Investigate ecosystems as systems of biotic and abiotic interactions; analyse energy flow, biogeochemical cycles, and the spatial distribution of major terrestrial and aquatic ecosystems

Note: This page is part of the HSC Geography 11-12 (2022) syllabus, first examined in HSC 2025. The legacy 2009 syllabus "Biophysical Interactions" and "Ecosystems at Risk" content is preserved in sibling folders.

What this dot point is asking

This dot point asks you to treat an ecosystem as a system: a defined set of biotic and abiotic components linked by flows of energy, matter and information. You need to explain how the components interact (food webs, nutrient cycling), and you need to be able to locate the major ecosystem types on a world map and describe why they sit where they do. The geographical concepts to lean on are interconnection (between living and non-living parts), scale (a single rock pool through to a whole biome), and change (succession, disturbance, climate-driven shifts).

The answer

Defining an ecosystem

An ecosystem is a community of living organisms (the biotic component) interacting with the non-living environment (the abiotic component) within a defined area. The boundary is set by the question being asked: a rotting log, a coral bommie, a catchment, or an entire biome can all be analysed as ecosystems.

Biotic components. Producers (autotrophs such as plants, algae, phytoplankton); consumers (herbivores, carnivores, omnivores); decomposers (fungi, bacteria, detritivores).

Abiotic components. Climate (temperature, precipitation, sunlight, wind); substrate (soil, rock, sediment); water chemistry; topography; disturbance regimes (fire, flood, tropical cyclone).

The interaction between biotic and abiotic components produces the spatial pattern, structure and function of every ecosystem on Earth.

Energy flow and trophic levels

Energy enters most ecosystems through photosynthesis by producers, which convert solar radiation into chemical energy in organic compounds. (Deep-sea hydrothermal vent ecosystems are the exception; they rely on chemosynthesis.) Energy then moves up trophic levels:

• Trophic level 1. Primary producers (grasses, trees, phytoplankton).
• Trophic level 2. Primary consumers (herbivores: kangaroos, krill, caterpillars).
• Trophic level 3. Secondary consumers (carnivores eating herbivores: dingoes, small reef fish).
• Trophic level 4 and higher. Tertiary and apex consumers (sharks, eagles, large carnivores).

A food chain is a single linear pathway; a food web is the more accurate picture of many interlinked chains. Energy is lost at each transfer (largely as heat through respiration), so ecosystems can typically support only a small number of trophic levels and far fewer apex predators than producers. This is the ecological basis for why top predators are usually the first species lost when an ecosystem is disturbed.

Biogeochemical cycles

Unlike energy, matter is recycled. The major cycles to know:

Carbon cycle

CO2 is fixed by photosynthesis, transferred through food webs, returned by respiration and decomposition. Long-term storage occurs in vegetation, soils, ocean dissolved inorganic carbon, sediments and fossil fuels. Human combustion of fossil fuels is shifting carbon from long-term storage into the atmosphere on geological-scale timeframes.

Nitrogen cycle

Atmospheric N2 is fixed by lightning and by nitrogen-fixing bacteria (some free-living, some in legume root nodules). Nitrogen moves through nitrification, assimilation by plants, consumption, decomposition, and denitrification back to N2. Industrial Haber-Bosch fertiliser production has roughly doubled the rate of nitrogen fixation globally, contributing to eutrophication of waterways.

Water cycle

Evaporation, transpiration, condensation, precipitation, runoff, infiltration, groundwater flow. Ecosystems both depend on and shape water cycling: forests transpire water that supports downwind rainfall; wetlands moderate flood flows; mangroves trap sediment.

These cycles operate at scales from a single soil profile to the whole biosphere. They are the link between ecosystem function and the global sustainability challenges of climate change, pollution and freshwater stress.

