Organisation of an Ecosystem¶
Part of 4.7 Ecology.
An ecosystem is more than a list of organisms. It is a system in which matter is recycled, energy is transferred and environmental changes alter the balance between populations. Understanding these cycles explains why ecosystems can sustain themselves over millions of years.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4.7.2-lo-1 |
4.7.2.1 Students should understand that photosynthetic organisms are the producers of biomass for life on Earth. 4.7.2.1 Feeding relationships within a community can be represented by food chains. All food chains begin with a producer which synthesises molecules. This is usually a green plant or alga which makes glucose by photosynthesis. 4.7.2.1 A range of experimental methods using transects and quadrats are used by ecologists to determine the distribution and abundance of species an ecosystem. 4.7.2.1 In relation to abundance of organisms students should be able to: 4.7.2.1 • understand the terms mean, mode and median 4.7.2.1 • calculate arithmetic means 4.7.2.1 • plot and draw appropriate graphs selecting appropriate scales for the axes. 4.7.2.1 Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers and then tertiary consumers. 4.7.2.1 Consumers that kill and eat other animals are predators, and those eaten are prey. In a stable community the numbers of predators and prey rise and fall in cycles. 4.7.2.1 Students should be able to interpret graphs used to model these cycles. |
Levels of Organisation |
4.7.2-lo-2 |
4.7.2.2 Students should: 4.7.2.2 • recall that many different materials cycle through the abiotic and biotic components of an ecosystem 4.7.2.2 • explain the importance of the carbon and water cycles to living organisms. 4.7.2.2 All materials in the living world are recycled to provide the building blocks for future organisms. 4.7.2.2 The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis. |
The Carbon Cycle, The Water Cycle |
4.7.2-lo-3 |
4.7.2.2 The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated. 4.7.2.2 Students are not expected to study the nitrogen cycle. 4.7.2.2 Students should be able to explain the role of microorganisms in cycling materials through an ecosystem by returning carbon to the atmosphere as carbon dioxide and mineral ions to the soil. 4.7.2.3 Students should be able to explain how temperature, water and availability of oxygen affect the rate of decay of biological material. 4.7.2.3 Students should be able to: 4.7.2.3 • calculate rate changes in the decay of biological material 4.7.2.3 • translate information between numerical and graphical form 4.7.2.3 • plot and draw appropriate graphs selecting appropriate scales for the axes. 4.7.2.3 Gardeners and farmers try to provide optimum conditions for rapid decay of waste biological material. The compost produced is used as natural fertiliser for growing garden plants or crops. 4.7.2.3 Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel. |
Decomposition |
4.7.2-lo-4 |
4.7.2.4 Students should be able to evaluate the impact of environmental changes on the distribution of species in an ecosystem given appropriate information. 4.7.2.4 Environmental changes affect the distribution of species in an ecosystem. These changes include: 4.7.2.4 • temperature 4.7.2.4 • availability of water 4.7.2.4 • composition of atmospheric gases. 4.7.2.4 The changes may be seasonal, geographic or caused by human interaction. |
Impact of Environmental Change |
Levels of Organisation¶
A food chain or food web shows who eats whom. A food web is a more realistic picture than a single food chain because most organisms eat (and are eaten by) more than one species.
- Producers — photosynthetic organisms (plants, algae) that convert light energy into biomass.
- Primary consumers — herbivores that eat producers.
- Secondary consumers — carnivores that eat primary consumers.
- Tertiary consumers — carnivores that eat secondary consumers.
- Decomposers — bacteria and fungi that break down dead organisms and waste, returning minerals to the soil.
Because organisms are linked in food webs, removing one species can affect many others. For example, removing a keystone predator may allow a prey species to over-reproduce, consuming all available vegetation and collapsing the ecosystem.
The Carbon Cycle¶
Carbon is constantly transferred between the atmosphere, living organisms, and the ground. The key processes are:
Carbon enters living organisms: - Photosynthesis — plants absorb CO₂ from the atmosphere and incorporate carbon into glucose and other organic molecules.
Carbon passes between organisms: - Feeding — animals eat plants, transferring organic carbon along food chains.
Carbon returns to the atmosphere: - Respiration — all living organisms release CO₂ during aerobic respiration. - Decomposition — decomposers (bacteria and fungi) break down dead organisms and waste, releasing CO₂ through their own respiration. - Combustion — burning of wood, fossil fuels and organic matter releases stored carbon as CO₂.
Long-term carbon storage: - When organisms die in conditions that prevent decomposition (e.g. compressed underwater sediment), their carbon is stored as fossil fuels (coal, oil, natural gas) over millions of years. Burning these fuels releases carbon that has been locked away for a very long time, adding extra CO₂ to the atmosphere.
The Water Cycle¶
Water is recycled continuously through the environment. The main stages are:
- Evaporation — solar energy causes water to evaporate from oceans, lakes and rivers, forming water vapour.
- Transpiration — plants release water vapour through their leaves (transpiration stream).
- Condensation — water vapour cools as it rises and forms clouds (liquid water droplets).
- Precipitation — water falls as rain, snow or hail onto land and into water bodies.
- Surface runoff — water flows across land surface into streams and rivers, returning to the sea.
- Infiltration — some water soaks into the ground and is stored in underground rocks (aquifers).
