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Spec Mapping — OCR H420 Module 6.3.1 — Ecosystems, content statements covering the ecosystem concept (biotic and abiotic components), trophic levels, food chains and food webs, gross and net primary productivity, energy transfer between trophic levels and the construction of ecological pyramids (numbers, biomass, energy) (refer to the official OCR H420 specification document for exact wording). This lesson opens Module 6.3 and supplies the quantitative framework for the nutrient-cycles and populations lessons that follow.
An ecosystem is the fundamental unit of ecology — a community of living organisms together with the non-living environment with which they interact. OCR A-Level Biology A specification 6.3.1 requires you to define the key terms, describe trophic levels, construct and interpret ecological pyramids, and calculate the efficiency of biomass and energy transfer between trophic levels.
The intellectual history of ecosystem ecology is unusually traceable. The English botanist Sir Arthur Tansley coined the term ecosystem in 1935, defining it (paraphrased) as the whole system of biotic community and physical environment considered as a single interacting unit; this was a deliberate move away from the earlier "superorganism" framing of Frederic Clements and toward a more mechanistic, energy- and matter-flow-oriented view. Raymond Lindeman at Minnesota applied this framework in his 1942 study of Cedar Bog Lake, producing the first quantitative trophic-dynamic analysis of an ecosystem; paraphrased, Lindeman's central insight was that the energy flow from producers to consumers could be calculated as a fraction (typically far less than half) of the energy available at each preceding trophic level, with the residue lost as respiration heat. Eugene Odum, at the University of Georgia, systematised these ideas in his influential 1953 textbook Fundamentals of Ecology, and his brother Howard T. Odum developed the energy-systems framework that underpins modern ecological energetics. Charles Elton at Oxford had introduced the concept of the food web and the ecological niche in the 1920s. These four schools of thought — Tansley's ecosystem, Lindeman's trophic dynamics, Odum's ecological energetics and Elton's food webs and niches — together constitute the conceptual base of the OCR ecosystems syllabus.
Key Definitions:
- Ecosystem — a community of living organisms interacting with their abiotic environment.
- Biotic factors — living components (predators, prey, competitors, pathogens).
- Abiotic factors — non-living components (light, temperature, water, pH, nutrients).
- Population — all individuals of one species in a defined area at the same time.
- Community — all the populations of different species in the same area.
- Niche — the role of an organism in its ecosystem, including everything it eats, where it lives, and how it interacts.
- Habitat — the place where an organism lives.
- Trophic level — a position in a food chain (producer, primary consumer, etc.).
- GPP (Gross Primary Productivity) — total energy fixed by producers through photosynthesis.
- NPP (Net Primary Productivity) — GPP minus energy lost as respiration.
The term ecosystem was coined by Arthur Tansley in 1935 to emphasise that organisms cannot be understood in isolation from their physical environment. An ecosystem can be as large as a tropical rainforest or as small as a rotting log. It contains:
All these components interact. Change one, and the rest shift in response.
| Factor type | Examples | Effect on populations |
|---|---|---|
| Biotic | Predation, competition (intra- and interspecific), parasitism, disease, food supply, mutualism, pollination | Density-dependent (stronger at high N) |
| Abiotic | Light intensity, temperature, water availability, soil pH, dissolved oxygen, salinity, wind, fire, mineral nutrient availability | Density-independent (mostly) |
The biotic / abiotic distinction is the most fundamental in ecology. Biotic factors involve living organisms and are typically density-dependent — they act more strongly when the affected population is dense, providing the negative feedback that allows populations to reach equilibrium. Abiotic factors involve the non-living environment and are typically density-independent — a cold winter or a forest fire affects a sparse population just as much as a dense one (in absolute, though not necessarily proportional, terms). The OCR specification expects you to apply this distinction to specific scenarios and to recognise that real ecosystems integrate both kinds of pressure.
The niche of a species is its full role in the ecosystem — not just where it lives but what it eats, when it is active, who eats it, and how it interacts with every other species. Two species cannot occupy exactly the same niche in the same place for long — this is the competitive exclusion principle (Gause, 1934, classically demonstrated with two Paramecium species competing in laboratory culture). One will always out-compete the other. Charles Elton's introduction of the niche concept in the 1920s, refined by Hutchinson's 1957 n-dimensional hypervolume formulation, is the conceptual foundation for understanding why species partition resources rather than directly competing across all dimensions.
Energy flows through an ecosystem from producers to consumers in a food chain:
flowchart LR
A[Sun] -->|Light energy| B[Producers: plants]
B -->|10%| C[Primary consumers: herbivores]
C -->|10%| D[Secondary consumers: carnivores]
D -->|10%| E[Tertiary consumers: top carnivores]
E --> F[Decomposers: bacteria, fungi]
B --> F
C --> F
D --> F
Each level is a trophic level. The arrows represent the direction of energy flow. Only about 10% of energy is transferred from one level to the next; the rest is lost to respiration, heat, faeces and excretion.
Real ecosystems contain food webs rather than simple chains, because most consumers eat (and are eaten by) multiple species. A food web gives a more realistic picture of ecosystem connections.
GPP is the total quantity of chemical energy fixed by producers through photosynthesis per unit area per unit time, conventionally expressed in kJ m⁻² yr⁻¹ or g of dry biomass m⁻² yr⁻¹. It represents the gross intake of energy into the living part of the ecosystem before any losses to respiration. GPP is measured experimentally by various techniques including eddy-covariance flux towers (for whole-ecosystem CO₂ exchange), ¹⁴C-labelled CO₂ uptake assays in aquatic systems, and remote-sensing estimates from satellite-measured leaf-area index combined with light-use-efficiency models. Order-of-magnitude estimates for terrestrial ecosystems range from ~100 g m⁻² yr⁻¹ in deserts to ~3,500 g m⁻² yr⁻¹ in tropical rainforests.
Only a small fraction (typically 1–3%) of incident sunlight is captured by plants. The rest is:
Producers use some of the energy they capture for their own respiration. The rest is stored as new biomass and is available to consumers. This is NPP:
NPP=GPP−R
where R is respiratory loss. Typically NPP is 50–80% of GPP for herbaceous plants, falling to 30% in mature forests.
NPP varies dramatically between ecosystems:
| Ecosystem | NPP (g m⁻² yr⁻¹) |
|---|---|
| Desert | 3–10 |
| Tundra | 100–400 |
| Temperate grassland | 200–1500 |
| Temperate forest | 600–2500 |
| Tropical rainforest | 1000–3500 |
| Coral reef | 500–4000 |
| Open ocean | 2–400 |
Tropical rainforests and coral reefs are the most productive ecosystems on Earth; deserts and the open ocean the least.
On average, only about 10% of the energy available at one trophic level is passed on to the next. The rest (about 90%) is lost in one of four ways:
Because of the 10% rule, food chains rarely exceed 4–5 trophic levels. By the top of a 5-level chain, only about 0.01% of the original producer energy remains.
efficiency=energy at trophic level nenergy at trophic level n+1×100%
For example, if grass produces 100,000 kJ m⁻² yr⁻¹ of NPP, cattle eating it might gain 10,000 kJ m⁻² yr⁻¹ — a transfer efficiency of 10%.
Transfer efficiency is typically 10–20% between producer and primary consumer, but can reach 20–30% between animal trophic levels (especially in warm ecosystems where less energy is lost to thermoregulation). Real measured efficiencies range from ~1% (cold-water ectotherms) to ~25% (warm marine plankton-grazer systems), so the canonical "10% rule" is a textbook simplification, not a universal constant.
Three kinds of ecological pyramid summarise how ecosystems are structured.
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