Ecosystem: NEET notes

1. Ecosystem: meaning and types

NCERT §12 (introduction)

• An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical surroundings.
• An ecosystem can be as small as a pond or as large as a whole forest or sea.
• Many ecologists treat the whole biosphere as one global ecosystem, made up of all the local ecosystems on Earth.
• Since the biosphere is too large and complex to study at once, it is divided into two basic kinds: terrestrial and aquatic.
• Terrestrial examples: forest, grassland, desert. Aquatic examples: pond, lake, wetland, river, estuary.
• Crop fields and an aquarium count as man-made ecosystems.
• The chapter follows the input (productivity), the transfer of energy (food chains and webs) and the output (degradation and loss of energy).

2. Structure and function

NCERT §12.1

• An ecosystem has abiotic (non-living) and biotic (living) components. Their interaction gives each type of ecosystem a characteristic physical structure.
• Identifying and counting the plants and animals present tells us the species composition of an ecosystem.
• Stratification is the vertical distribution of species at different levels.
• In a forest, trees form the top layer, shrubs the second, and herbs and grasses the bottom layers.
• The two chief features of an ecosystem's structure are its species composition and its stratification.
• An ecosystem works as a unit through four functional aspects: productivity, decomposition, energy flow and nutrient cycling.

3. The pond ecosystem

NCERT §12.1

• A pond is a shallow water body and a fairly self-sustaining, simple example that shows all four basic functions of an ecosystem.
• Abiotic component: the water, the inorganic and organic matter dissolved in it, and the nutrient-rich soil on the pond floor.
• Solar input, the temperature cycle, day length and other climatic conditions regulate how fast the whole pond functions.
• Autotrophs (producers): phytoplankton, some algae, and plants that float, stay submerged or grow along the margins.
• Consumers: zooplankton, and free-swimming and bottom-dwelling forms.
• Decomposers: fungi, bacteria and flagellates, most abundant at the bottom of the pond.
• The pond does what every ecosystem does: autotrophs turn inorganic into organic matter using solar energy, heterotrophs consume the autotrophs, and dead matter is decomposed and mineralised for the autotrophs to reuse.
• Energy moves in one direction towards higher trophic levels and is lost as heat to the surroundings.

4. Productivity

NCERT §12.2

• Every ecosystem needs a steady supply of solar energy to keep working.
• Primary production is the organic matter (biomass) that plants make by photosynthesis on a unit of area over a stated period. It is expressed as weight (g m⁻²) or energy (kcal m⁻²).
• Productivity is the rate of biomass production, expressed as g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹, so that different ecosystems can be compared.
• Gross primary productivity (GPP) is the rate at which organic matter is made in photosynthesis. Plants use a considerable part of it in respiration.
• Net primary productivity: GPP − R = NPP, where R is the respiration loss. NPP is the biomass available to heterotrophs (herbivores and decomposers).
• Secondary productivity is the rate at which consumers form new organic matter.
• Primary productivity depends on the plant species of an area, environmental factors, nutrient availability and the photosynthetic capacity of plants, so it differs between ecosystems.
• The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter.
• Oceans cover about 70 per cent of the surface but produce only 55 billion tons of it; the rest is produced on land.

5. Decomposition

NCERT §12.3

• Decomposers break complex organic matter down into inorganic substances such as carbon dioxide, water and nutrients. This is decomposition.
• The earthworm is called the farmer's friend because it helps break down organic matter and loosens the soil.
• Detritus, the raw material, is made of dead plant remains (leaves, bark, flowers) and dead animal remains, including fecal matter.
• Fragmentation: detritivores such as the earthworm break detritus into smaller particles.
• Leaching: water-soluble inorganic nutrients move down into the soil horizon and are precipitated as unavailable salts.
• Catabolism: enzymes of bacteria and fungi break detritus down to simpler inorganic compounds. These three steps run at the same time on the detritus.
• Humification forms humus, a dark, amorphous substance that resists microbes and decomposes very slowly. Being colloidal, it acts as a nutrient reservoir.
• Mineralisation: some microbes degrade the humus further and release inorganic nutrients.
• Decomposition mostly needs oxygen. It is slower when detritus is rich in lignin and chitin, and faster when it is rich in nitrogen and water-soluble substances such as sugars.
• Of the climatic factors, temperature and soil moisture matter most, acting through soil microbes. Warm, moist conditions speed it up; low temperature and anaerobiosis slow it, so organic matter builds up.

