What you'll learn
Energy flow and nutrient cycles explains how energy moves through an ecosystem and why nutrients, unlike energy, are recycled. The two behave quite differently, and the distinction is the organising idea of the topic: energy enters as light, passes through organisms once and is ultimately lost as heat, whereas the atoms of carbon and nitrogen are used repeatedly. At CAPE level you must be able to calculate efficiencies of energy transfer, explain why transfers are so inefficient, and describe the carbon and nitrogen cycles including the roles of the named microorganisms. By the end of this topic you should be able to construct and interpret pyramids, calculate net and gross production and percentage efficiency, explain energy losses at each level, describe both nutrient cycles with the organisms responsible, and relate all of this to agricultural practice.
Key terms and definitions
Producer — an autotroph that converts light energy into chemical energy by photosynthesis
Consumer — a heterotroph that obtains energy by eating other organisms
Decomposer — an organism that obtains energy from dead organic matter, releasing nutrients
Trophic level — the position of an organism in a food chain
Gross primary production — the total chemical energy fixed by producers in a given area and time
Net primary production — gross primary production minus the energy lost in respiration
Biomass — the mass of living material, usually expressed as dry mass per unit area
Pyramid of energy — a diagram showing energy flow at each trophic level per unit area per unit time
Saprobiont — an organism feeding on dead material by extracellular digestion
Nitrogen fixation — the conversion of atmospheric nitrogen into ammonium compounds
Nitrification — the oxidation of ammonium to nitrite and then to nitrate
Denitrification — the conversion of nitrate back to atmospheric nitrogen
Ammonification — the production of ammonium compounds from nitrogen-containing organic matter
Core concepts
Energy flow
Almost all the energy entering an ecosystem comes from sunlight, fixed by producers in photosynthesis.
Only about one to three per cent of the light energy falling on a plant is converted into chemical energy in organic molecules. The reasons are examinable: much of the light misses the chloroplasts altogether, some is reflected from the leaf surface, some passes straight through, some is of a wavelength that chlorophyll cannot absorb — notably green light, which is reflected — and some is lost as heat. In addition, carbon dioxide concentration, water availability and temperature may all be limiting.
Gross primary production is the total chemical energy fixed. Net primary production is what remains after the plant's own respiration, and it is net primary production that is available to the next trophic level.
Energy is then transferred along the food chain, but the transfer between consumer levels is only about ten per cent efficient, and typically between five and twenty per cent.
Why transfers are inefficient
Losses occur at every step, and a full answer should distinguish energy that is never consumed from energy that is consumed but not incorporated.
Not all of the organism at one level is eaten. Roots, woody stems and bark are frequently not consumed, and some individuals die without being eaten, their energy passing instead to decomposers.
Not all of what is eaten is digested and absorbed. Cellulose, bone, hair and chitin resist digestion, and this energy is egested in faeces.
Not all of what is absorbed is incorporated into new tissue. Much is released in respiration to drive movement, active transport and other processes, and is eventually lost as heat to the surroundings. This is the largest single loss in most animals.
Energy is also lost in excretory products such as urea.
The proportion lost in respiration is much greater in mammals and birds than in reptiles, fish and invertebrates, because endotherms expend a great deal of energy maintaining a constant body temperature. Transfer efficiency is therefore higher in a chain of ectotherms, which is why fish farming can be more efficient than cattle farming.
Because so much is lost at each step, food chains rarely exceed four or five trophic levels: beyond that there is insufficient energy to support a viable population.
Calculating efficiency
Percentage efficiency of energy transfer is the energy incorporated at a trophic level divided by the energy available from the previous level, multiplied by 100.
Net production of a consumer is calculated as the chemical energy in ingested food, minus the energy lost in faeces and urine, minus the energy lost in respiration.
Units matter and are frequently credited. Energy flow is expressed per unit area per unit time, typically kilojoules per square metre per year, because it is a rate.
Pyramids
A pyramid of numbers shows the number of organisms at each trophic level. It is easy to construct but can be inverted or oddly shaped, because it takes no account of size: a single tree supporting thousands of insects gives a very narrow base.
A pyramid of biomass shows the dry mass of organisms at each level and is usually a true pyramid. Dry mass is used because water content varies and contains no energy. However, it represents a single moment, so it can also be inverted — in open water, where phytoplankton reproduce very rapidly and are eaten as fast as they are produced, the standing biomass of producers may be lower than that of the zooplankton feeding on them.
A pyramid of energy shows energy flow per unit area per unit time and can never be inverted, because energy is always lost between levels and a level cannot transfer more energy than it received. This is why the pyramid of energy is the most accurate representation, and being able to say why it can never be inverted is a standard question.
