CLASS 12 BIOLOGY · CHAPTER 12 · STUDY NOTES
Ecosystem
A comprehensive study of ecosystem structure, function, productivity, decomposition, and energy flow, explaining how biotic and abiotic components interact to sustain life.
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Ecosystem: Structure and Function
Look for relationships in a habitat, not just a list of organisms.Read an ecosystem as interacting living and physical components
- Ecosystem
- A functional unit involving interactions among organisms and with their physical environment.
| Component | Pond example | Role or condition |
|---|---|---|
| Physical environment | Water, light, sediments | Conditions and material pools |
| Producers | Plants and algae | Formation of organic matter |
| Consumers | Zooplankton and fish | Use of organic material |
| Decomposers | Microbes in detritus and sediments | Breakdown and nutrient return |
An ecosystem is a functional unit in which organisms interact with one another and with their physical environment. Forests and ponds are natural examples; crop fields and aquariums are managed examples. An ecosystem’s boundary depends on the question being studied.
Species composition describes the species present. Stratification describes their vertical arrangement: a forest can have a tree canopy, shrubs and a lower herb layer. These height layers are different from feeding levels.
In a pond, water, sediments, light and temperature provide the physical setting. Plants and algae produce organic matter; consumers use it; decomposers help return nutrients from dead material. Productivity, decomposition, energy flow and nutrient cycling connect the system.
Let’s look a little closer.
Go deeper: Go beyond the first view
A pond exchanges matter and energy with its surroundings. Calling it relatively self-sustaining does not mean it is a sealed container or that its usable energy is endlessly recycled.
Functional categories describe roles. Many organisms participate in several relationships; identifying a producer or consumer does not replace studying its actual habitat and feeding links.
Take a moment. What do you think?
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Question
Do the canopy, shrub and herb layers of a forest represent three trophic levels? Explain.
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Productivity: GPP and NPP
Measure how fast new organic matter forms, with matching area and time units.Distinguish light input, organic-matter production and respiration
- Net primary productivity (NPP)
- Gross primary productivity minus the producers’ respiration losses, expressed over the same area and interval.
Primary productivity is the rate of production of organic matter by producers. Gross primary productivity (GPP) includes the total photosynthetic production; producers use some of this in respiration (R). Net primary productivity is NPP = GPP − R.
For a hypothetical annual budget, GPP = 1500 and R = 600 kJ m⁻² year⁻¹ give NPP = 900 kJ m⁻² year⁻¹. NPP supports herbivores and decomposers; this does not mean that every unit is eaten by herbivores.
Secondary productivity is the rate at which consumers form new organic matter. Food intake is different: some material is unassimilated and some energy supports metabolism. Plant species, light, nutrients and photosynthetic capacity help explain differences in primary productivity.
Go deeper: Go beyond the first view
A standing biomass is an amount present at a moment; productivity is a rate of formation. Compare rates using consistent units such as grams of dry matter per square metre per year, or energy per square metre per year.
Photosynthetically active radiation usually refers to approximately 400–700 nm. Photosynthetic ecosystems need continuing energy input; not all incoming light becomes organic matter. Chemosynthetic systems obtain energy differently, so sunlight is not a universal requirement for every ecosystem.
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Question
An original ecosystem budget has GPP of 1500 and producer respiration of 600 kJ m⁻² year⁻¹. Calculate NPP and state whether it is a standing stock.
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Decomposition: The Recycling Unit
Follow changes in dead material while keeping the processes distinct.Inspect processes that can overlap in the same detritus
- Mineralisation
- Release of inorganic nutrients through the breakdown of organic matter.
Detritus includes dead plant and animal material and faecal matter. Earthworms and other detritivores fragment it into smaller particles. Water can leach soluble material through soil, while microbial enzymes catalyse breakdown into simpler substances.
Humification contributes to dark, relatively resistant organic humus. Mineralisation releases inorganic nutrients from organic matter. Humus can be broken down slowly; it is neither an inorganic salt nor permanently immune to decomposition.
These processes can overlap in the same detritus. Warm, suitably moist and aerated conditions generally favour microbial activity. Cold, oxygen limitation and resistant material such as lignin or chitin can slow breakdown; no single decomposition time applies to every substrate.
Go deeper: Go beyond the first view
Fragmentation increases exposed surface but does not, by itself, turn a leaf into inorganic nutrients. Leaching describes movement in water; dissolved nutrients can become unavailable after precipitation or other interactions.
