediate; reeds at low salinity/low inundation) [0.5 marks].
Activity 2 — Predicting Distribution Changes
(a) (1) Treeline moves upward 150-200 m as temperature limitation is relaxed [1 mark]. (2) Alpine herbfield area decreases as snow gum woodland invades [1 mark]. (3) Increased fire risk in alpine zones due to drier conditions and more woody fuel [1 mark].
(b) As water evaporates from a shrinking river, dissolved salts become more concentrated (same solute mass in less solvent volume) [1 mark]. Consequence 1: freshwater fish (e.g. Murray cod) experience osmotic stress as external salinity rises above their tolerance range; they must expend more energy on osmoregulation [1 mark]. Consequence 2: reduced dissolved O2 (warmer water holds less O2) causes hypoxia, stressing fish and invertebrates [1 mark].
(c) CO2 dissolves in seawater to form carbonic acid (H2CO3), which dissociates to release H+ ions, lowering pH [0.5 marks]. Increased H+ reacts with carbonate ions (CO3 2-) to form bicarbonate (HCO3-), reducing the concentration of carbonate available for shell formation [1 mark]. Shell-forming organisms (oysters, corals, foraminifera) require carbonate ions to precipitate CaCO3; with less carbonate, shell growth slows or shells dissolve [0.5 marks]. Connects to Module 2: calcium carbonate is a structural component of shells and coral skeletons.
(d) (1) Water table drops — aquatic organisms (fish, frogs, waterbirds) lose habitat [0.5 marks]. (2) Salinity increases as evaporation concentrates salts — salt-sensitive plants die [0.5 marks]. (3) Soil oxidation — drained peat soils release CO2 and sulfides become oxidised to sulfuric acid, dropping pH to 2-3 and killing vegetation [0.5 marks]. (4) Loss of shade and humidity — microclimate becomes drier and hotter, favouring terrestrial weeds over wetland specialists [0.5 marks].
Multiple Choice
1. B — Soil pH is abiotic; the others are biotic interactions.
2. D — Correctly identifies optimal, stress and lethal zones.
3. A — Liebig's Law: nitrogen is limiting despite other factors being optimal.
4. C — Lower pH reduces carbonate ion concentration, the chemical basis of shell formation.
5. B — Abiotic sets fundamental niche; biotic determines realised niche. Both are essential.
Short Answer Model Answers
Q6 (5 marks): (a) Abiotic: temperature below 10 degrees C (for hibernation) and reliable winter snow cover (insulation) [1 mark]. Biotic: bogong moths as food source [1 mark]. (b) Temperature and snow cover: if winter temperatures rise above the possum's tolerance for hibernation (upper stress zone), metabolic rates during hibernation increase and fat reserves are depleted before spring [1 mark]. Snow cover reduction removes insulation, exposing possums to lethal cold snaps and temperature fluctuations [0.5 marks]. Moth timing: if moths arrive earlier due to warming, they may not coincide with the possum's post-hibernation feeding period — a phenological mismatch [0.5 marks]. The combination of three changed factors (temperature, snow, food timing) means multiple abiotic and biotic factors simultaneously move toward stress zones, creating a compounding threat [1 mark]. Total: 5 marks.
Q7 (5 marks): (i) Temperature affects enzyme activity (Module 1: enzymes have optimal temperatures; denaturation at extremes) [0.5 marks]. Australian example: eastern water dragons bask in morning sun to raise body temperature for digestion; on cold days they are torpid and vulnerable to predators [0.5 marks]. (ii) Water availability affects osmosis and turgor pressure (Module 1: water potential, osmotic adjustment) [0.5 marks]. Australian example: saltbush (Atriplex) accumulates salt in bladder cells to maintain water uptake in saline, dry soils [0.5 marks]. (iii) Salinity creates osmotic stress across cell membranes (Module 1: hypertonic environments cause water loss; osmoregulation required) [0.5 marks]. Australian example: barramundi can tolerate fresh to brackish water but migrate to estuaries to spawn; they osmoregulate using specialised gill cells [0.5 marks]. Clear structure and Module 1 connections: [1 mark]. Total: 5 marks.
Q8 (6 marks): (a) Temperature is currently most limiting: the growing season above 1,800 m is too short for snow gums to accumulate sufficient carbohydrates for growth and reproduction [1 mark]. (b) Warming effect on temperature: longer growing season, reduced frost damage to seedlings [0.5 marks]. Warming effect on wind: wind speeds remain high but wind chill decreases, reducing desiccation stress [0.5 marks]. Warming effect on soil: increased microbial activity speeds decomposition and nutrient release, but also increases erosion risk [0.5 marks]. (c) Other eucalypts (e.g. Eucalyptus delegatensis, E. dalrympleana) might colonise faster than snow gums because they grow taller and shade out the slower-growing snow gum seedlings [1 mark]. Snow gums are adapted to the harshest conditions but are poor competitors in milder environments [0.5 marks]. (d) Implications: reduced herbfield area = less habitat for mountain pygmy possum (requires rock heaps in herbfields) [0.5 marks]. Warmer, drier conditions also reduce bogong moth numbers (less snowmelt moisture = fewer moth breeding sites) [0.5 marks]. Evaluated conclusion: warming would affect both the treeline position AND the community composition below it, with cascading effects on endemic alpine fauna. Protecting refugia (deep rock piles, north-facing slopes) may be critical [1 mark]. Total: 6 marks.