Biophysical interactions in ecosystems, including the role of energy flows, nutrient cycling, and biotic and abiotic components

What this dot point is asking

NESA wants you to know the foundational ecology of ecosystems: what their components are, how those components interact, and how energy and nutrients move through them. This is the conceptual platform on which the rest of the Ecosystems at Risk topic is built. Strong responses define the components precisely and use one or two named ecosystems to ground the explanation.

Ecosystem definition

An ecosystem is a community of living organisms (biotic components) interacting with each other and with their non-living environment (abiotic components) within a defined space. The boundary of an ecosystem is functional rather than absolute. A river, a forest, a coral reef, an alpine bog, a city park, and the entire Great Barrier Reef can each be considered ecosystems at different scales.

Biotic components

Biotic components are the living parts of the ecosystem, organised by their role in energy and nutrient flow.

Producers (autotrophs)

Convert solar energy (or chemical energy in some deep-sea ecosystems) into biomass through photosynthesis or chemosynthesis. Plants, algae, photosynthetic bacteria.

In the Murray-Darling, river red gums, lignum, common reed, and aquatic algae are the dominant producers. In the Great Barrier Reef, coral zooxanthellae, seagrasses, and reef algae are the producers. In Australian woodlands, eucalypts and acacias dominate.

Consumers (heterotrophs)

Obtain energy by eating other organisms.

• Primary consumers (herbivores). Eat producers. Kangaroos, wombats, leaf-eating insects, parrotfish.
• Secondary consumers (carnivores). Eat herbivores. Dingoes, foxes, raptors, small reef fish.
• Tertiary consumers (top carnivores). Eat other carnivores. Wedge-tailed eagles, sharks, saltwater crocodiles.
• Omnivores. Eat both plants and animals. Magpies, brushtail possums, humans.

Decomposers (detritivores and saprotrophs)

Break down dead organisms and waste, releasing nutrients back into the ecosystem. Bacteria, fungi, termites, earthworms, dung beetles, crabs (in marine systems).

Decomposers are often overlooked but are critical to nutrient cycling. The Australian dung beetle introduction in the 1960s (CSIRO program) addressed the lack of native decomposers adapted to cattle dung; success eliminated bush fly plagues in eastern Australia.

Abiotic components

Abiotic components are the non-living environmental conditions that constrain biotic activity.

Atmosphere

Temperature, humidity, atmospheric CO2, wind, precipitation, sunlight.

Hydrosphere

Water availability, water chemistry (pH, salinity, dissolved oxygen, nutrients), water movement (currents, tides).

Lithosphere

Soil (texture, pH, organic matter, mineral nutrients), topography (slope, aspect, altitude), substrate (rock type, sediment).

Energy

Solar radiation is the primary energy source for nearly all ecosystems. Australian ecosystems receive around 5 kWh/m2/day on average. Total solar input to the Great Barrier Reef is roughly 3 x 10^15 kJ/year.

Energy flow through ecosystems

Energy enters as solar radiation and flows through the food web from producers to consumers to decomposers. Around 1-2 percent of incoming solar energy is fixed into plant biomass through photosynthesis. Each subsequent trophic level captures only around 10 percent of the energy from the level below (the "10 percent rule"); the rest is lost as heat, used in metabolism, or stored in non-edible structures.

This is why food chains rarely exceed four or five levels: insufficient energy remains at the top. It is also why a vegetarian diet uses around 10 percent of the land and water required by a meat-based diet for the same caloric output.

Energy flow is one-way. Once dissipated as heat, energy leaves the ecosystem and cannot be recycled.

Nutrient cycling through ecosystems

Unlike energy, nutrients cycle. Carbon, nitrogen, phosphorus, sulfur, and other elements move between biotic and abiotic components and back. Five major cycles:

Carbon cycle

Atmospheric CO2 is fixed into biomass through photosynthesis. Biomass is consumed by other organisms, or returns to soil as litter, or is burned. Respiration, decomposition, fire, and combustion return CO2 to the atmosphere. Long-term storage occurs in fossil fuels, peat bogs, and ocean sediments.

Nitrogen cycle

Atmospheric nitrogen (N2) is unusable by most plants. Nitrogen-fixing bacteria (in legume root nodules and free-living in soils) convert N2 to ammonium. Nitrifying bacteria convert ammonium to nitrate, which plants can absorb. Denitrifying bacteria return nitrogen to the atmosphere. Lightning also fixes nitrogen.

Australian native vegetation is adapted to low soil nitrogen because the continent has not been geologically renewed by glaciation. Industrial fertiliser application (around 1.5 Mt N applied to Australian crops per year) has disrupted natural nitrogen patterns.

Phosphorus cycle

Unlike nitrogen, phosphorus has no significant atmospheric phase. Plants take phosphorus from soil solution; animals eat plants; decomposition returns phosphorus to soil. Long-term storage in marine sediments. Australian native ecosystems are even more phosphorus-poor than the global average; this is why phosphorus fertiliser killed native banksia in WA.

Water cycle

Covered in the Biophysical Interactions topic, but ecosystems are tied closely to water availability. Evapotranspiration by vegetation is a major component of water return to the atmosphere.

Calcium cycle

Important for coral reefs (calcium carbonate skeletons), shellfish, and bones. Ocean acidification (lower pH from dissolved CO2) reduces calcium carbonate availability and weakens reef builders.

Trophic structure and food webs

Trophic structure describes the relative biomass at each level. In productive ecosystems (tropical rainforests, kelp forests), producer biomass is high and supports many consumer levels. In nutrient-poor ecosystems (Australian heathlands, deep-sea ecosystems), producer biomass is low and food chains are short.

Food webs describe the actual eating relationships, often complex networks rather than simple chains. The Great Barrier Reef food web has been mapped to include over 1,500 fish species, 5,000 mollusc species, and hundreds of coral species. Murray-Darling food webs are simpler but include native fish (Murray cod, golden perch, silver perch), introduced species (carp, redfin), aquatic invertebrates, and riparian vegetation.

How biotic and abiotic components interact

The interaction is two-way. Biotic components are shaped by abiotic conditions: coral reefs only form in warm clear shallow water; alpine ecosystems only persist where freezing temperatures exclude lowland species. Abiotic components are shaped by biotic activity: reef-building corals create the reef substrate; vegetation alters soil chemistry, structure, and microclimate; algae produce around 50 percent of atmospheric oxygen.

When biophysical interactions are disrupted (warming oceans bleaching coral; clearing vegetation degrading soil), the whole ecosystem function changes.