Ecosystem

The One Thing

An ecosystem is sustained by one-way energy flow from the Sun through trophic levels, together with the continuous recycling of nutrients through decomposition. Energy decreases at successive trophic levels, whereas nutrients such as carbon, nitrogen and phosphorus are repeatedly reused.

Reactions, Processes and Experiments

Key Terms

• Ecosystem: A self-sustaining functional unit made up of biotic and abiotic components interacting through energy flow and nutrient cycling.
• Abiotic components: Non-living factors such as sunlight, temperature, water, soil, minerals, pH and atmospheric gases.
• Biotic components: Living organisms of an ecosystem, including 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.
• Primary consumers: Herbivores that feed directly on producers, such as deer, grasshoppers and zooplankton.
• Secondary consumers: Consumers that feed on primary consumers, commonly including small carnivores and omnivores.
• Tertiary consumers: Higher-level carnivores that feed on secondary consumers.
• Decomposers: Organisms such as bacteria and fungi that break down dead organic matter into simpler substances.
• Detritivores: Organisms such as earthworms and termites that ingest and fragment dead organic matter.
• Trophic level: A feeding position in an ecosystem; producers occupy the first trophic level, herbivores the second, and successive consumers higher levels.
• Energy flow: The one-way movement of energy from the Sun to producers and through successive trophic levels, with energy lost as heat at each transfer.
• Food chain: A linear sequence showing the transfer of food, energy and matter from one organism to another.
• Grazing food chain: A food chain beginning with living green plants, followed by herbivores and then carnivores.
• Detritus food chain: A food chain beginning with dead organic matter and proceeding through detritivores, decomposers and their consumers.
• Food web: A network of interconnected food chains providing alternative feeding pathways.
• Standing crop: The mass of living material present in an ecosystem or trophic level at a particular time, often expressed as biomass or number.
• Gross primary productivity: The total rate at which producers capture solar energy and synthesise organic matter through photosynthesis.
• Net primary productivity: The organic matter available to consumers after producers use part of the captured energy for respiration.
• Secondary productivity: The rate of formation of new organic matter by consumers.
• Ecological pyramid: A graphical representation of the number, biomass or energy at successive trophic levels.
• Pyramid of numbers: A pyramid showing the number of organisms at each trophic level; its shape may be upright or inverted.
• Pyramid of biomass: A pyramid showing the total dry mass of organisms at each trophic level; it is usually upright in terrestrial ecosystems but may be inverted in aquatic ecosystems.
• Pyramid of energy: A pyramid showing energy available at each trophic level per unit area per unit time; it is always upright.
• Lindeman’s ten percent law: Approximately 10 percent of the energy at one trophic level is transferred to the next, while most is lost through respiration, movement, waste and heat.
• Decomposition: The breakdown of complex dead organic matter into simpler inorganic substances by decomposers.
• Detritus: Dead plant remains, animal remains, faecal matter and other non-living organic material.
• Fragmentation: The physical breakdown of detritus into smaller particles by detritivores such as earthworms.
• Leaching: The downward movement of water-soluble inorganic nutrients from detritus into the soil.
• Catabolism: The enzymatic conversion of complex organic substances into simpler compounds by decomposers.
• Humification: The formation of a dark, amorphous and highly resistant organic substance called humus.
• Mineralisation: The release of inorganic nutrients from organic matter during decomposition.
• Humus: A dark, resistant organic material that decomposes slowly and improves soil fertility and water-holding capacity.

Easily Confused

• Energy flow vs nutrient cycling: Energy moves one way through an ecosystem and is ultimately lost as heat, whereas nutrients such as carbon, nitrogen and phosphorus are recycled.
• Producers vs primary consumers: Producers manufacture organic food, whereas primary consumers obtain food by feeding directly on producers.
• Decomposers vs detritivores: Decomposers chemically break down dead organic matter, whereas detritivores ingest and physically fragment it.
• Grazing food chain vs detritus food chain: A grazing food chain begins with living green plants, whereas a detritus food chain begins with dead organic matter.
• Gross primary productivity vs net primary productivity: GPP is total photosynthetic production; NPP is the biomass remaining after plant respiration, expressed as NPP = GPP - Respiration (R).
• Pyramid of numbers vs pyramid of biomass: A pyramid of numbers represents organism numbers, whereas a pyramid of biomass represents total dry mass.
• Pyramid of biomass vs pyramid of energy: A biomass pyramid may be inverted in aquatic ecosystems, whereas an energy pyramid is always upright.
• Humification vs mineralisation: Humification forms resistant humus, whereas mineralisation releases inorganic nutrients from organic matter.
• Fragmentation vs catabolism: Fragmentation is physical particle reduction by detritivores, whereas catabolism is enzymatic chemical breakdown by decomposers.
• Food chain vs food web: A food chain is a linear feeding sequence, whereas a food web consists of interconnected food chains.

What Gets Asked

• Define and distinguish ecosystem components: Questions may require examples of abiotic components, such as sunlight, temperature, water, soil, minerals, pH and atmospheric gases, and biotic components, including producers, consumers and decomposers. Marks are lost by treating non-living factors as biotic components.
• Describe energy flow through trophic levels: The required sequence is Sun -> producers -> primary consumers -> secondary consumers -> tertiary consumers. The key slip is stating that energy is recycled; energy is unidirectional and is lost as heat.
• Calculate or apply productivity equations: Questions may require use of NPP = GPP - Respiration (R) or the trophic transfer efficiency formula. Marks are lost by confusing GPP with the biomass remaining after respiration.
• Apply Lindeman’s ten percent law: From 10,000 units in producers, the expected approximate values are 1,000 units in primary consumers, 100 units in secondary consumers and 10 units in tertiary consumers. The common error is transferring all, rather than approximately 10 percent, of the energy.
• Interpret ecological pyramids: Questions may compare pyramids of numbers, biomass and energy. The specific distinctions are that a pyramid of numbers may be inverted in a tree ecosystem, a pyramid of biomass may be inverted in aquatic ecosystems, and a pyramid of energy is always upright.
• Explain decomposition and its controlling factors: Answers should include fragmentation, leaching, catabolism, humification and mineralisation, together with temperature, moisture, oxygen availability, detritus composition and decomposer activity. Marks are lost by omitting the release of inorganic nutrients or by claiming that cold, dry or oxygen-poor conditions accelerate decomposition.