Major ecosystem types and their distribution

Terrestrial biomes are distributed primarily by climate (temperature and precipitation patterns set by latitude, elevation and ocean currents):

• Tropical rainforest. Equatorial belt (Amazon, Congo, South-East Asia, parts of north Queensland). High biomass, high biodiversity, year-round warmth and rainfall.
• Savanna. Tropical and subtropical zones with a marked wet-dry season (East Africa, northern Australia, Cerrado in Brazil). Grass-dominated with scattered trees; fire is a key process.
• Desert. Mid-latitude high-pressure belts and continental interiors (Sahara, Arabian, Atacama, Australian arid zone). Low precipitation; high variability.
• Temperate forest. Mid-latitudes with adequate rainfall (eastern North America, Europe, parts of eastern Asia and south-eastern Australia).
• Boreal forest (taiga). High northern latitudes (Canada, Russia, Scandinavia). Conifer-dominated; cold winters.
• Tundra. Polar and high-alpine zones. Short growing seasons; permafrost in many areas; low species diversity but distinct cold-adapted communities.

Aquatic ecosystems sort by salinity, depth, light and temperature:

• Coral reef. Warm, shallow, low-nutrient tropical and subtropical seas (Great Barrier Reef, Coral Triangle, Caribbean). Highest marine biodiversity per area.
• Mangrove and estuary. Tropical and subtropical intertidal zones. Nurseries for many fish; coastal protection; substantial carbon storage in sediments.
• River and freshwater wetland. Globally distributed; structured by catchment hydrology and gradient.
• Open ocean (pelagic) and deep sea. The largest biome by volume; productivity concentrated in surface waters and at upwelling zones.

The geographical concept of spatial distribution asks you to explain not just where ecosystems are but why: latitude, prevailing winds, ocean currents, rain shadows, soil parent material, fire and flood regimes.

Examples in context

Example 1. Northern Australian savanna and fire regime. Australia's tropical savanna stretches across the Top End and northern Queensland. The system is structured by a sharp wet-dry seasonal cycle and by fire, which has been managed by Traditional Owners through patch-burning for many thousands of years. Without low-intensity early-dry-season burning, fuel loads build through the wet season and produce destructive late-dry-season wildfires. The Indigenous-led West Arnhem Land Fire Abatement (WALFA) project reintroduced traditional burning to reduce greenhouse-gas emissions and protect biodiversity. The case illustrates how a single abiotic factor (fire) and an Indigenous management practice (cultural burning) jointly shape ecosystem structure and function.

Example 2. The Coral Triangle and ocean current distribution. The Coral Triangle (Indonesia, Philippines, Malaysia, Papua New Guinea, Solomon Islands, Timor-Leste) holds the highest marine biodiversity on the planet. The pattern is explained by warm tropical sea-surface temperatures, complex tectonics producing a dense network of shallow shelves, and ocean currents that connect populations across the archipelago. Spatial distribution analysis (using GIS overlays of bathymetry, temperature and current data) shows why this region, rather than the Caribbean or Indian Ocean, became the global centre of marine species richness. The case illustrates how abiotic factors at multiple scales produce biotic patterns.

Try this

Q1. Define an ecosystem and identify two biotic and two abiotic components, with reference to a named ecosystem. [4 marks]

• Cue. Definition (biotic plus abiotic interactions in a defined area). Biotic examples for a named system (e.g. Great Barrier Reef: corals, reef fish). Abiotic examples (sea-surface temperature, salinity, light, currents).

Q2. Explain energy flow and one biogeochemical cycle in an ecosystem of your choice. [6 marks]

• Cue. Producers fix solar energy; energy moves up trophic levels with loss at each transfer; food web rather than chain. Pick one cycle (carbon, nitrogen, water) and walk through fluxes and stores in your named ecosystem.

Q3. Analyse the spatial distribution of two major ecosystem types, explaining the abiotic factors that shape their location. [8 marks]

• Cue. Choose one terrestrial and one aquatic. Latitude, temperature, precipitation, ocean currents, topography. Reference geographical tools (climate maps, GIS layers, biome maps) and the geographical concept of spatial distribution.