NCERT Solutions for Class 12 Biology Chapter 12: Ecosystem

Each of these five blanks tests a core concept from the structure, productivity, and nutrient-cycling framework of this chapter. A full, descriptive answer to each blank, explaining the underlying ecological reasoning rather than just naming the missing word, is given below.

(a) Producers (autotrophs). Plants, along with photosynthetic bacteria, are called producers because they are the only organisms in an ecosystem capable of fixing atmospheric carbon dioxide into organic (carbon-containing) compounds, using the radiant energy of the sun during photosynthesis. This single ability is what makes them the foundation of every food chain and food web: every other organism in the ecosystem — every herbivore, carnivore, omnivore and decomposer — ultimately depends, directly or indirectly, on the organic matter first synthesised by producers. In a terrestrial ecosystem the major producers are herbaceous and woody plants; in an aquatic ecosystem, producers are phytoplankton, algae and various higher plants. Because they occupy this foundational position, producers are always placed at the first trophic level of every ecological pyramid.

(b) Inverted. In a typical grassland or herb-dominated ecosystem, the pyramid of numbers is upright — a very large number of small plants supports progressively fewer herbivores and still fewer carnivores. But in an ecosystem dominated by trees, this relationship flips at the base: a single large tree, though it counts as just one producer ‘individual’, is physically massive enough to support a very large number of insects feeding on its leaves, bark, sap and fruit. Those insects, in turn, support a smaller (but still substantial) number of insect-eating birds, and those birds support an even smaller number of larger predatory birds. The result is a pyramid that starts narrow at the base (few producer individuals) and widens before narrowing again towards the top — making the pyramid of numbers for a tree-dominated ecosystem an inverted (or partially inverted/spindle-shaped) type, the classic exception NCERT itself highlights to the usual ‘upright pyramid’ generalisation.

(c) Light (light penetration into the water column). On land, productivity is most often limited by water, nutrients or temperature, but in aquatic ecosystems the single most important limiting factor is usually the availability of light. Sunlight entering a water body is progressively absorbed and scattered as depth increases, so photosynthesis by phytoplankton and submerged plants can only occur efficiently in a shallow, sunlit surface zone (the photic zone); below this depth, there is simply not enough light energy to drive photosynthesis, regardless of how abundant nutrients or carbon dioxide might be. This is also the underlying reason the chapter asks you to ‘discuss the main reason for the low productivity of oceans’ with your teacher — despite covering about 70% of the Earth’s surface, oceans contribute only about 55 of the biosphere’s 170 billion tons of annual net primary productivity, precisely because only their thin, sunlit surface layer is productive, while the vast, deep, dark majority of ocean volume contributes almost nothing.

(d) Earthworms (along with other detritivores such as termites, millipedes, woodlice and certain soil mites and beetles). The chapter specifically singles out the earthworm, describing it as the farmer’s ‘friend’, because it performs two simultaneous services: it physically breaks down (fragments) complex dead organic matter — fallen leaves, bark, dead roots and other detritus — into much smaller particles with a correspondingly larger surface area (which in turn speeds up further microbial decomposition), and it simultaneously loosens and aerates the soil as it burrows, improving soil structure, drainage and root penetration for growing plants. This fragmentation step is the very first of the five sequential steps of decomposition (fragmentation, leaching, catabolism, humification, mineralisation), making detritivores like the earthworm absolutely essential gatekeepers for the entire decomposition process to proceed.

(e) Atmosphere and hydrosphere (oceans). Nutrient cycling in an ecosystem is of two broad types: gaseous cycles, whose reservoir is the atmosphere or hydrosphere, and sedimentary cycles, whose reservoir is the Earth’s crust. Carbon follows a gaseous-type cycle: it exists as carbon dioxide gas in the atmosphere and as dissolved carbon (bicarbonate/carbonate) in the oceans in enormous quantities, both of which are continuously exchanged with living organisms through photosynthesis (carbon fixed into organic matter) and respiration/decomposition (carbon released back as CO₂). Because these two reservoirs together hold by far the largest readily-cycling store of carbon on the planet, the atmosphere and hydrosphere together are considered carbon’s major reservoir — in direct contrast to an element like phosphorus, which follows a sedimentary cycle with the Earth’s crust (rocks and sediments) as its principal reservoir instead.

Conclusion: together these five blanks revise five distinct pillars of this chapter — the defining role of producers, the exception-proving-the-rule inverted pyramid of a tree-dominated ecosystem, light as the key limiting factor underwater, earthworms as the classic detritivore beginning decomposition, and the atmosphere/hydrosphere as carbon’s gaseous-cycle reservoir.

Correct option: (a) Producers

To understand why, it helps to revisit exactly how energy and numbers move through a food chain. In the overwhelming majority of ecosystems studied, the pyramid of numbers is upright — a broad base of producers narrowing progressively towards the apex, exactly like the shape of a physical pyramid. This shape arises directly from the 10% law of energy transfer: at every step from one trophic level to the next, only about 10% of the energy present at the lower level is actually transferred and made available to the level above it; the remaining roughly 90% is lost as heat through respiration, in the course of digestion, and in the incomplete consumption of biomass. Since each trophic level can only sustain a population whose total energy demand is met by the (shrinking) energy supply available to it, each successive level is forced to support dramatically fewer individuals (or, at minimum, far less standing biomass) than the level below it.

The chapter grounds this abstract rule in a vivid, concrete example from a grassland ecosystem: an estimated nearly 6 million individual plants at the producer level are needed, cumulatively, to generate just enough energy — after being diminished at every successive 10%-transfer step through herbivores and then carnivores — to ultimately support only three top carnivores at the very apex of that food chain’s pyramid of numbers. This dramatic numerical collapse, from millions down to single digits across just three or four trophic transfers, is the clearest possible illustration of why producers, sitting at the very base where all of the ecosystem’s primary energy capture happens, must necessarily have, by a very wide margin, the largest population of any trophic level.

Conclusion: producers have, by a wide margin, the largest population in any food chain, because the upright pyramid of numbers and the 10% law of energy transfer together mean that each higher trophic level can only ever support a small fraction of the individuals sustained by the level immediately beneath it.

Correct option: (b) Zooplankton

Trophic levels are assigned strictly according to an organism’s source of nutrition, not according to its taxonomic identity or where it physically lives. By this rule, every producer in an ecosystem, regardless of what kind of organism it is, is always placed at the first trophic level, because it is the point at which solar energy first enters the biological system via photosynthesis. In a lake, the autotrophic producers are the phytoplankton — microscopic, free-floating, photosynthesising algae and cyanobacteria suspended in the sunlit surface waters. Because they are producers, phytoplankton occupy the first trophic level of the lake’s food chain, not the second, which immediately rules out option (a).

The organisms that feed directly on these phytoplankton — consuming them as their primary and direct food source — are the lake’s primary consumers, and by definition, primary consumers always occupy the second trophic level, precisely one step above the producers they depend on. In a lake’s plankton community, this grazing role is filled by the zooplankton — small, often microscopic, free-swimming or floating animals (such as small crustaceans like copepods and cladocerans, and the larval stages of many aquatic animals) that filter-feed or actively graze on the suspended phytoplankton. This phytoplankton–zooplankton relationship in a lake’s open water is the direct aquatic analogue of the grass–herbivore relationship at the start of a terrestrial grazing food chain.

Conclusion: zooplankton occupy the second trophic level in a lake because, as the primary consumers that graze directly on the phytoplankton producers occupying the first trophic level, they are the aquatic equivalent of herbivores in a terrestrial grazing food chain.

Correct option: (d) None of the above

This question is a deliberate test of precise terminology, and answering it well requires carefully distinguishing between two superficially similar but conceptually very different terms that this chapter introduces: ‘secondary productivity’ (a real, well-defined ecological rate) and ‘secondary producers’ (a phrase that does not correspond to any organism category recognised in this chapter, or in ecology more broadly).

The chapter explicitly defines secondary productivity as “the rate of formation of new organic matter by consumers”. Note carefully who this rate applies to: it describes how fast consumers — organisms that eat other organisms rather than photosynthesising — build new biomass by assimilating the food energy they ingest. It is a rate, a quantitative measure of biomass formation, not a label for a type of organism.

By contrast, the entire organism-classification framework of this chapter recognises only two fundamental categories at the base of any food chain analysis: producers — the autotrophic plants, algae and photosynthetic bacteria that are the sole organisms capable of fixing solar energy into organic matter in the first place — and consumers, every heterotrophic organism that depends (directly or indirectly) on producers for its energy, further subdivided by their position in the food chain into primary consumers (herbivores, which eat producers directly), secondary consumers (primary carnivores, which eat primary consumers), and so on up to tertiary consumers. There is no third parallel category of ‘secondary producers’ anywhere in this scheme. Herbivores and carnivores are both explicitly types of consumers, not producers of any kind, ‘secondary’ or otherwise; and the organisms that genuinely are producers are simply called ‘producers’, full stop, with no ‘primary/secondary’ subdivision applied to them at all in this framework. Since none of options (a) herbivores, (b) producers, or (c) carnivores can correctly be re-labelled ‘secondary producers’ under this terminology, the only logically correct choice is (d).

