Year 11 Biology Module 4 · IQ2 Lesson 11 of 18 ~35 min

Comparing Ecosystems — Abiotic and Biotic Differences

The Great Barrier Reef hosts over 1,600 fish species across 2,300 kilometres of coastline. The Southern Ocean surrounding Antarctica, by contrast, supports some of the most productive plankton blooms on Earth — yet far fewer species. Why does high productivity not always mean high biodiversity? The answer lies in how abiotic conditions shape the relationships between organisms, determining which species can survive, compete, and cooperate in each environment.

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Think First

Before you read, commit to a prediction. You will revisit these at the end.

Q1. A tropical rainforest and a semi-arid desert receive vastly different amounts of rainfall. Predict how this difference would affect: (a) the number of species living in each ecosystem, (b) the type of competition most common in each, and (c) the importance of mutualistic relationships such as pollination and mycorrhizae.

Q2. The waters around Antarctica are extremely cold but nutrient-rich, supporting enormous phytoplankton blooms. The Great Barrier Reef is warm but nutrient-poor. Predict which ecosystem would have higher biodiversity and explain your reasoning.

Terrestrial Ecosystems — How Climate Shapes Communities

On land, the dominant abiotic factors are rainfall, temperature, and seasonality. These three variables alone explain most of the variation in biodiversity, productivity, and the types of species interactions found across the world’s biomes.

🌳 Tropical Rainforest

Abiotic: High rainfall (>2,000 mm/year), year-round warmth (25–28°C), minimal seasonality
Biodiversity: Highest of any terrestrial ecosystem; up to 300 tree species per hectare
Competition: Intense interspecific competition for light drives vertical stratification — canopy, understory, shrub layer, forest floor
Mutualism: Extremely important — pollination by specialised insects/birds, mycorrhizae on nearly all trees, seed dispersal by fruit-eating animals
Decomposers: Highly active due to warmth and moisture; nutrients recycled rapidly; soils are surprisingly nutrient-poor because biomass is locked in living tissue

🏜️ Semi-Arid Scrubland

Abiotic: Low, unpredictable rainfall (250–500 mm/year), extreme temperature swings (hot days, cold nights), high evaporation
Biodiversity: Lower species richness; dominated by drought-tolerant generalists
Adaptations: Succulence (cacti, saltbush), deep taproots, reduced leaf surface area, dormancy during dry periods
Competition: Primarily intraspecific competition for water; spacing between plants reflects root zones of exclusion
Mutualism: Less specialised and more facultative; relationships break down during drought when partners cannot meet obligations

🌿 Temperate Grassland / Woodland

Abiotic: Seasonal rainfall with dry summers or winters; moderate temperatures; fire is a regular disturbance
Biodiversity: Moderate; dominated by grasses with scattered trees and herbs
Trophic structure: Grass → grazer → predator dominates; kangaroos, wallabies, and introduced grazers are key herbivores
Competition: Intraspecific competition intensifies in dry season when grass biomass is lowest
Fire adaptation: Many Australian grasses resprout from lignotubers; eucalypts have epicormic buds beneath bark

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Australian context: Australia spans all three terrestrial types. The Daintree Rainforest (Queensland) contains ancient lineages found nowhere else. The Mulga scrublands of the interior are dominated by Acacia aneura with sparse understory. The temperate woodlands of the Murray-Darling Basin, once vast, have been heavily cleared for agriculture — making them one of Australia’s most threatened ecosystems.

Aquatic Ecosystems — Light, Nutrients and Stratification

In water, the critical abiotic gradients are light availability, nutrient concentration, temperature, and salinity. These gradients create vertical and horizontal zones that structure aquatic communities just as rainfall and temperature structure land communities.