The carbon cycle
Carbon dioxide is removed from the atmosphere by photosynthesis in plants and algae, and the carbon is incorporated into carbohydrates, lipids and proteins.
It passes along food chains as organisms feed.
It returns to the atmosphere by three main routes.
Respiration by plants, animals and microorganisms releases carbon dioxide continuously.
Decomposition by saprobiotic bacteria and fungi breaks down dead organisms and waste, and these decomposers respire as they do so, releasing carbon dioxide.
Combustion of wood and fossil fuels releases carbon that had been locked away, in the case of fossil fuels for hundreds of millions of years.
Carbon is also exchanged with the oceans, dissolving in sea water and being incorporated into the shells and skeletons of marine organisms, which may form sedimentary rock such as limestone.
Human activity has disturbed the balance. Burning fossil fuels releases carbon that was removed from the atmosphere long ago and returns it far faster than natural processes remove it, while deforestation both releases carbon and reduces the capacity for photosynthetic removal. The result is a rising atmospheric concentration of carbon dioxide, contributing to the enhanced greenhouse effect.
The nitrogen cycle
Nitrogen is required for amino acids, proteins and nucleic acids. The atmosphere is about 78 per cent nitrogen, but the triple bond of the nitrogen molecule makes it extremely unreactive, so plants cannot use it directly and it must first be fixed.
Four processes and their microorganisms must be known, and marks are usually awarded for naming the organism as well as the process.
Nitrogen fixation converts atmospheric nitrogen into ammonium compounds. Free-living nitrogen-fixing bacteria in the soil, such as Azotobacter, do this, as do mutualistic bacteria of the genus Rhizobium living in root nodules of leguminous plants such as peas, beans and clover. The relationship is mutualistic: the bacteria receive carbohydrates from the plant and supply the plant with fixed nitrogen. Fixation also occurs to a small extent through lightning and industrially through the Haber process.
Ammonification is carried out by saprobiotic bacteria and fungi, which digest proteins, nucleic acids, urea and faeces from dead organisms and waste, releasing ammonium compounds into the soil.
Nitrification is carried out by two groups of nitrifying bacteria in two steps. Nitrosomonas oxidises ammonium to nitrite, and Nitrobacter oxidises nitrite to nitrate. Both are aerobic, and they obtain energy from these oxidations. Because they require oxygen, nitrification proceeds best in well-aerated soil, which is the reason ploughing and good drainage improve fertility.
Plants absorb nitrate from the soil by active transport through their root hairs, and use it with carbohydrate to synthesise amino acids and then proteins. Nitrogen then passes along the food chain.
Denitrification is carried out by denitrifying bacteria such as Pseudomonas, which convert nitrate back into atmospheric nitrogen. These bacteria are anaerobic, so denitrification occurs in waterlogged, compacted or poorly drained soils. This is why waterlogging reduces fertility both by inhibiting the aerobic nitrifying bacteria and by favouring the anaerobic denitrifying ones.
Agricultural applications
The cycles explain much agricultural practice, and questions often ask candidates to make the connection.
Ploughing aerates the soil, which favours the aerobic nitrifying bacteria and inhibits the anaerobic denitrifying bacteria, raising the nitrate concentration available to crops.
Crop rotation including a leguminous crop introduces Rhizobium in root nodules, increasing the fixed nitrogen in the soil. Ploughing the legume back in returns that nitrogen directly.
Natural fertilisers such as manure and compost supply organic nitrogen, which decomposers convert to ammonium and nitrifying bacteria to nitrate. They release nutrients slowly, improve soil structure and water retention, and use a waste product, but their composition is variable and the release may be too slow for a rapidly growing crop.
Artificial fertilisers supply nitrate directly in a known, controlled composition that is immediately available, but they are soluble and therefore readily leached into waterways, they do not improve soil structure, and their manufacture consumes substantial energy.
Leaching of nitrate into rivers and lakes causes eutrophication. The added nutrients cause algae to grow rapidly and form a surface layer, which blocks light from the plants below so that they die. Decomposing bacteria feed on the dead material and multiply, and because they respire aerobically they consume the dissolved oxygen. With the oxygen depleted, fish and other aquatic organisms die. Giving the full chain rather than stopping at the algae is what earns the marks.
Worked examples
Example 1: Calculating transfer efficiency (4 marks)
Producers in an ecosystem have a net primary production of 87,000 kilojoules per square metre per year. The primary consumers incorporate 6,090 kilojoules per square metre per year. Calculate the percentage efficiency of transfer and suggest two reasons why it is not higher.
Percentage efficiency is the energy incorporated divided by the energy available, multiplied by 100. That is 6,090 divided by 87,000, multiplied by 100, which gives 7.0 per cent.
The efficiency is low partly because not all of the plant material is eaten: roots and woody tissue are often not consumed, and some plants die and pass to decomposers instead.
