QCE Biology - Unit 3 - Functioning ecosystems and succession

Energy Flow and Productivity | QCE Biology

Learn energy transfer, biomass, ecological pyramids, gross productivity, net productivity and transfer efficiency.

Updated 2026-05-18 - 6 min read

Energy flow and productivity is part of the way QCE Biology turns living systems into evidence students can describe, analyse and evaluate. The safest way to study it is to connect each term to a data pattern, a biological mechanism and a limitation.

Core explanation

Energy enters ecosystems

Most ecosystems are powered by photosynthesis, where producers convert light energy into chemical energy stored in organic molecules.

Biomass

Biomass is living or recently living organic matter. When producers make biomass, carbon is stored in biological molecules and becomes available to consumers.

Transfer efficiency

Energy transfer between trophic levels is inefficient because organisms use energy for respiration, movement, heat loss and waste. This is why upper trophic levels usually contain less biomass.

Gross and net productivity

Gross productivity is the total energy fixed by producers. Net productivity is the energy left after respiration, so it is the portion available for growth and consumption.

Food chains, food webs and productivity calculations

A food chain shows one pathway of energy transfer. A food web shows multiple feeding relationships in the same ecosystem, so it is usually a better model for real communities. Arrows show the direction of energy transfer, from the organism being eaten to the organism receiving the energy.

| Term | Meaning | Formula or cue | | --- | --- | --- | | Gross primary productivity | Total chemical energy fixed by producers | GPP | | Respiration loss | Energy producers use for cellular respiration | R | | Net primary productivity | Producer energy available for growth and consumers | NPP = GPP - R | | Gross secondary productivity | Energy assimilated by consumers | Food eaten minus faecal loss | | Net secondary productivity | Consumer energy stored as growth or reproduction | GSP - respiration | | Transfer efficiency | Percentage of energy passed to next trophic level | energy at higher level / energy at lower level x 100 |

Energy transfer is limited because not all biomass is eaten, not all eaten material is digested, and much assimilated energy is used in respiration. This is why food chains rarely contain many trophic levels: by the fourth or fifth level, too little energy remains to support a large population.

Food webs can also show indirect effects. If a top predator declines, herbivores may increase and reduce producer biomass. In an exam, connect the direction of the arrow to the direction of energy flow, then explain the likely population effect separately.

Producers, consumers and decomposers

Producers are autotrophs that make organic molecules, usually by photosynthesis. Consumers obtain energy by eating producers or other consumers. Decomposers break down dead organisms and waste, releasing nutrients that producers can use again. Detritivores physically consume dead material, while decomposers such as bacteria and fungi chemically break it down.

Energy flow and nutrient cycling are linked but not the same. Energy enters mostly as light, moves through trophic levels and is eventually dissipated as heat. Matter is recycled through decomposers and biogeochemical cycles. This is why removing decomposers would not just leave dead matter behind; it would also reduce nutrient availability for producers and lower productivity over time.

Food webs are more stable when consumers have multiple food sources and when energy pathways are not dependent on one species. However, a highly connected food web can still be disrupted if a keystone species or major producer is removed.

When calculating transfer efficiency, make sure both values refer to the same unit and time period. Comparing annual producer energy with monthly consumer energy is invalid unless the data are converted first.

How to use this in data questions

Start by identifying what has been measured. In Biology, a graph or table is rarely just asking for a trend; it is asking whether you can connect the trend to a process. Quote enough data to show the pattern, then use the concept language from the syllabus. If the evidence is limited, name the limitation precisely: sample size, sampling method, uncontrolled variables, measurement precision, population choice or the time scale of the data.

A useful study habit is to turn each heading into a data prompt. Ask what you would expect to happen if the relevant variable increased, decreased or was removed. For ecology topics, think about abundance, distribution, biodiversity, biomass and carrying capacity. For genetics topics, think about genotype, phenotype, gene expression, allele frequency and inheritance pattern. For evolution topics, think about variation, selection pressure, gene flow, isolation and relatedness.

When a question asks you to evaluate, do not just list problems with the experiment. Link the limitation to the confidence of the conclusion. For example, a small sample size matters because a few unusual individuals can distort the pattern. An uncontrolled abiotic factor matters because it gives another possible explanation for the same biological trend. This is the difference between naming a limitation and using it scientifically.

Worked example

Common exam traps

Other traps to watch for:

• using a general word when a syllabus term is available
• ignoring units, sample size or time scale
• treating a model as a perfect copy of the real ecosystem or cell
• writing a memorised paragraph that does not use the given data

Quick check

Sources

• QCAA Biology subject page
• QCAA Biology 2025 syllabus
• OpenStax Biology 2e: Energy Flow through Ecosystems
• Khan Academy: Energy flow and primary productivity
• Biology LibreTexts: Energy flow through ecosystems/03%3A_Ecology/3.02%3A_Energy_Flow_through_Ecosystems)