Ecosystems and Biomass Transfer

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 (a)–(c) 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.

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.

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The Ecosystem Concept

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:

• Producers (autotrophs) — usually plants and photosynthetic microbes that trap solar energy.
• Consumers (heterotrophs) — animals, fungi and many bacteria that feed on other organisms.
• Decomposers — bacteria and fungi that break down dead material, recycling nutrients.
• Abiotic environment — temperature, water, light, soil, nutrients, pH.

All these components interact. Change one, and the rest shift in response.

Biotic and Abiotic Factors

Factor type	Examples
Biotic	Predation, competition, parasitism, disease, food supply
Abiotic	Light, temperature, water, pH, oxygen, mineral availability, wind

Niche

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). One will always out-compete the other.

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Trophic Levels

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.

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Primary Productivity

Gross Primary Productivity (GPP)

GPP is the total quantity of chemical energy fixed by producers through photosynthesis per unit area per unit time. It represents the gross intake of energy into the living part of the ecosystem.

Only a small fraction (typically 1–3%) of incident sunlight is captured by plants. The rest is:

• Reflected from leaves.
• Transmitted through leaves without being absorbed.
• The wrong wavelength (plants use mostly red and blue light).
• Absorbed but not used in photosynthesis (lost as heat).
• Not absorbed by leaves because they have reached their maximum photosynthetic rate.

Net Primary Productivity (NPP)

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:

$\text{NPP} = \text{GPP} - R$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.

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Energy Transfer Between Trophic Levels

The 10% Rule

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: