How does energy flow and nutrients cycle through ecosystems, and how can resources be used sustainably?
6.1.5 Ecosystems and sustainability: the flow of energy through ecosystems (gross and net primary productivity and trophic efficiency); the recycling of nutrients (the nitrogen and carbon cycles); primary and secondary succession; and the principles of managing ecosystems sustainably and conservation.
A focused answer to the OCR H420 6.1.5 dot point on ecosystems. Covers energy flow (gross and net primary productivity and trophic efficiency), the nitrogen and carbon cycles, primary and secondary succession, and the principles of sustainable management and conservation.
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What this dot point is asking
OCR wants you to explain the flow of energy through ecosystems (gross and net primary productivity and trophic efficiency), describe the nitrogen and carbon cycles, explain primary and secondary succession, and explain the principles of managing ecosystems sustainably and conservation.
The answer
Energy flow through ecosystems
Energy enters an ecosystem when producers fix light energy in photosynthesis:
- Gross primary productivity (GPP): the total energy fixed by producers.
- Net primary productivity (NPP): the energy left for growth after respiration: . NPP is the energy available to the next trophic level.
Energy passes along the food chain through trophic levels (producer, primary consumer, secondary consumer, and so on), but is lost at each transfer: not all of an organism is eaten or digestible (losses in faeces), and much energy is lost as heat in respiration and in excretion. So only about 10 percent of the energy is passed to the next level (the trophic efficiency), which is why food chains are short and biomass decreases up the chain.
The nitrogen cycle
Microorganisms recycle nitrogen between the air, soil and organisms:
- Nitrogen fixation: bacteria (Rhizobium in root nodules; free-living Azotobacter) and lightning convert nitrogen gas to ammonium.
- Ammonification: decomposers break down proteins and urea in dead matter and waste to ammonium.
- Nitrification: nitrifying bacteria oxidise ammonium to nitrite (Nitrosomonas), then nitrite to nitrate (Nitrobacter); plants absorb nitrate.
- Denitrification: denitrifying bacteria convert nitrate back to nitrogen gas in anaerobic (waterlogged) soils.
The carbon cycle
Carbon moves between the atmosphere (as carbon dioxide) and organisms: photosynthesis removes carbon dioxide and fixes it into organic molecules; respiration, decomposition and combustion (including of fossil fuels) return it. Burning fossil fuels and deforestation are raising atmospheric carbon dioxide, contributing to climate change.
Succession
Succession is the change in a community over time:
- Primary succession begins on bare ground with no soil (for example bare rock or a new sand dune). Pioneer species (such as lichens) colonise, and as they die they build soil, allowing larger plants to establish; the community changes through stages (seres) to a stable climax community.
- Secondary succession occurs where soil already exists (for example after a fire or clearance), so it is faster.
Each stage changes the environment, making it suitable for the next; diversity generally increases towards the climax community.
Sustainability and conservation
Sustainable management uses resources so they are available for the future without depleting or damaging the ecosystem (for example coppicing and selective felling in forestry, controlled fishing quotas and net-mesh sizes). Conservation is the active management of ecosystems and species to maintain biodiversity (for example protected areas, captive breeding and seed banks). Conservation may conflict with the needs of local people, so management must balance ecological, economic and social interests.
Examples in context
Example 1. Why there are few top predators. Because only about 10 percent of energy passes between trophic levels, there is far less energy available high up the food chain, so top predators are few and food chains rarely exceed four or five links.
Example 2. Sustainable forestry by coppicing. Coppicing cuts trees to the base so they regrow, providing a renewable supply of wood while keeping the woodland and its habitats intact, a practical example of sustainable management.
Try this
Q1. Write the equation linking net and gross primary productivity. [1 mark]
- Cue. Net primary productivity = gross primary productivity minus respiratory losses.
Q2. Explain why only about 10 percent of energy is transferred between trophic levels. [2 marks]
- Cue. Energy is lost as heat in respiration, in faeces (egestion) and excretion, and not all of an organism is eaten or digested, so only a small proportion passes to the next level.
Q3. Name the process by which nitrifying bacteria convert ammonium into nitrate. [1 mark]
- Cue. Nitrification.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/02 20194 marksA producer fixes 20000 kJ per square metre per year as gross primary productivity, and loses 8000 kJ per square metre per year in respiration. Calculate the net primary productivity and explain why only a small percentage of this energy reaches the secondary consumers.Show worked answer →
Do the subtraction, then explain energy loss between trophic levels.
