Ecosystem

NCERT defines an ecosystem as a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment. The two words that the examiner looks for are functional unit: an ecosystem is not a place but a working system in which biotic and abiotic components exchange energy and matter. Ecosystems vary greatly in size, from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as one global ecosystem, but since that is too big and complex to study at one time, it is convenient to divide it into two basic categories, terrestrial and aquatic. Forest, grassland and desert are examples of terrestrial ecosystems; pond, lake, wetland, river and estuary are examples of aquatic ecosystems. Crop fields and an aquarium are man-made ecosystems. Keep these lists ready: a 1-mark question often asks for one natural and one artificial ecosystem.

The chapter is organised around four processes, and every question in the paper maps onto one of them: productivity (how fast organic matter is made), decomposition (how dead matter is broken back down), energy flow (how energy moves one way through food chains) and nutrient cycling (how matter goes round and round). The rationalised NCERT chapter (2023 onwards) describes the first three in detail and only names the fourth.

A word on the syllabus, because most question banks and old papers were written for the earlier, longer chapter. The rationalised chapter has five sections: 12.1 Ecosystem: Structure and Function, 12.2 Productivity, 12.3 Decomposition, 12.4 Energy Flow and 12.5 Ecological Pyramids. The earlier sections on ecological succession (hydrarch, xerarch, pioneer species, climax community), nutrient cycling (the carbon and phosphorus cycles, gaseous and sedimentary cycles, reservoirs) and ecosystem services (Robert Costanza's estimate) have been deleted and are not asked in the board paper. If you meet those questions in an old paper, skip them. Everything that remains is asked, and asked often, so the chapter rewards exact NCERT wording more than any other chapter of the ecology unit.

The interaction of the biotic and abiotic components gives an ecosystem a physical structure, and NCERT names two things that describe that structure. First, species composition: the identification and enumeration of the plant and animal species of an ecosystem. Second, stratification: the vertical distribution of different species occupying different levels. In a forest, trees occupy the top vertical strata or layer, shrubs the second, and herbs and grasses occupy the bottom layer. That one sentence is the complete answer to the 2-mark question 'How is stratification represented in a forest ecosystem?' (asked in 2014).

The components of the ecosystem are seen as a functional unit when you consider the four functional aspects: (i) productivity, (ii) decomposition, (iii) energy flow and (iv) nutrient cycling. Learn them as a list; a 1-mark question asks for the four and a 5-mark question asks you to explain them.

NCERT then uses a pond as the model of a small, shallow, self-sustainable ecosystem, and this is the example the board expects (asked as 'taking the example of a small pond, explain how the four components function as an ecosystem', 2016). The abiotic component is the water with all the dissolved inorganic and organic substances and the rich soil deposit at the bottom of the pond. The solar input, the cycle of temperature, day length and other climatic conditions regulate the rate of functioning of the whole pond. The autotrophic (producer) component is the phytoplankton, some algae and the floating, submerged and marginal plants found at the edges. The consumers are the zooplankton, the free-swimming and the bottom-dwelling forms. The decomposers are the fungi, bacteria and flagellates, especially abundant in the bottom of the pond.

The pond performs all the functions of any ecosystem and of the biosphere as a whole: (1) conversion of inorganic into organic material with the help of the radiant energy of the sun by the autotrophs; (2) consumption of the autotrophs by the heterotrophs; (3) decomposition and mineralisation of the dead matter to release them back for reuse by the autotrophs; and (4) unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment. When you answer the pond question, write these four numbered points; they are the marking scheme.

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is called productivity, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ so that the productivity of different ecosystems can be compared. Note the distinction the examiner wants: production is an amount, productivity is a rate.

Productivity can be divided into gross primary productivity (GPP) and net primary productivity (NPP). GPP of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R) is the net primary productivity: GPP − R = NPP. NPP is the available biomass for the consumption of heterotrophs (herbivores and decomposers). This relation is asked almost every year in some form: 'What does R represent?' (respiration, 2014), 'Write the equation for NPP' (2013), 'Difference between GPP and NPP' (2011), and 'Write the relationship between productivity, GPP, NPP and secondary productivity' (2019, 3 marks).