The Nitrogen Cycle¶
Nitrogen is an essential element for making proteins and nucleic acids. Although 78% of the atmosphere is nitrogen gas (N₂), most organisms cannot use it directly — it must be converted into usable forms.
The key processes are:
| Process | What happens | Organisms involved |
|---|---|---|
| Nitrogen fixation | Atmospheric N₂ → ammonia/nitrates in soil | Nitrogen-fixing bacteria (in root nodules of legumes and free in soil); lightning |
| Nitrification | Ammonium ions (NH₄⁺) → nitrites → nitrates (NO₃⁻) | Nitrifying bacteria in soil |
| Assimilation | Plants absorb nitrates through roots → used to build amino acids and proteins; animals gain nitrogen by eating plants | Plants and animals |
| Ammonification | Dead organisms and excretion → ammonia returned to soil | Decomposers (bacteria and fungi) |
| Denitrification | Nitrates in soil → N₂ returned to atmosphere | Denitrifying bacteria (in waterlogged soil) |
Leguminous plants (peas, beans, clover) have nitrogen-fixing bacteria in root nodules that convert N₂ into ammonia — this is why farmers rotate crops with legumes to restore soil nitrogen.
Decomposition¶
Decomposers (bacteria and fungi) are essential for recycling — they break down dead organisms and waste products, releasing mineral ions back into the soil where plants can reabsorb them.
Decomposers break down molecules by secreting enzymes outside their cells. Soluble products are then absorbed.
Factors affecting the rate of decomposition:¶
| Factor | Effect on decomposition rate |
|---|---|
| Temperature | Higher temperature → faster decomposition (up to optimum); too high → enzymes denature |
| Moisture | More moisture → faster decomposition; dry conditions preserve organisms (mummification) |
| Oxygen availability | Aerobic decomposers work faster with more oxygen; anaerobic decomposers work in absence of O₂ |
| pH | Each decomposer species has an optimum pH range |
Practical applications:¶
Compost heaps — garden waste decomposes aerobically to produce compost, a nutrient-rich material that improves soil structure and fertility. Turning a compost heap adds oxygen and speeds decomposition.
Biogas generators — organic waste (animal dung, crop waste) is decomposed anaerobically by bacteria. This produces biogas, a mixture mainly of methane (CH₄) and CO₂. Methane can be burned as a fuel for cooking, heating, or generating electricity. Biogas is a renewable energy source.
Food preservation — decomposition can be slowed by: - Refrigeration/freezing — low temperature slows enzyme activity in decomposers. - Drying — removes water, preventing microbial growth. - Airtight packaging — removes oxygen needed by aerobic decomposers; also sterilised at high temperature to kill microorganisms.
Impact of Environmental Change¶
Changes to abiotic conditions can shift where organisms live:
- Temperature change — species distributions shift towards cooler regions or higher altitudes. Many British bird species now breed earlier in the year because spring arrives sooner. Tropical insects are extending their range northward.
- Water availability — drought reduces plant populations; flooding creates new wetland habitats. Migrating animals (e.g. wildebeest in the Serengeti) follow seasonal rainfall.
- Atmospheric composition — air pollution (e.g. sulphur dioxide) can kill sensitive species such as lichens, which are used as pollution indicators. Rising CO₂ affects photosynthesis rates and is linked to climate change.
Environmental change can have indirect effects through the food web: if one species declines, its predators may also decline while its prey may increase — cascading through the entire ecosystem.
Common Confusions¶
- Decomposers and detritivores: Decomposers (bacteria, fungi) break down organic matter by releasing enzymes. Detritivores (e.g. earthworms, woodlice) eat dead matter and physically break it into smaller pieces, speeding up decomposition — but they are not the same thing.
- The carbon cycle and combustion: Students sometimes omit combustion from the carbon cycle. It is a major pathway for carbon to return to the atmosphere, especially since the Industrial Revolution.
- Nitrogen fixation vs nitrification: Nitrogen fixation converts atmospheric N₂ into ammonia or nitrates (involves specific bacteria in root nodules or soil). Nitrification converts ammonium ions into nitrates in the soil. These are different processes with different bacteria.
- Aerobic vs anaerobic decomposition: Most decomposers work best with oxygen (aerobic). Biogas generators deliberately use anaerobic conditions to produce methane. Waterlogged soils are anaerobic, which is why peat bogs preserve bodies for thousands of years.
Key Terms¶
- Ecosystem: the interaction between a community of organisms and all the non-living components of their environment.
- Decomposer: a microorganism (bacterium or fungus) that breaks down dead organic matter and waste, releasing minerals.
- Decay: the breakdown of dead material by decomposers.
- Nitrogen fixation: the conversion of atmospheric nitrogen gas into ammonia or nitrates by bacteria.
- Nitrification: the conversion of ammonium ions into nitrates by nitrifying bacteria in the soil.
- Denitrification: the conversion of soil nitrates back to nitrogen gas by denitrifying bacteria.
- Compost: decomposed organic material used to enrich soil.
- Biogas generator: a sealed vessel in which organic waste is broken down anaerobically by bacteria to produce methane-rich biogas.
- Transpiration: the loss of water vapour from plant leaves; contributes to the water cycle.
- Precipitation: water falling from the atmosphere as rain, snow, hail or sleet.
- Detritivore: an organism (e.g. earthworm, woodlouse) that feeds on dead organic material, physically breaking it into smaller pieces.