Conclusion: the correct answer is (d) None of the above, because this chapter’s terminology recognises only producers and consumers (primary, secondary, tertiary) as organism categories, with ‘secondary productivity’ referring only to a rate of biomass formation by consumers — ‘secondary producers’ is not a category that exists in this framework at all.

Correct option: (b) 50%

This question hinges on carefully separating two different percentages that this chapter introduces back-to-back, in the very same discussion of how solar energy enters an ecosystem, and which are very easy to conflate if read quickly.

The chapter states precisely: “Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR).” This sentence is describing the first of two successive energy ‘filters’ that sunlight passes through before it can be converted into chemical energy by a plant. Not all wavelengths of sunlight reaching the Earth are usable by the photosynthetic pigments (like chlorophyll) in plant cells — only a specific band of wavelengths, roughly corresponding to visible light, is biologically usable for photosynthesis, and this usable band is called Photosynthetically Active Radiation (PAR). Of all the solar radiation that actually reaches the Earth’s surface (across the full spectrum, including wavelengths plants cannot use at all), only under 50% falls within this usable PAR band — so PAR constitutes approximately half of total incident solar radiation.

The chapter then introduces an entirely separate, second percentage in the next sentence: “Plants capture only 2–10 per cent of the PAR” — and this small percentage is explicitly described as sustaining “the entire living world”. This second figure describes the second filter: of the PAR that is already available and usable in principle, plants, limited by factors like leaf area, chlorophyll content, and the efficiency of the photosynthetic machinery itself, are only able to actually absorb and convert a very small fraction — just 2 to 10 percent — of that available PAR into stored chemical (food) energy. The remainder of the PAR striking a leaf is reflected, transmitted through, or lost as heat, without ever being captured as useful biological energy.

So the correct sequence of these two numbers, applied one after another, is: total incident sunlight → (less than 50% of it is PAR) → (plants capture only 2–10% of that PAR). The question specifically asks for the first of these two percentages — ‘the percentage of PAR in the incident solar radiation’ — which is therefore 50%, making option (d), 2–10%, an incorrect answer to this specific question, even though 2–10% is itself a genuine, correctly-remembered figure from the very same paragraph (just answering a different question — how much PAR plants capture, not how much of sunlight is PAR).

Conclusion: the correct answer is (b) 50%, since photosynthetically active radiation constitutes just under half of all incident solar radiation reaching Earth, while the separate figure of 2–10% describes only how much of that already-filtered PAR plants are actually able to capture and convert into usable chemical energy during photosynthesis.

(a) Grazing food chain (GFC) vs Detritus food chain (DFC). A grazing food chain begins with living green plants (producers) at its base, and energy flows from them to herbivores (primary consumers), then to primary carnivores (secondary consumers), and potentially on to secondary carnivores (tertiary consumers) — the classic sequence Grass → Goat → Man. A detritus food chain, in sharp contrast, begins not with living plants but with dead organic matter (detritus); it is driven by decomposers — mainly fungi and bacteria, also called saprotrophs — which secrete digestive enzymes onto dead and waste material, breaking it down into simple inorganic substances that they then absorb. Ecologically, their relative importance differs sharply by ecosystem type: in an aquatic ecosystem, the GFC is the major conduit through which energy flows; but in a terrestrial ecosystem, a much larger fraction of total energy flow actually passes through the DFC rather than the GFC. The two chains are not isolated from each other either — some organisms of the DFC become prey for GFC animals, and omnivores such as cockroaches and crows straddle both, interconnecting the two chains into a single food web.

(b) Production vs Decomposition. Production is the building-up process of an ecosystem: it is carried out exclusively by producers (autotrophs), which use solar energy during photosynthesis to synthesise complex organic matter (biomass) from simple inorganic raw materials such as carbon dioxide and water, measured as primary productivity (GPP, NPP). Decomposition is the opposite, breaking-down process: it is carried out by decomposers acting on detritus (dead organic matter from any trophic level), converting complex organic compounds back into simple inorganic substances — carbon dioxide, water and mineral nutrients — via the sequential steps of fragmentation, leaching, catabolism, humification and mineralisation. In short, production synthesises organic matter from inorganic raw materials; decomposition does precisely the reverse, regenerating inorganic nutrients from organic matter so that producers can use them again — the two processes together complete the cycle of matter in an ecosystem.