It is also low because much of what is eaten is not incorporated into consumer tissue. Cellulose is difficult to digest and is egested in faeces, and a large proportion of the energy absorbed is released in respiration to support movement and other processes and is ultimately lost as heat to the surroundings.
Example 2: Explaining an inverted pyramid (4 marks)
In an open ocean ecosystem, a pyramid of biomass is inverted, with a smaller mass of phytoplankton than of zooplankton. Explain how this is possible, and state why a pyramid of energy could not be inverted.
A pyramid of biomass records the mass present at one moment in time. Phytoplankton have very short life cycles and reproduce extremely rapidly, and they are consumed by zooplankton almost as fast as they are produced.
The standing biomass of phytoplankton at any instant is therefore small, even though the total quantity produced over a year is very large. The zooplankton are longer-lived and accumulate biomass, so the mass present at that moment is greater.
A pyramid of energy cannot be inverted because it records energy flow per unit area per unit time rather than a standing quantity. Energy is lost at every transfer through respiration, egestion and excretion, so a trophic level can never pass on more energy than it received, and each level must therefore be smaller than the one below.
Example 3: Explaining the effect of waterlogging (5 marks)
Explain why waterlogged soil is less fertile than well-drained soil, referring to named bacteria.
Nitrifying bacteria, Nitrosomonas and Nitrobacter, oxidise ammonium to nitrite and nitrite to nitrate. Both are aerobic and require oxygen for these oxidations.
In waterlogged soil the air spaces are filled with water, so the oxygen concentration is very low. The activity of the nitrifying bacteria is therefore reduced and less nitrate is produced.
At the same time, denitrifying bacteria such as Pseudomonas are anaerobic and are favoured by these conditions. They convert nitrate in the soil back into atmospheric nitrogen, which plants cannot use.
The combined effect is that nitrate is produced more slowly and removed more rapidly, so the concentration of nitrate available for absorption by plant roots falls. Since nitrate is required for amino acid and protein synthesis, plant growth is limited and the soil is less fertile.
Common mistakes and how to avoid them
The most frequent error is describing nutrients as flowing through an ecosystem and energy as being recycled. Energy flows through and is lost; nutrients are recycled.
Students often state that plants absorb nitrogen from the air. Plants absorb nitrate from the soil; only nitrogen-fixing bacteria use atmospheric nitrogen.
Another common slip is naming the wrong bacteria for a process. Learn the four pairings: Rhizobium and Azotobacter for fixation, saprobionts for ammonification, Nitrosomonas and Nitrobacter for nitrification, Pseudomonas for denitrification.
Many candidates explain low transfer efficiency only by respiration. Material not eaten and material egested are separate points and are usually credited separately.
In eutrophication questions, answers frequently stop at the algae blocking light. The chain through plant death, bacterial multiplication, oxygen depletion and fish death is where the marks lie.
Finally, candidates often omit units from energy flow figures. The quantity is a rate and requires units of energy per unit area per unit time.
Exam technique for "Energy flow and nutrient cycles"
Quote units in every calculation and in every stated figure, since energy flow is a rate and the units are commonly worth a mark.
When explaining inefficiency, group the losses into not eaten, not digested and used in respiration, and give an example of each.
For the nitrogen cycle, name the process, the organism and the conversion in every case. A process without its organism is usually a partial answer.
When a question links the cycles to agriculture, identify the process being promoted or inhibited before giving the practical consequence.
For pyramid questions, state what the pyramid measures before explaining its shape. Numbers, standing biomass and energy flow behave differently for that reason.
Quick revision summary
Energy enters as light, of which only one to three per cent is fixed, and net primary production is gross primary production minus respiration. Transfers between consumers are about ten per cent efficient because not all material is eaten, not all that is eaten is digested, and much of what is absorbed is respired and lost as heat, with endotherms losing most. Percentage efficiency is energy incorporated divided by energy available, times 100, expressed per unit area per unit time. Pyramids of numbers and biomass can be inverted, but a pyramid of energy cannot, because energy is lost at every transfer. In the carbon cycle, photosynthesis removes carbon dioxide and respiration, decomposition and combustion return it, with fossil fuel burning and deforestation raising atmospheric concentrations. In the nitrogen cycle, Rhizobium in legume root nodules and free-living Azotobacter fix atmospheric nitrogen; saprobionts ammonify dead material; aerobic Nitrosomonas and Nitrobacter nitrify ammonium to nitrite and nitrate; plants absorb nitrate by active transport; and anaerobic Pseudomonas denitrifies nitrate back to nitrogen. Ploughing aerates soil to favour nitrification, legume rotation adds fixed nitrogen, and leached fertiliser causes eutrophication through algal growth, light blocking, decomposer respiration and oxygen depletion.