🌊 Marine Open Ocean

Light: Photosynthesis only in the photic zone (top ~200 m); below this is permanent darkness
Primary producers: Phytoplankton (microscopic algae and cyanobacteria) — base of the planktonic food web
Nutrients: Tropical surface waters are nutrient-poor because the thermocline (a steep temperature gradient) blocks vertical mixing; nutrients sink and stay below
Productivity: High in upwelling zones (cold, nutrient-rich water rises); low in tropical gyres
Biodiversity: Surprisingly high species richness in the deep sea, but low abundance per unit area in surface waters

🐚 Coral Reef

Abiotic: Warm, clear, shallow water (<50 m); stable salinity; low dissolved nutrients — yet paradoxically the most biodiverse marine ecosystem
Foundation species: Coral polyps in mutualism with zooxanthellae (photosynthetic dinoflagellates) — corals provide shelter, zooxanthellae provide 90% of coral energy via photosynthesis
Biodiversity: Highest marine biodiversity; 25% of all marine species on <1% of ocean floor
Competition: Intense interspecific competition for substrate — space on the reef is the limiting resource; corals, sponges, and algae constantly overgrow each other
Three-dimensional structure: Complex physical habitat supports micro-habitats for specialised species

🐟 Freshwater Lake

Stratification: Temperature layers form in summer — warm surface (epilimnion), cold bottom (hypolimnion), with a thermocline between
Seasonal turnover: In autumn and spring, surface water cools or warms to match bottom water, allowing wind-driven mixing that redistributes oxygen and nutrients
Benthic zone: Bottom-dwelling decomposers break down organic matter; oxygen can become depleted in deep, stratified lakes
Riparian connections: Lakes are linked to surrounding land by nutrient input from runoff; vegetation along shorelines provides habitat and stabilises banks

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The thermocline explained: In tropical oceans and deep lakes, warm surface water sits atop cold deep water. Because warm water is less dense, it does not mix downward. Nutrients that sink from surface plankton are trapped below the thermocline, starving surface waters. This is why tropical oceans are often called “blue deserts” despite their clarity — they are nutrient-poor, not life-poor, because the life that is present is concentrated at the reef.

The Diversity-Productivity Paradox

Here is a puzzle that confuses many students: the Southern Ocean around Antarctica is one of the most productive marine ecosystems on Earth, with phytoplankton blooms so dense they are visible from space. Yet it supports far fewer species than the nutrient-poor Great Barrier Reef. Why?

Great Barrier Reef

Productivity: Moderate per unit area (zooxanthellae photosynthesis)

Biodiversity: Extremely high (>1,600 fish species, 400+ types of coral)

Key factors:

Stable, warm temperatures year-round allow specialisation
Complex 3D structure provides countless micro-habitats
Mutualisms (coral-zooxanthellae, cleaner fish, anemonefish) reduce competition pressure
Low nutrients select for efficient nutrient recycling within tight symbiotic loops
Long evolutionary time in stable conditions allowed adaptive radiation

Southern Ocean

Productivity: Very high per unit area (upwelling, nutrient-rich water)

Biodiversity: Low to moderate (krill, penguins, seals, whales — few species, huge populations)

Key factors:

Extreme seasonality: winter darkness stops photosynthesis entirely; ice cover restricts habitat
Harsh conditions select for generalists that tolerate wide ranges, not specialists
Simple habitat structure: open water and ice edge offer fewer niches than a reef
High productivity supports massive populations of few species (krill swarms)
Evolutionary time in unstable conditions favours ecological flexibility over specialisation

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The resolution: Biodiversity depends not just on energy availability, but on:

Environmental stability — stable conditions allow niche specialisation
Habitat complexity — more physical structure means more niches
Evolutionary time — older, undisturbed ecosystems accumulate species
Resource partitioning — finely divided resources support more species

The Southern Ocean has energy but lacks stability and habitat complexity. The Great Barrier Reef has less energy per unit area but converts it efficiently through mutualisms and supports extraordinary specialisation through stable conditions and complex structure.

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Australian connection: Australia is unique in bordering both ecosystems. The Great Barrier Reef Marine Park Authority monitors reef biodiversity through long-term transect surveys, while the Australian Antarctic Division tracks Southern Ocean productivity via satellite chlorophyll measurements and krill biomass estimates. Both datasets inform international climate and fisheries policy.

