How does energy move through ecosystems and how are nutrients recycled to sustain life?
Explain energy flow through trophic levels and the cycling of carbon, nitrogen and water, and interpret food webs and ecological pyramids.
Trophic levels, energy transfer and the ten percent rule, ecological pyramids, and the carbon, nitrogen and water cycles with Tasmanian examples, for TCE Environmental Science and Society Level 3.
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What this dot point is asking
This dot point asks you to explain how energy flows one way through an ecosystem and how matter cycles around within it. You should be able to read food webs and ecological pyramids, apply the idea that energy is lost between trophic levels, and describe the carbon, nitrogen and water cycles, using Tasmanian examples where you can.
Energy flow and trophic levels
Almost all ecosystems run on energy from the Sun. Producers (autotrophs) such as plants, algae and phytoplankton use photosynthesis to convert light energy into chemical energy stored in organic molecules. This forms the first trophic level. Consumers then obtain energy by eating other organisms: herbivores are primary consumers, the carnivores that eat them are secondary consumers, and so on. Decomposers such as fungi and bacteria break down dead matter and wastes, releasing nutrients back into the system.
Energy flow is one way. At each transfer between trophic levels, only about 10 percent of the energy is passed on, because organisms use most of the energy for respiration, movement and maintaining body heat, and lose it to the surroundings as heat. This 10 percent rule explains why food chains rarely have more than four or five links and why top predators such as the wedge-tailed eagle are relatively few.
Food webs and ecological pyramids
A food chain shows one feeding pathway, but real ecosystems are linked into food webs because most organisms eat, and are eaten by, several others. Food webs reveal how a change to one species can ripple through many others.
Ecological pyramids summarise an ecosystem quantitatively. A pyramid of numbers counts organisms at each level, a pyramid of biomass measures the mass of living material, and a pyramid of energy shows the energy stored at each level. Energy pyramids are always widest at the base and narrow upwards, directly reflecting the loss of energy between trophic levels.
The carbon cycle
Carbon moves between the atmosphere, living things, oceans and rocks. Photosynthesis removes carbon dioxide from the air and fixes it into organic compounds; respiration by all organisms returns it. Decomposition releases carbon from dead matter, while combustion of fossil fuels and bushfires add carbon dioxide rapidly. Tasmania's vast forests and the Southern Ocean act as important carbon sinks, storing carbon that would otherwise warm the atmosphere.
The nitrogen cycle
Nitrogen is needed for proteins and DNA, but the nitrogen gas that makes up most of the atmosphere cannot be used directly by plants. Nitrogen-fixing bacteria convert it into ammonia, nitrification turns ammonia into nitrates that plants absorb, and consumers gain nitrogen by eating plants. Decomposers release nitrogen back into the soil through ammonification, and denitrifying bacteria return nitrogen gas to the air. Excess nitrogen from fertiliser running into Tasmanian waterways can cause eutrophication.
The water cycle
Water cycles through evaporation, transpiration from plants, condensation, precipitation and runoff. Tasmania's high rainfall and forested catchments, such as those feeding hydro-electric storages, depend on this cycle, and forest cover influences how much water is held and slowly released.
Exam-style practice questions
Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
TCE 20216 marksAn energy pyramid for a Tasmanian grassland shows producers fixing kJ m yr. Primary consumers contain kJ m yr and secondary consumers kJ m yr. Calculate the percentage efficiency of energy transfer at each step and explain why so little energy reaches the secondary consumers.Show worked answer →
A 6 mark calculation-and-explain question rewards two efficiency figures plus the mechanism.
Producers to primary consumers. .
Primary to secondary consumers. .
Both transfers are close to the rule-of-thumb 10 percent.
Why so little passes on. At each transfer most energy is lost: organisms use the bulk of it for respiration, movement and maintaining body heat, and lose it to the surroundings as heat, while some food is never eaten or digested. After two transfers only about 0.8 percent of the original energy remains, which is why food chains are short and top predators are few.
Markers reward the two correct percentages and the respiration-and-heat-loss explanation.
TCE 20195 marksExplain the difference between the way energy and the way matter move through an ecosystem, using the carbon cycle as an example of matter movement.Show worked answer →
A 5 mark question wants the energy-versus-matter contrast made explicit.
Energy flows one way. Energy enters as sunlight, is fixed by producers, passes through trophic levels once and is lost as heat at every step, so it must be continually resupplied by the Sun and is never reused.
Matter cycles. Atoms are not lost. In the carbon cycle, photosynthesis fixes atmospheric carbon dioxide into organic compounds, respiration and decomposition return it to the air, and combustion and bushfires release it rapidly, so the same carbon atoms are used over and over.
Markers reward the one-way-energy / cycling-matter contrast and a correct carbon-cycle example.