Paper I · 7 Sustainable Development & Environment · 7.1.1 Ecology and ecosystems
7.1.1.2 Ecosystem structure: biotic and abiotic components
An ecosystem is a functional unit of nature in which living organisms interact with each other and with their non-living surroundings, linked by energy flow and nutrient cycling.
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Core facts
- Ecosystem: a functional unit of nature where living organisms interact among themselves and with the physical environment. Term coined by A.G. Tansley (1935).
- Two parts:
- Biotic: producers, consumers and decomposers.
- Abiotic: light, temperature, water, soil, air, minerals and dead organic matter.
- Producers (autotrophs): make their own food. Green plants, algae and cyanobacteria do it by photosynthesis; some bacteria do it by chemosynthesis.
- Consumers (heterotrophs): herbivores (primary), carnivores (secondary and tertiary) and omnivores.
- Decomposers: mainly bacteria and fungi. They break down dead matter and release nutrients.
- Detritivores: animals such as the earthworm that feed on detritus and break it into smaller pieces.
- Ecosystems are natural (forest, grassland, desert, pond, lake, river, estuary, sea) or man-made (crop field, aquarium, fish pond, plantation).
- Four functional aspects (NCERT): productivity, decomposition, energy flow and nutrient cycling.
- Stratification: the vertical layering of species, such as trees at the top, then shrubs, then herbs and grasses at the bottom. It is a structural feature.
Explained
Components of an ecosystem
| Component | Type | Examples |
|---|---|---|
| Inorganic substances | Abiotic | Carbon dioxide, water, nitrogen, phosphates |
| Organic substances | Abiotic | Proteins, carbohydrates, lipids, humus |
| Climatic factors | Abiotic | Light, temperature, rainfall, humidity, wind |
| Edaphic and topographic factors | Abiotic | Soil type, slope, altitude |
| Producers | Biotic | Grass, trees, phytoplankton |
| Consumers | Biotic | Deer, tiger, zooplankton, fish |
| Decomposers | Biotic | Bacteria, fungi |
The pond as a model ecosystem (NCERT's example)
- Abiotic: water, dissolved inorganic and organic matter, and the soil deposit at the bottom.
- Producers: phytoplankton, some algae, and floating, submerged and marginal plants.
- Consumers: zooplankton, and free-swimming and bottom-dwelling animals.
- Decomposers: fungi, bacteria and flagellates, which are especially plentiful at the bottom.
- The pond shows all four functions: producers convert sunlight to food, consumers eat it, decomposers mineralise dead matter, and nutrients return to the producers.
Ecosystem services
- These are the benefits people get from ecosystems.
- Examples: forests purify air and water, reduce droughts and floods, cycle nutrients, form fertile soil, provide habitat, pollinate flowers and store carbon.
- Purification of air and water by forests is a classic example of an ecosystem service.
- Robert Costanza and colleagues (1997) put the value of global ecosystem services at an average of about US$ 33 trillion a year. This was nearly twice the global GNP of the time (about US$ 18 trillion).
- In NCERT's summary, soil formation accounts for about 50% of the total. (Answer exam questions by NCERT's figures.)
- Recreation and nutrient cycling were each less than 10%.
- Climate regulation and habitat for wildlife were about 6% each.
Common traps
- ✗ "Stratification is one of the four functional aspects." ✓ It is structural. The four functions are productivity, decomposition, energy flow and nutrient cycling.
- ✗ "An estuary is a man-made ecosystem." ✓ An estuary is natural. Crop fields, aquaria and fish ponds are man-made.
- ✗ "Earthworms are decomposers in the strict sense." ✓ Earthworms are detritivores; the true decomposers are bacteria and fungi.
- ✗ "Costanza valued ecosystem services at about US$ 18 trillion." ✓ About US$ 33 trillion; US$ 18 trillion was global GNP.
- ✗ "All producers use sunlight." ✓ Chemosynthetic bacteria at hydrothermal vents use chemical energy.
Quick revision
- Ecosystem: Tansley, 1935; biotic + abiotic.
- Biotic: producers, consumers, decomposers.
- Decomposers: bacteria and fungi; earthworm = detritivore.
- Four functions: productivity, decomposition, energy flow, nutrient cycling.
- Stratification = vertical layering = structure.
- Man-made: crop field, aquarium, fish pond.
- Costanza (1997): about US$ 33 trillion a year; soil formation about 50%.
- MEA 2005: provisioning, regulating, cultural, supporting.
- Liebig: law of minimum; Shelford: law of tolerance.
Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)
Deeper Advanced
Millennium Ecosystem Assessment (2005) categories
| Category | Meaning | Examples |
|---|---|---|
| Provisioning | Products obtained | Food, fresh water, timber, fibre, medicine |
| Regulating | Control of natural processes | Climate regulation, flood control, water purification, pollination |
| Cultural | Non-material benefits | Recreation, spiritual value, tourism |
| Supporting | Needed for all the others | Soil formation, nutrient cycling, primary production |
Laws of limiting factors
- Liebig's law of the minimum (1840): growth is controlled by the essential nutrient in shortest supply, not by the total of resources.
- Shelford's law of tolerance (1913): each organism has a minimum and a maximum limit for each factor. Its range between them is its range of tolerance. Too much of a factor limits it as surely as too little.
Autotroph types
- Photoautotrophs use sunlight.
- Chemoautotrophs use energy from oxidising inorganic chemicals such as hydrogen sulphide, ammonia or iron. They support the deep-sea hydrothermal vent communities, the main ecosystems not driven by sunlight.
Ecosystems as open systems. Energy enters (mainly as sunlight) and leaves (as heat). Matter is largely recycled within. Ecosystems show homeostasis: feedback (for example, predator and prey numbers) keeps them within limits, until a disturbance crosses a threshold.
Mains writing Mains writing
- Frame: treating ecosystems as natural capital makes their services visible in planning. Most services have no market price, so they are lost without being counted.
- Analysis points:
- Valuation: the Costanza estimate showed services were worth more than the world economy. Critics say a single price hides irreplaceable value.
- Externalities: a dam or mine counts its revenue but not the lost flood control, groundwater recharge or fisheries.
- Payment for ecosystem services: funds such as CAMPA's compensatory afforestation, and the Finance Commission's use of forest cover in tax devolution, are attempts to pay for services.
- Equity: forest-dwelling and coastal communities depend most on provisioning services and lose most when they decline.
- Way forward:
- natural capital accounting (India's environmental accounts published by MoSPI under the UN SEEA framework)
- valuation of services in environmental impact assessment
- community rights over local ecosystems
- Example: mangroves are often cleared for aquaculture for short-term profit, but they provide nursery grounds for fish and protection against storms, which are worth more over time.
Practise this topic. Questions on Ecology & biodiversity, with answers explained.
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