Secondary productivity is defined as the rate of formation of new organic matter by consumers. The word 'consumers' is the key: it is the rate at which heterotrophs build their own biomass from what they eat.

Primary productivity depends on the plant species inhabiting a particular area, on a variety of environmental factors, on the availability of nutrients and on the photosynthetic capacity of the plants. Therefore it varies in different types of ecosystems (the 2-mark 'two reasons why primary productivity varies' question). Two numbers must be memorised. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans is only 55 billion tons. The rest comes from land. When asked why the oceans produce so little for their area, reason from the factors above: light penetrates only the surface layers, and nutrients are limited in open water. The same reasoning answers 'Why are green algae not likely to be found in the deepest strata of the ocean?' (2013): light does not reach there, and green algae need light for photosynthesis.

NCERT opens with the earthworm: you may have heard that earthworms are the farmer's friend, because they help in the breakdown of complex organic matter as well as in loosening the soil. Similarly, decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, and the process is called decomposition. Dead plant remains such as leaves, bark and flowers, and the dead remains of animals including faecal matter, constitute detritus, which is the raw material for decomposition.

The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation. Learn the five in order and be ready to write one line on each; this is a standard 3-mark question and the core of the 5-mark 'define decomposition and describe its processes and products'.

1. Fragmentation: detritivores (e.g. earthworms) break down detritus into smaller particles.
2. Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances. It is important to note that fragmentation, leaching and catabolism operate simultaneously on the detritus.
4. Humification: leads to the accumulation of a dark coloured amorphous substance called humus, which is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.
5. Mineralisation: the humus is further degraded by some microbes, and the release of inorganic nutrients occurs by this process.

Two distinctions come up in 2-mark questions. A detritivore (earthworm) breaks detritus into smaller pieces by feeding on it, which is physical fragmentation; a decomposer (bacteria, fungi) secretes enzymes and chemically degrades detritus into inorganic substances. And litter is the dead plant material lying on the soil surface (above ground), while detritus is all dead organic matter, above and below ground, including animal remains and faecal matter.

Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by the chemical composition of detritus and by climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker if detritus is rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favours decomposition, whereas low temperature and anaerobiosis inhibit decomposition, resulting in a build-up of organic materials. These three controls (oxygen, chemical composition, climate) are the 3-mark question 'describe how oxygen and the chemical composition of detritus control decomposition' (2011).

Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. That exception is a favourite 1-mark item. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria (the autotrophs) fix the sun's radiant energy to make food from simple inorganic materials. Plants capture only 2 to 10 per cent of the PAR, and this small amount of energy sustains the entire living world. So when the board asks 'what percentage of PAR is captured by plants', the answer is 2 to 10 per cent; when it asks 'what fraction of incident solar radiation is PAR', the answer is less than 50 per cent. Do not mix the two.

Energy flow is unidirectional: from the sun to the producers and then to the consumers. All organisms are dependent for their food on producers, either directly or indirectly. Ecosystems are not exempt from the laws of thermodynamics. They obey the First law (energy is neither created nor destroyed, only converted from one form to another) and the Second law (in every transfer some energy is degraded to heat and lost). Ecosystems therefore need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness. This is why energy, unlike nutrients, cannot be recycled and must keep entering the system.

The number of trophic levels in the grazing food chain is restricted because the transfer of energy follows the 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. The rest is used in the metabolism of the organisms at that level, lost as heat, or passes to the detritus food chain as unconsumed or undigested matter. With 10,000 units of energy in the producers, the herbivores receive about 1,000, the primary carnivores about 100 and the secondary carnivores about 10; after four or five levels there is too little energy left to support another. That arithmetic is exactly what the pyramid of energy draws.

Decomposers sit outside the linear chain but are part of the energy flow. Whatever is not eaten at any trophic level, plus everything that dies at every level, becomes detritus and its energy is released by the decomposers, again with heat loss. So the full energy-flow diagram has the sun on the left, producers, primary consumers, secondary consumers and tertiary consumers in a line, an arrow from every box down to decomposers, and a wavy arrow of heat leaving every box. A 5-mark 'give an account of energy flow' answer needs the numbers, the law, the diagram and the thermodynamics sentence.