Net primary productivity (NPP) = gross primary productivity minus respiratory losses kJ per square metre per year.
Only a small percentage reaches secondary consumers because energy is lost at each trophic level: not all of an organism is eaten or digested (some is lost in faeces and as indigestible material), and a large proportion of the energy taken in is lost as heat through respiration (and in excretion). So only about 10 percent passes to the next level, and after two transfers (producer to primary to secondary consumer) only a small fraction remains.
Markers reward the correct NPP calculation and energy losses through respiration (heat), faeces and not all being eaten, reducing the energy passed on at each level.
OCR H420/02 20215 marksDescribe the roles of microorganisms in the nitrogen cycle, naming the processes they carry out.Show worked answer →
Name each process, the bacteria and what they convert.
- Nitrogen fixation
- nitrogen-fixing bacteria (for example Rhizobium in root nodules, and free-living Azotobacter) convert atmospheric nitrogen gas into ammonia / ammonium.
- Ammonification
- decomposers (saprobionts) break down proteins and urea in dead organisms and waste into ammonium.
- Nitrification
- nitrifying bacteria oxidise ammonium to nitrite (Nitrosomonas) and then nitrite to nitrate (Nitrobacter); nitrate is the form plants absorb.
- Denitrification
- denitrifying bacteria convert nitrate back to nitrogen gas (in anaerobic, waterlogged soils), returning nitrogen to the atmosphere.
Markers reward the four named processes with the correct conversions (and ideally named bacteria), especially nitrification producing the nitrate that plants absorb.
Related dot points
- 6.1.4 Cloning and biotechnology: natural and artificial cloning of plants (including micropropagation and tissue culture) and animals; the use of microorganisms in biotechnology and the conditions in an industrial fermenter; the principles and advantages of using immobilised enzymes; and the asepsis and growth curve of a microbial culture.
A focused answer to the OCR H420 6.1.4 dot point on cloning and biotechnology. Covers natural and artificial cloning of plants and animals, micropropagation and tissue culture, the use of microorganisms and the conditions in an industrial fermenter, immobilised enzymes, and the microbial growth curve.
- 6.1.2 Populations and evolution: the meaning of a gene pool and allele frequency; the use of the Hardy-Weinberg principle to calculate allele and genotype frequencies; the factors that change allele frequencies (natural selection, genetic drift, the founder effect and migration); and the process of speciation (allopatric and sympatric).
A focused answer to the OCR H420 6.1.2 dot point on populations and evolution. Covers gene pools and allele frequency, the Hardy-Weinberg principle and its calculations, the factors that change allele frequencies including genetic drift and the founder effect, and allopatric and sympatric speciation.
- 4.2.1 Biodiversity: the levels of biodiversity (habitat, species and genetic); how to sample plants and animals (random sampling, quadrats, transects and mark-release-recapture); the calculation and interpretation of Simpson's index of diversity; and the ecological, economic and aesthetic reasons for maintaining biodiversity.
A focused answer to the OCR H420 4.2.1 dot point on biodiversity. Covers habitat, species and genetic diversity, sampling methods including quadrats, transects and mark-release-recapture, the calculation and interpretation of Simpson's index of diversity, and the reasons for maintaining biodiversity.
- 5.2.1 Photosynthesis: the structure of the chloroplast; the light-dependent stage (photolysis of water, photophosphorylation and the reduction of NADP); the light-independent stage (the Calvin cycle, fixing carbon dioxide using RuBP, forming GP and TP and regenerating RuBP); and the effect of limiting factors (light intensity, carbon dioxide concentration and temperature).
A focused answer to the OCR H420 5.2.1 dot point on photosynthesis. Covers chloroplast structure, the light-dependent stage (photolysis, photophosphorylation and reduced NADP), the light-independent stage (the Calvin cycle with RuBP, GP and TP), and the effect of limiting factors.
- 5.2.2 Respiration: the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation); the role of decarboxylation, dehydrogenation, reduced NAD and FAD, the electron transport chain, chemiosmosis and ATP synthase; the synthesis of ATP and the role of oxygen as the final electron acceptor; and anaerobic respiration in animals (lactate) and in yeast (ethanol).
A focused answer to the OCR H420 5.2.2 dot point on respiration. Covers the four stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), chemiosmosis and ATP synthase, the role of oxygen, and anaerobic respiration producing lactate or ethanol.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)