Aerobic decomposition is prominent in the chapter’s examples. Anaerobic breakdown also occurs, with different pathways and products. Distinguish slower breakdown under a particular condition from the claim that no decomposition can occur.
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Question
A leaf is broken into small pieces by a detritivore. Has all of it been mineralised? Explain.
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Energy Flow and Trophic Levels
Read a feeding arrow as food or energy moving to a consumer.Follow feeding arrows and separate transferred energy from recycled nutrients
- Trophic level
- A feeding position defined by an organism’s role in a particular energy-transfer pathway.
| Quantity | What it describes | Necessary context |
|---|---|---|
| Number | Individual counts | Defined feeding chain and area |
| Biomass | Standing organic matter | Common area; stated time and mass units |
| Energy | Energy transfer or production rate | Common area and time interval |
A grazing chain starts with living producers; a detritus chain starts with dead organic material. Both connect to wider food webs. The arrow grass → insect → bird means that material passes from grass to the insect and then to the bird.
Trophic levels describe feeding roles in a pathway. A seed-eating bird is a primary consumer in one chain; when it eats a herbivorous insect, it acts as a secondary consumer. A species does not always belong to one fixed feeding level.
The chapter’s simplified 10% transfer model makes successive consumer production smaller. From an original hypothetical producer rate of 20000 kJ m⁻² year⁻¹, it gives 2000, 200 and 20 at the next three levels. Actual efficiencies vary.
Go deeper: Go beyond the first view
Energy flows through organisms and is progressively dissipated as heat. It does not return from heat to the producers as usable sunlight. Nutrient elements can instead circulate between organisms and environmental pools.
Untransferred energy is not all immediate heat. Uneaten material and unassimilated waste can retain chemical energy that supports detrital pathways. The 10% model is an approximate teaching relationship rather than an exact universal law.
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Question
Use the simplified 10% model with an original producer rate of 30000 kJ m⁻² year⁻¹. What are the primary- and secondary-consumer production rates?
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Ecological Pyramids
Identify the quantity before interpreting the width of a pyramid.Choose the quantity before interpreting a pyramid
- Standing biomass
- The amount of living organic material present in a specified area or volume at a particular time.
A pyramid of numbers compares counts, a biomass pyramid compares standing biomass, and an energy pyramid compares energy transfer or production rates. The base represents producers; successive levels represent feeding positions in the stated chain.
A tree can support many herbivorous insects, giving a small count at the base. In some aquatic systems, consumer standing biomass can exceed producer biomass at a snapshot because producers are replaced rapidly. Neither observation reverses energy flow.
An energy-rate pyramid remains upright when the same area and interval are compared across feeding levels. The interactive diagrams use original hypothetical data and a common linear width scale; thin upper bars represent smaller quantities.
Go deeper: Go beyond the first view
A stock and a flow answer different questions. A small phytoplankton stock can generate substantial new material over time, just as a small quantity present at a moment is not a measure of its replacement rate.
A single pyramid simplifies the system. It does not fully represent omnivory, connected food chains or decomposer pathways. Label the quantity and units before drawing conclusions from the shape.
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Question
At a snapshot, hypothetical producer and primary-consumer dry biomasses are 2 and 8 g m⁻². Does this establish an inverted energy pyramid?
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Limitations and Nutrient Cycling
Use pyramids as summaries, then restore the relationships they leave out.See what a simple pyramid leaves out
- Nutrient cycling
- Movement of nutrient elements among organisms and environmental reservoirs through connected processes.
A food web includes species that feed along several pathways, so one organism may occupy more than one trophic position. Decomposers connect to material from many levels. Simple pyramids do not capture all these links.
Nutrient elements move into organisms and return to environmental pools through processes such as decomposition and mineralisation. Carbon fixation and respiratory return, or phosphate release from rock and uptake, illustrate different connections rather than one identical cycle for every nutrient.
Ecosystem functions can support water regulation, soil formation, pollination and other services. A wetland may retain or process some material through its plants, soils and microbes. The effectiveness depends on actual conditions; an illustration does not guarantee removal of every contaminant.
Go deeper: Go beyond the first view
The partial carbon and phosphorus pictures make only the illustrated connections. They are not complete global budgets, do not imply identical timescales, and do not assign unsupported numerical fluxes.
Keep energy input separate from element recycling. Reuse of a nutrient atom does not mean the usable chemical energy that once accompanied it has been completely restored.
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Question
Why can a nutrient loop be drawn while usable energy does not form the same loop?
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