Ecosystem

The One Thing

An ecosystem functions through interactions between abiotic factors and biotic components, with energy flowing one way from the Sun while nutrients are continuously recycled. Productivity supplies organic matter to higher trophic levels, and decomposition returns nutrients to the physical environment.

Reactions, Processes and Experiments

Key Terms

• Ecosystem: A self-sustaining functional unit consisting of living organisms and their physical environment, interacting through energy flow and nutrient cycling.
• Abiotic components: Non-living factors of an ecosystem, including sunlight, temperature, water, air, soil, pH, humidity, and mineral nutrients.
• Biotic components: Living organisms of an ecosystem, grouped mainly into producers, consumers, and decomposers.
• Producers: Autotrophs, mainly green plants, algae, and some bacteria, that manufacture organic food using light or chemical energy.
• Consumers: Heterotrophs that obtain food by eating producers or other organisms. They include primary, secondary, and tertiary consumers.
• Decomposers: Organisms such as bacteria and fungi that break down dead organic matter into simpler substances and release nutrients.
• Detritivores: Organisms such as earthworms and some insects that feed on dead organic fragments and help in their breakdown.
• Trophic level: A feeding position in an ecosystem. Producers form the first trophic level, herbivores the second, and successive carnivores occupy higher levels.
• Food chain: A linear sequence showing the transfer of food, matter, and energy from one organism to another.
• Food web: A network of interconnected food chains that represents several feeding relationships in an ecosystem.
• Ecological pyramids: Diagrams showing the number, biomass, or energy present at successive trophic levels.
• Standing crop: The amount of living biomass or number of organisms present in an ecosystem at a particular time.
• Productivity: The rate at which organic matter or biomass is produced in an ecosystem.
• Gross primary productivity: The total rate of organic matter production by producers through photosynthesis before respiratory losses.
• Net primary productivity: The organic matter stored by producers after subtracting their respiratory loss from gross primary productivity.
• Secondary productivity: The rate of formation of new biomass by consumers.
• Decomposition: The physical and chemical breakdown of dead organic matter into simpler inorganic substances by decomposers and detritivores.
• Detritus: Dead plant and animal remains, including fallen leaves, animal wastes, and other organic debris.
• Fragmentation: The breaking of detritus into smaller pieces by detritivores, increasing its surface area for microbial action.
• Leaching: The movement of water-soluble inorganic nutrients from detritus into deeper soil layers or water.
• Catabolism: The enzymatic conversion of complex organic substances into simpler molecules by decomposers, usually releasing energy.
• Humification: The formation of dark, resistant, and partially decomposed organic matter called humus.
• Mineralisation: The release of inorganic nutrients from organic matter during decomposition.
• Energy flow: The movement of energy through an ecosystem from the Sun to producers and then through successive consumer levels, with heat lost at each transfer.
• Ecological efficiency: The percentage of energy transferred from one trophic level to the next.
• Ten percent law: On average, only about ten percent of the energy at one trophic level becomes available to the next trophic level.

Easily Confused

• Gross primary productivity and net primary productivity: GPP is the total organic matter produced by producers; NPP is the amount remaining after plant respiratory loss, expressed as NPP = GPP - R.
• Producers and primary consumers: Producers manufacture organic food, whereas primary consumers obtain food by eating producers.
• Decomposers and detritivores: Decomposers, such as bacteria and fungi, chemically break down dead organic matter; detritivores, such as earthworms and some insects, physically consume dead organic fragments.
• Food chains and food webs: A food chain is a linear feeding sequence; a food web consists of interconnected food chains.
• Energy flow and nutrient cycling: Energy is unidirectional and eventually lost as heat; nutrients such as carbon, nitrogen, phosphorus, and sulphur are repeatedly recycled.
• Grazing-based and detritus-based food chains: A grazing-based chain begins with living green plants, whereas a detritus-based chain begins with dead organic matter.
• Standing crop and productivity: Standing crop is the amount of living biomass or number of organisms present at a particular time; productivity is the rate of organic matter or biomass formation.
• Pyramid of energy and pyramids of number or biomass: The pyramid of energy is always upright, whereas pyramids of number and biomass may be upright, inverted, or spindle-shaped.

What Gets Asked

• Define and classify ecosystem components: Questions may require the two major components—abiotic and biotic—or the classification of biotic components into producers, consumers, and decomposers. Marks are lost by omitting named abiotic factors or confusing consumers with decomposers.
• Write and interpret productivity relationships: Questions may ask for the photosynthesis equation, the definitions of GPP and NPP, or NPP = GPP - R. The specific error is treating GPP as the organic matter available to consumers without subtracting respiratory loss.
• Describe decomposition: Questions may require the sequence of fragmentation, leaching, catabolism, humification, and mineralisation. Marks are lost by leaving out stages or assigning the formation of humus to mineralisation.
• Explain factors affecting decomposition: Warm, moist, well-aerated conditions increase decomposition, whereas low temperature, dryness, waterlogging, and poor oxygen supply slow it. Nitrogen-rich detritus decomposes faster than detritus containing substantial lignin, wax, or other resistant compounds.
• Apply the ten percent law: A question may ask students to calculate energy available at successive trophic levels, such as 10,000 units in producers, followed by approximately 1,000, 100, and 10 units. The common slip is treating energy as recycled rather than progressively lost as heat.
• Compare ecological pyramids and feeding pathways: Questions may test why the pyramid of energy is always upright, why pyramids of number and biomass may be inverted or spindle-shaped, or how food webs provide greater stability than simple food chains. The key distinction is between the type of quantity represented and the structure of feeding relationships.