6. Energy flow

NCERT §12.4

• Apart from deep-sea hydrothermal ecosystems, every ecosystem on Earth runs on energy from the sun.
• Less than 50 per cent of the incident solar radiation is photosynthetically active radiation (PAR).
• Of this PAR, plants trap just 2-10 per cent, yet that small share keeps the whole living world going.
• All organisms depend on producers for food, directly or indirectly, so energy flows one way: from the sun to producers and then to consumers.
• Ecosystems obey the second law of thermodynamics. They need a constant energy supply to build the molecules they need and to counter the universal trend towards disorder.
• Producers in terrestrial ecosystems are mainly herbaceous and woody plants; in aquatic ecosystems they are phytoplankton, algae and higher plants.
• Energy trapped in an organism does not stay there for ever. It passes to a consumer, or the organism dies, and death starts the detritus food chain.

7. Food chains and food webs

NCERT §12.4

• Consumers (heterotrophs) depend on plants directly or indirectly for food.
• Primary consumers eat producers, so they are herbivores. Examples: insects, birds and mammals on land; molluscs in water.
• Secondary consumers eat herbivores; they are primary carnivores. Animals that eat primary carnivores are secondary carnivores (tertiary consumers).
• A simple grazing food chain (GFC): grass (producer) → goat (primary consumer) → man (secondary consumer).
• The detritus food chain (DFC) starts from dead organic material. Its decomposers, mainly fungi and bacteria, are heterotrophs also called saprotrophs.
• Decomposers secrete digestive enzymes that break dead and waste materials into simple inorganic materials, which they then absorb.
• In aquatic ecosystems the GFC is the main channel of energy flow. On land, a much larger share of energy flows through the DFC than through the GFC.
• The DFC links with the GFC at some levels: some DFC organisms are eaten by GFC animals, and some animals, such as cockroaches and crows, are omnivores. These links make a food web.

8. Trophic levels and the 10 per cent law

NCERT §12.4

• An organism's trophic level is its place in the food chain, based on its source of food.
• Producers form the first trophic level, herbivores (primary consumers) the second and carnivores (secondary consumers) the third.
• The amount of energy decreases at each successive trophic level.
• When any organism dies it becomes detritus, dead biomass that is an energy source for decomposers.
• The standing crop is the mass of living material at a trophic level at a given time, measured as biomass or as number per unit area.
• Biomass is given as fresh or dry weight; dry weight is the more accurate measure.
• The 10 per cent law: each trophic level receives just a tenth of the energy held by the level below it.
• This limits the number of levels in a grazing food chain: producer, herbivore, primary carnivore, secondary carnivore.

9. Ecological pyramids

NCERT §12.5

• Food or energy relationships between trophic levels, drawn as bars, give a pyramid shape: broad at the base, narrow at the apex.
• The base stands for producers (first trophic level) and the apex for the top consumers.
• Three kinds are usually studied: the pyramid of number, of biomass and of energy.
• In a grassland, the pyramid of number shows only three top carnivores supported by the production of nearly 6 million plants.
• The pyramid of biomass usually falls sharply at higher trophic levels.
• In most ecosystems all three pyramids are upright: producers outnumber and outweigh herbivores, which outnumber and outweigh carnivores.
• Exceptions: counting the insects on one big tree, and then the small and larger birds that depend on them, gives an inverted pyramid of number.
• The pyramid of biomass in the sea is generally inverted: a small standing crop of phytoplankton supports a much larger biomass of zooplankton and fishes.
• The pyramid of energy is always upright and can never be inverted, because some energy is lost as heat at every transfer. Producers convert only about 1 per cent of the sunlight available to them into NPP.