Activity: Analyse and Connect

Use the ecosystem comparison framework to analyse two contrasting Australian environments.

Part A — Analyse Two Ecosystems

Complete the comparison table for the Daintree Rainforest (Queensland) and the Simpson Desert (central Australia). For each row, explain why the difference exists.

Factor	Daintree Rainforest	Simpson Desert
Annual rainfall
Dominant producer type
Most important limiting factor
Type of competition most common
Role of mutualism

Part B — Predict and Justify

A new lake is formed in a temperate region of Tasmania. In its first 10 years, the lake is deep, clear, and low in nutrients (oligotrophic). Over the next 50 years, agricultural runoff increases nutrient input.

Predict how the lake’s productivity and biodiversity would change as nutrient levels increase. (2 marks)
Explain one positive effect and one negative effect of this change for the lake ecosystem. (2 marks)
Connect your answer to the diversity-productivity paradox discussed in this lesson. Under what conditions might high nutrients reduce biodiversity? (2 marks)

Copy Into Your Books

Key comparison

Tropical rainforest: High rainfall, year-round warmth, high biodiversity, intense interspecific competition for light, abundant mutualism (pollination, mycorrhizae), rapid decomposition, nutrient-poor soils.

Key comparison

Semi-arid scrubland: Low unpredictable rainfall, temperature extremes, lower biodiversity, drought-tolerant adaptations, intraspecific competition for water, facultative rather than obligate mutualism.

Key comparison

Coral reef: Warm, clear, shallow, nutrient-poor water; highest marine biodiversity; coral-zooxanthellae mutualism is foundational; intense interspecific competition for substrate; complex 3D habitat structure creates niches.

Key comparison

Marine open ocean: Photosynthesis only in photic zone (<200 m); phytoplankton base of food web; tropical surface waters nutrient-poor due to thermocline blocking upwelling; high productivity in upwelling zones.

Paradox resolution

High productivity does not guarantee high biodiversity. Biodiversity depends on environmental stability, habitat complexity, evolutionary time, and resource partitioning — not just energy availability.

Syllabus link

ACSBL051, ACSBL052: Compare and analyse differences in relationships between organisms in different ecosystems; predict how distribution and abundance are affected by abiotic and biotic factors across ecosystems.

Revisit Your Predictions

Now that you have completed the lesson, review your initial answers. What did you get right? What surprised you?

Lesson Summary

In this lesson you learned:

Terrestrial ecosystems differ primarily in rainfall, temperature, and seasonality, which determine biodiversity, competition type, and mutualism importance.
Tropical rainforests have the highest terrestrial biodiversity, intense interspecific competition for light, and abundant specialised mutualisms.
Semi-arid scrublands select for drought-tolerant generalists, intraspecific competition for water, and facultative mutualisms.
Temperate grasslands/woodlands show seasonal resource pulses, fire-adapted species, and grass-grazer-predator trophic dominance.
Aquatic ecosystems are structured by light availability, nutrient concentration, and the thermocline, which blocks vertical mixing in stratified waters.
Coral reefs achieve extraordinary biodiversity through stable conditions, complex 3D structure, and efficient coral-zooxanthellae mutualism despite low nutrient levels.
The diversity-productivity paradox shows that biodiversity depends on stability, habitat complexity, evolutionary time, and resource partitioning — not just energy input.

I have completed this lesson and copied the key points into my notes.

Practice Questions

Test your understanding of ecosystem comparisons

Q1. Which factor best explains why tropical rainforests have higher biodiversity than semi-arid scrublands?

A Rainforests have more decomposers, which directly produce new species

B Rainforest soils contain more nitrogen, allowing more plants to grow

C Higher rainfall and stable temperatures allow niche specialisation and complex layering

D Rainforest plants do not compete with each other because mutualism eliminates competition

Q2. In a coral reef ecosystem, what is the primary limiting resource that drives intense interspecific competition?

A Sunlight, because the water is too deep for photosynthesis

B Substrate space on the reef surface

C Dissolved oxygen, because warm water holds less oxygen

D Food particles, because filter feeders consume all plankton

Q3. Why are tropical open ocean surface waters typically nutrient-poor despite receiving abundant sunlight?