Producers are the green plants (and photosynthetic bacteria). Animals cannot make food and depend on plants directly or indirectly; they are consumers or heterotrophs. Those that feed on producers are primary consumers (herbivores); those that eat herbivores are secondary consumers (primary carnivores); those that eat the primary carnivores are tertiary consumers (secondary carnivores). NCERT gives two grazing food chains that you should quote: Grass → Goat → Man on land, and Phytoplankton → Zooplankton → Fish in water. A food chain that starts with a living plant is the grazing food chain (GFC).

The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers, which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading dead organic matter or detritus, and are also known as saprotrophs (sapro: to decompose). Decomposers secrete digestive enzymes that break down dead and waste materials into simple inorganic materials, which are subsequently absorbed by them. The comparison the board loves: in an aquatic ecosystem the GFC is the major conduit for energy flow, whereas in a terrestrial ecosystem a much larger fraction of energy flows through the DFC than through the GFC ('Name the type of food chain responsible for the flow of a larger fraction of energy in a terrestrial ecosystem', 2010: the detritus food chain).

The DFC may be connected with the GFC at some levels: some of the organisms of the DFC are prey to the GFC animals, and in a natural ecosystem some animals like cockroaches and crows are omnivores. These natural interconnections of food chains make a food web. A food chain is a single linear sequence; a food web is the network of interconnected chains, closer to what really happens.

Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain known as their trophic level. Producers belong to the first trophic level, herbivores (primary consumers) to the second and carnivores (secondary consumers) to the third. The important point, asked in 2013 and 2016, is that a given organism may occupy more than one trophic level simultaneously. A sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms. Hence a trophic level represents a functional level, not a species as such.

Each trophic level has a certain mass of living material at a particular time called the standing crop. The standing crop is measured as the biomass of living organisms (biomass in a unit area) or as the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight, and measurement in dry weight is more accurate, because water content varies. 'State what the standing crop of a trophic level represents' (2013) and 'List two ways of measuring standing crop' (2010) are both 2-mark questions answered by this paragraph.

You may have seen a pyramid of numbers in a tribe or an army: the base is broad and it narrows at the top. The same shape describes trophic structure. An ecological pyramid is the graphic representation of the relationship between different trophic levels of an ecosystem, in which the base represents the producers or the first trophic level and the apex represents the tertiary or top-level consumers. Each bar's length (or area) is proportional to the quantity at that level. The relationship can be expressed in terms of number, biomass or energy, giving the three pyramids.

The pyramid of numbers in a grassland ecosystem is upright: an enormous number of grass plants support fewer grasshoppers, fewer still lizards or birds, and only a handful of top carnivores. The pyramid of biomass in a grassland or a forest is also upright, because the producers weigh more than the herbivores and the herbivores weigh more than the carnivores. The pyramid of energy is upright everywhere, because energy at a lower trophic level is always more than at a higher level.

In most ecosystems, therefore, all three pyramids are upright. The exceptions are what the examiner asks. The pyramid of biomass in the sea is generally inverted, because the biomass of fishes far exceeds that of phytoplankton. The phytoplankton have a small standing crop but a very high rate of reproduction and are eaten almost as fast as they are produced, so a small biomass of producers supports a larger biomass of consumers at any one moment. The pyramid of numbers in a tree-dominated ecosystem is inverted: a single large tree supports many fruit-eating birds and a far larger number of insects. NCERT's exercise fill-in-the-blank uses this exact case. The pyramid of energy, however, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step. The 3-mark 'why is the pyramid of energy always upright' (2013, 2015, 2019) wants exactly that sentence plus a labelled upright pyramid.

Ecological pyramids have limitations, and these are a standard 3-mark list. (1) They do not take into account the same species belonging to two or more trophic levels. (2) They assume a simple food chain, something that almost never exists in nature; they do not accommodate a food web. (3) Saprophytes (decomposers) are not given any place in ecological pyramids, even though they play a vital role in the ecosystem. Write all three; the answer is incomplete without the decomposers point.

When you draw pyramids, always draw four bars, label the trophic levels (producers, primary consumers, secondary consumers, tertiary consumers) on the left and write the unit of the quantity (number per unit area, g m⁻², or kJ m⁻² yr⁻¹) below. An upright pyramid gets narrower going up; an inverted one gets wider. Unlabelled pyramids lose a mark every time.