Ecosystem

1. Chapter at a glance

• An ecosystem is a functional unit of nature where biotic and abiotic components interact, resulting in characteristic species composition and stratification.
• Ecosystem functions comprise four processes: productivity, decomposition, energy flow and nutrient cycling.
• Primary productivity (GPP – R = NPP) is the rate of biomass production by autotrophs; secondary productivity is the rate of new organic matter formation by consumers.
• Decomposition converts detritus into inorganic substances through fragmentation, leaching, catabolism, humification and mineralisation.
• Energy flow is unidirectional from the sun → producers → consumers, with only ~10 % transferred to each successive trophic level.
• Organisms occupy specific trophic levels (producers → herbivores → carnivores) forming grazing food chains (GFC), detritus food chains (DFC) and interconnected food webs.
• Ecological pyramids (numbers, biomass, energy) represent trophic relationships; most are upright, but biomass pyramids may be inverted (e.g., ocean).
• The biosphere’s annual net primary productivity is ~170 billion tons (dry weight), with terrestrial ecosystems contributing the majority.

2. Definitions and laws

• Primary production: “the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.” (expressed as gm⁻² or kcal m⁻²)
• Productivity: “the rate of biomass production” (gm⁻² yr⁻¹ or kcal m⁻² yr⁻¹).
• Gross primary productivity (GPP): “the rate of production of organic matter during photosynthesis.”
• Net primary productivity (NPP): GPP – R = NPP (R = respiration losses); “available biomass for the consumption to heterotrophs.”
• Secondary productivity: “the rate of formation of new organic matter by consumers.”
• Decomposition: process by which “decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients.”
• Detritus: “Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter.”
• 10 per cent law: “only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.”
• Energy flow obeys the first and second laws of thermodynamics (unidirectional flow; constant energy input required to counteract increasing disorder).

3. Important diagrams and activities

• Figure 12.1 – Diagrammatic representation of decomposition cycle in a terrestrial ecosystem (shows simultaneous operation of fragmentation, leaching, catabolism, humification and mineralisation).
• Figure 12.2 – Diagrammatic representation of trophic levels in an ecosystem (illustrates producers at first level, herbivores at second, carnivores at third).
• Figure 12.3 – Energy flow through different trophic levels (demonstrates unidirectional flow and progressive energy loss).
• Figure 12.4 (a–d) – Ecological pyramids: (a) pyramid of numbers in grassland, (b) upright pyramid of biomass, (c) inverted pyramid of biomass in ocean, (d) upright pyramid of energy (students must draw all four types and note which can be inverted).

4. Common misconceptions and exam pitfalls

• Assuming all ecological pyramids are upright — biomass pyramid can be inverted; energy pyramid is always upright.
• Treating a species as belonging to only one trophic level — a species (e.g., sparrow, human) can occupy more than one level simultaneously.
• Confusing food chain with food web or omitting the detritus food chain in terrestrial ecosystems.
• Forgetting that saprophytes/decomposers are not assigned a trophic level in ecological pyramids.
• Writing GPP = NPP without subtracting respiration losses.
• Stating that energy flow is cyclic (it is strictly unidirectional).

5. Formula sheet