Describe energy flow through ecosystems including food chains, food webs and trophic levels, and explain biomass, productivity (GPP and NPP) and the 10 per cent rule of trophic efficiency

What this dot point is asking

QCAA wants you to track energy from the sun through producers and consumers, to read and construct food chains and food webs, and to compute productivity using the 10 per cent rule. Numerical questions on GPP, NPP and trophic efficiency appear most years.

The answer

Ecosystems run on a one-way flow of energy from the sun, fixed by producers and transferred (with heavy losses) through consumers. Matter cycles through these same pathways, but energy does not.

Producers (autotrophs)

Producers fix energy from an abiotic source into chemical energy in organic molecules.

• Photoautotrophs. Use light. Plants, algae and cyanobacteria. The dominant producers in most ecosystems.
• Chemoautotrophs. Use chemical energy from inorganic compounds. Mostly bacteria and archaea at hydrothermal vents, in deep soils and around sulfur springs.

Producers occupy trophic level 1.

Consumers (heterotrophs)

Consumers obtain energy by eating other organisms.

• Primary consumers (herbivores) eat producers. Trophic level 2.
• Secondary consumers (carnivores) eat primary consumers. Trophic level 3.
• Tertiary consumers (top carnivores) eat secondary consumers. Trophic level 4.
• Quaternary consumers appear in some long food chains. Trophic level 5.
• Omnivores feed at more than one trophic level.
• Decomposers (detritivores) consume dead organic matter and excretory waste, returning nutrients to the abiotic pool. Bacteria, fungi, earthworms, dung beetles.

Food chains and food webs

A food chain is a single linear path of energy transfer. Example: grass to kangaroo to dingo. Useful as a diagram but rarely the full picture.

A food web is the network of all feeding relationships in a community. Each species sits at one or more trophic levels and connects to multiple prey and multiple predators. Food webs reveal redundancy: if one prey species declines, predators can switch to alternatives.

When building a food web on an exam, arrows always point from the organism being eaten to the organism that eats it (the direction of energy flow), with producers at the bottom.

Biomass

Biomass is the total mass of living organic matter (dry mass, in g or kg) per unit area or volume.

• Standing crop biomass. The mass present at a moment in time.
• Pyramid of biomass. Diagram of biomass at each trophic level. Usually a pyramid with producers having the largest biomass and top consumers the smallest, but inverted pyramids occur in aquatic systems where phytoplankton turn over rapidly (low standing biomass, high turnover).

Productivity: GPP and NPP

Productivity is the rate at which producers fix energy (or organic matter) per unit area per unit time.

• Gross primary productivity (GPP). The total energy fixed by photosynthesis in a given time. Includes energy the producers themselves use for respiration.
• Net primary productivity (NPP). The energy remaining after producers' respiration. NPP equals GPP minus plant respiration. NPP is the energy actually available to primary consumers and accumulates as plant growth.

Units are typically kJ per m squared per year, or g of dry mass per m squared per year.

Secondary productivity is the rate at which consumers convert ingested food into their own biomass.

The 10 per cent rule

On average, only around 10 per cent of the energy at one trophic level is incorporated into biomass at the next level. The other 90 per cent is lost as:

• Heat from respiration. All living cells respire continuously, releasing CO2, water and heat. This is the single largest loss.
• Movement, growth and maintenance. Energy expended on muscular work, biosynthesis, immune function.
• Indigestible material. Cellulose, lignin, bone, fur, chitin pass through the gut and are lost as faeces.
• Heat in endotherms. Maintaining body temperature in mammals and birds costs additional energy and reduces transfer efficiency compared with ectotherms.

The rule is a useful approximation. Real values range from about 1 per cent (cold-blooded predators feeding inefficiently) to about 25 per cent (insect larvae on high-quality food). Aquatic food webs often have higher transfer efficiencies than terrestrial ones.

Worked numerical example

Suppose a Queensland savanna has:

• GPP = 20 000 kJ per m squared per year.
• Plant respiration = 8 000 kJ per m squared per year.

NPP = 20 000 minus 8 000 = 12 000 kJ per m squared per year.

Applying 10 per cent at each transfer:

• Trophic level 2 (kangaroos): 1 200 kJ per m squared per year.
• Trophic level 3 (dingoes feeding on kangaroos): 120 kJ per m squared per year.
• Trophic level 4 (large raptors feeding on dingo prey): 12 kJ per m squared per year.

After three transfers, only 12 kJ per m squared per year is available to top consumers, which sets the upper bound on their population density.

Why food chains are short

The compounding effect of the 10 per cent rule means each added trophic level captures a tenth of the energy of the level below. After four or five transfers, the residual energy is too small to support a population of mobile predators. Productive ecosystems (estuaries, reefs) can support longer chains; unproductive ones (deserts, deep ocean) sustain shorter chains and fewer top predators.

Energy pyramids

Three pyramid forms appear in QCAA stimulus.

• Pyramid of numbers. Counts individuals at each level. Can be inverted when a single large plant feeds many herbivores.
• Pyramid of biomass. Standing biomass at each level. Almost always pyramidal on land; can be inverted in aquatic plankton communities.
• Pyramid of energy. Energy fixed or transferred per unit area per unit time. Always pyramidal (never inverted) because energy flow is one-way and lost at each transfer.

The pyramid of energy is the most informative because it cannot be inverted by short-term turnover differences.

Common traps

Drawing arrows the wrong way. Arrows in food webs follow energy flow, from prey to predator.

Confusing GPP and NPP. GPP is total photosynthesis; NPP is what is left after plant respiration. Only NPP is available to herbivores.

Treating 10 per cent as exact. It is an average. State it as approximate, and use it where the question specifies.

Mixing up biomass pyramids and energy pyramids. Biomass can invert in plankton systems; energy cannot.

Forgetting decomposers. Decomposers receive energy from every trophic level (through dead matter and waste) and return inorganic nutrients to the abiotic pool. They appear in any complete food web.

In one sentence

Energy enters ecosystems through producers (GPP), is reduced to NPP after plant respiration, and is then transferred between trophic levels in food chains and webs with roughly 10 per cent efficiency at each step, with the remaining 90 per cent lost as respiratory heat, locomotion costs and indigestible faeces, which limits most food chains to four or five levels.