Ecology: Habitat, Ecosystem and Energy Flow

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Ecology studies the interactions between living organisms and their non-living (abiotic) surroundings.
A habitat is the specific natural place where an organism lives and finds food, shelter, and mates; a niche is the role it plays there.
An ecosystem = biotic factors (plants, animals, microbes) + abiotic factors (sunlight, water, temperature, soil) interacting as a functional unit.
Energy flow moves in one direction: Sun → producers (autotrophs) → consumers (heterotrophs) → decomposers, with heat lost at every step.
The 10% rule: only about 10% of energy at one trophic level is transferred to the next; the rest (≈90%) is lost as heat, in movement, or in undigested waste.
High-yield pointers for NECO SSCE: know the difference between a food chain (single linear pathway) and a food web (interlinked chains); recall the formula NPP = GPP − R; remember that energy pyramids are never inverted but biomass and number pyramids can be.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Components of an Ecosystem

An ecosystem has two inseparable parts. Biotic components are the living organisms classified into producers, consumers (primary, secondary, tertiary, omnivores) and decomposers (fungi and bacteria). Abiotic components include sunlight, water, temperature, soil pH, humidity, and mineral salts. The habitat provides these physical conditions; the ecological niche describes how an organism lives within that habitat.

Energy Flow and Trophic Levels

Sunlight is the ultimate energy source. Producers (green plants, algae, cyanobacteria) fix this energy by photosynthesis into chemical energy stored as glucose.

GPP = NPP + R, where:

GPP = Gross Primary Productivity (total energy fixed)
NPP = Net Primary Productivity (energy stored in plant tissue)
R = energy lost through respiration

The 10% rule quantifies transfer between levels: Eₙ = Eₙ₋₁ × 10/100. So if producers capture 20,000 kJ m⁻² yr⁻¹, primary consumers store roughly 2,000 kJ, secondary consumers about 200 kJ, and tertiary consumers only ~20 kJ. This is why food chains rarely exceed 4–5 trophic levels.

Food Chain, Food Web and Ecological Pyramids

A food chain is a single linear pathway such as Grass → Grasshopper → Lizard → Hawk. A food web is a network of such chains, giving a more realistic picture because most organisms eat several species.

Pyramid Type	What it Measures	Can it Invert?
Pyramid of energy	Energy (kJ m⁻² yr⁻¹) at each trophic level	No
Pyramid of biomass	Dry mass of organisms at each level	Yes (in aquatic ecosystems, e.g., plankton)
Pyramid of numbers	Count of organisms at each level	Yes (e.g., one tree supporting many insects)

Typical NECO Question Patterns

Expect Objective questions asking you to identify producers in a given chain, calculate energy at a level using the 10% rule, or pick the correct definition of habitat, niche, or ecosystem. Essay questions often test the unidirectional nature of energy flow versus the cyclic nature of nutrients (carbon, nitrogen cycles).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Edge Cases and Inverted Pyramids

The pyramid of energy is always upright because energy is continuously lost as heat (Second Law of Thermodynamics). However, the pyramid of numbers inverts in parasitic chains: a single host (e.g., a cow) supports many parasites, then more hyperparasites. The pyramid of biomass inverts in marine ecosystems where phytoplankton (small standing crop but high turnover) feed zooplankton of greater total mass at any instant — energy still flows correctly because productivity is measured per unit time.

Decomposers and Nutrient Cycling

Although energy flows one way and exits as heat, nutrients cycle. Decomposers (saprotrophs) secrete enzymes that break down dead organic matter into inorganic forms (nitrates, phosphates, CO₂) which producers reabsorb. Without this, ecosystems would exhaust available minerals.

Ecological Succession

On bare rock or cleared land, communities change over time: pioneer species (lichens, mosses) alter the substrate, allowing seral communities to replace them, until a stable climax community (e.g., tropical rainforest in wet zones, savanna in drier zones) is reached.

Common Mistakes

Confusing habitat (the place) with niche (the role). Two species may share a habitat but occupy different niches.
Assuming the 10% rule applies to numbers or biomass — it applies strictly to energy.
Saying energy is “recycled” — only matter is recycled; energy is dissipated.

Worked Example

A grassland ecosystem has producers storing 8,000 kJ m⁻² yr⁻¹. Using the 10% rule:

Primary consumers: 8,000 × 0.10 = 800 kJ m⁻² yr⁻¹
Secondary consumers: 800 × 0.10 = 80 kJ m⁻² yr⁻¹
Tertiary consumers: 80 × 0.10 = 8 kJ m⁻² yr⁻¹

Practice Prompts

Explain why a food web is considered more stable than a single food chain.
In an aquatic ecosystem, the pyramid of biomass is inverted yet the ecosystem still thrives. Justify this observation using energy flow concepts.

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Ecology: Habitat, Ecosystem and Energy Flow