10. Limits of trophic levels and pyramids

NCERT §12.5

• Any calculation of energy, biomass or number must include all the organisms at a trophic level; a few individuals cannot support a generalisation.
• A trophic level describes a role, not a species. A single species can work at two or more trophic levels in one ecosystem at once.
• Feeding on seeds, fruits and peas, a sparrow acts as a primary consumer; feeding on insects and worms, it acts as a secondary consumer.
• Ecological pyramids do not account for a species that belongs to two or more trophic levels.
• They assume a simple food chain, which almost never exists in nature, and cannot show a food web.
• Saprophytes are left out of ecological pyramids altogether, despite their vital role in the ecosystem.

Must-know facts

1. An ecosystem is a functional unit of nature: organisms interacting with each other and with their physical environment.
2. Terrestrial: forest, grassland, desert. Aquatic: pond, lake, wetland, river, estuary. Man-made: crop field, aquarium.
3. Structural features: species composition and stratification (trees on top, then shrubs, then herbs and grasses).
4. Four functional aspects: productivity, decomposition, energy flow, nutrient cycling.
5. Pond: phytoplankton and plants produce; zooplankton consume; fungi, bacteria and flagellates decompose.
6. Productivity units: g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹.
7. GPP − R = NPP; NPP is what heterotrophs can use. Secondary productivity is the rate of new organic matter formed by consumers.
8. Biosphere NPP about 170 billion tons dry weight a year; oceans, about 70 per cent of the surface, give only 55 billion tons.
9. Decomposition steps: fragmentation, leaching, catabolism, humification, mineralisation.
10. Lignin and chitin slow decomposition; nitrogen and sugars speed it; warm and moist favour it, cold and anaerobiosis inhibit it.
11. The sun powers all ecosystems except deep-sea hydrothermal ones.
12. PAR is under 50 per cent of incident sunlight; plants capture 2-10 per cent of PAR.
13. Energy flow is unidirectional: sun → producers → consumers.
14. Grazing food chain: grass → goat → man. Detritus food chain starts with dead organic matter.
15. Aquatic: GFC is the main energy channel. Terrestrial: more energy flows through the DFC.
16. 10 per cent law: only 10 per cent of energy passes to the next trophic level.
17. Standing crop: living biomass or number per unit area at a trophic level; dry weight is more accurate.
18. Pyramid of number inverted for a single tree; pyramid of biomass inverted in the sea; pyramid of energy always upright.
19. A sparrow is a primary consumer eating seeds and a secondary consumer eating insects.
20. Pyramids ignore food webs, species at two levels, and saprophytes.

Common traps

Key terms

Ecosystem

A functional unit of nature where organisms interact with each other and their physical surroundings.

Stratification

Vertical layering of species at different levels, such as trees, shrubs and herbs in a forest.

Species composition

The identity and number of plant and animal species in an ecosystem.

Primary production

Biomass made per unit area over a period by plants in photosynthesis.

Gross primary productivity (GPP)

Rate of production of organic matter in photosynthesis.

Net primary productivity (NPP)

GPP minus respiration losses; the biomass available to heterotrophs.

Secondary productivity

Rate of formation of new organic matter by consumers.

Detritus

Dead plant and animal remains, including fecal matter; the raw material of decomposition.

Detritivore

An organism such as the earthworm that breaks detritus into smaller pieces.

Humus

Dark, amorphous, colloidal matter that resists decay and stores nutrients.

Mineralisation

Release of inorganic nutrients as microbes break down humus.

PAR

Photosynthetically active radiation, under 50 per cent of incident sunlight.

Saprotroph

A decomposer that feeds by breaking down dead organic matter.

Food web

Interconnected food chains, including links between grazing and detritus chains.

Trophic level

An organism's place in a food chain based on its source of food.

Standing crop

Mass or number of living organisms at a trophic level at a given time.

10 per cent law

Only about 10 per cent of energy passes from one trophic level to the next.

Ecological pyramid

Bars showing number, biomass or energy at successive trophic levels.