A Phytoplankton consume nutrients faster than they can be replaced

B Nutrients are locked in the bodies of large marine mammals

C Salinity is too high for nutrient dissolution

D The thermocline prevents vertical mixing of nutrients from deep water

Q4. Which ecosystem would you expect to have the most highly specialised mutualistic relationships?

A Tropical rainforest

B Semi-arid scrubland

C Southern Ocean

D Temperate grassland

Q5. The Southern Ocean supports enormous phytoplankton blooms and krill populations but relatively few species. What best explains this diversity-productivity paradox?

A Krill outcompete all other species for food

B Extreme seasonality and simple habitat structure favour generalist species

C High productivity directly causes low biodiversity through competitive exclusion

D The cold temperature prevents mutation, which is needed for speciation

Short Answer

Q1. Explain how the abiotic conditions of a coral reef lead to three different biotic characteristics. In your answer, describe the abiotic condition and link it directly to a biotic outcome. (4 marks)

Q2. Compare the role of competition in a tropical rainforest and a semi-arid scrubland. Your answer should identify the type of competition most important in each ecosystem, the resource being competed for, and explain why the difference exists. (4 marks)

Q3. The Great Barrier Reef supports over 1,600 fish species but has relatively low nutrient levels in its surface waters. The Southern Ocean surrounding Antarctica has very high nutrient levels and massive phytoplankton productivity but far fewer species.

(a) Explain why the Great Barrier Reef has high biodiversity despite low nutrients. Refer to mutualism, habitat structure, and environmental stability. (3 marks)

(b) Explain why the Southern Ocean, despite being highly productive, has relatively low species richness. (3 marks)

Comprehensive Answers

Any three of the following, with clear links (1 mark each, max 4):

Low nutrients + high light: Selects for the coral-zooxanthellae mutualism, where corals recycle scarce nutrients tightly within the symbiosis rather than relying on external supply (1 mark).

Shallow, clear water: Allows sunlight to reach reef-building corals, enabling photosynthesis by zooxanthellae and supporting the high-energy foundation of the entire food web (1 mark).

Stable warm temperatures: Allow niche specialisation over evolutionary time, producing the extraordinary species diversity found on reefs (1 mark).

Hard substrate: Provides attachment points for corals, which then build complex three-dimensional structure that creates micro-habitats for thousands of associated species (1 mark).

Tropical rainforest: Interspecific competition for light is the dominant competitive force (1 mark). The dense canopy blocks most sunlight, so seedlings, understory shrubs, and climbing vines compete fiercely for the limited light that reaches lower layers. This drives vertical stratification, with different species occupying different height niches (1 mark).

Semi-arid scrubland: Intraspecific competition for water is dominant (1 mark). Rainfall is the limiting factor, and plants space themselves according to the reach of their root systems. Competition is primarily within species because all individuals require the same scarce resource at the same time; there is little opportunity for niche partitioning when the single limiting resource is water (1 mark).

(a) The Great Barrier Reef achieves high biodiversity through several mechanisms despite low nutrients. First, the coral-zooxanthellae mutualism creates an efficient internal nutrient cycle, conserving scarce nitrogen and phosphorus within the coral tissue rather than losing them to the water column (1 mark). Second, the complex three-dimensional structure of the reef provides countless micro-habitats — crevices, branches, caves, and sandy patches — each supporting specialised species (1 mark). Third, stable warm temperatures over millions of years have allowed extensive adaptive radiation, producing specialised species finely tuned to narrow niches (1 mark).

(b) The Southern Ocean has low species richness despite high productivity because harsh conditions select against specialisation. Extreme seasonality — months of winter darkness and ice cover — means any species must be a generalist capable of surviving drastic change, not a narrow specialist (1 mark). The habitat is simple: open water and ice edge offer far fewer structural niches than a reef (1 mark). Consequently, the high productivity supports enormous populations of a few highly successful generalist species (krill, some copepods) rather than many specialised species (1 mark).