(c) Upright pyramid vs Inverted pyramid. An upright pyramid has a broad base that narrows progressively towards its apex — mirroring the shape of a physical pyramid — and represents the typical, most common situation in nature where producers are more numerous/have greater biomass than herbivores, who in turn exceed carnivores; this is the usual shape for pyramids of number and biomass in most terrestrial ecosystems. An inverted pyramid shows the opposite pattern — a narrow base widening towards the top — and occurs in specific exceptional situations: the pyramid of biomass in the sea is typically inverted because the small standing crop (biomass present at any instant) of phytoplankton, despite its very rapid turnover/reproduction rate, supports a much larger standing crop of zooplankton; similarly, the pyramid of numbers in a tree-dominated ecosystem can be inverted because one large tree (a single producer individual) supports a great many insects. Notably, the pyramid of energy can never be inverted, under any circumstances, because energy is always lost as heat at every single transfer between trophic levels, guaranteeing that energy content must always decrease at each higher level.

(d) Food chain vs Food web. A food chain is a single, linear sequence of organisms, each feeding on the one before it and in turn being fed upon by the next — a simple, straight-line pathway such as Grass → Goat → Man. A food web, by contrast, is the much more realistic, complex network formed when multiple interconnected food chains overlap within the same community — arising naturally because many organisms do not restrict themselves to a single food source or a single predator, because the grazing food chain and detritus food chain interconnect at various points, and because omnivorous species (which feed at more than one trophic level) create additional cross-links between otherwise separate chains. A simple, isolated food chain essentially never exists in nature in practice; what actually operates in any real ecosystem is always a food web.

(e) Litter vs Detritus. Litter specifically refers to the fallen, dead plant material that accumulates on or near the ground — principally dead leaves, bark, twigs and flowers shed by plants. Detritus is the broader, more inclusive term for all the raw material that decomposition acts upon: it explicitly includes litter (dead plant remains) plus dead remains of animals and faecal matter from any organism. In other words, litter is essentially the plant-derived subset of the full detritus pool, while detritus as a whole draws from both plant and animal sources of dead/waste organic matter.

(f) Primary productivity vs Secondary productivity. Primary productivity is the rate at which producers (autotrophs) generate new biomass/organic matter through photosynthesis — it is further split into Gross Primary Productivity (GPP, the total rate of organic matter production) and Net Primary Productivity (NPP = GPP – respiration losses, the biomass actually available to heterotrophs). Secondary productivity, by contrast, is the rate at which consumers (heterotrophs — herbivores, carnivores, omnivores) form new organic matter, by assimilating and converting the food energy they obtain by consuming producers or other consumers. The key distinction is simply who is generating the new biomass: primary productivity belongs to producers; secondary productivity belongs to consumers.

Conclusion: these six pairs of terms cover the structural backbone of ecosystem ecology — how energy enters via living vs dead matter (GFC/DFC), how matter is built up and broken down (production/decomposition), how trophic relationships are shaped (upright/inverted pyramids, chain/web), and how dead organic matter is categorised and quantified (litter/detritus, primary/secondary productivity) — and a precise, example-grounded distinction for each is essential for full marks.

An ecosystem is a functional unit of nature in which living organisms interact among themselves and with the surrounding physical environment. Its components fall into two broad structural categories — abiotic and biotic — and are further understood through two structural features and four functional processes.

1. Abiotic components. These are the non-living, physico-chemical factors of the environment — inorganic materials such as air, water and soil, along with climatic conditions such as solar radiation input, temperature, day-length (photoperiod) and other weather factors that regulate the overall rate of functioning of the ecosystem. In the chapter’s pond example, the abiotic component is the water itself, together with all the dissolved inorganic and organic substances in it and the rich soil deposit at the bottom of the pond.
2. Biotic components, which are themselves divided into three functional groups:
   • Producers — autotrophic organisms (green plants, photosynthetic bacteria) that fix solar energy and convert inorganic raw material into organic matter. In the pond example: phytoplankton, some algae, and floating, submerged and marginal plants.
   • Consumers — heterotrophic organisms that depend on producers (directly or indirectly) for food; classified as primary, secondary and tertiary consumers depending on their trophic position. In the pond example: zooplankton, free-swimming and bottom-dwelling animal forms.
   • Decomposers — heterotrophic organisms (mainly fungi and bacteria, also called saprotrophs) that break down dead organic matter into simple inorganic substances. In the pond example: fungi, bacteria and flagellates, especially abundant at the bottom of the pond.
3. Structural features. The interaction of biotic and abiotic components produces a characteristic physical structure for each ecosystem type, described by two features: species composition (the identification and enumeration/count of the plant and animal species present) and stratification (the vertical distribution of different species occupying different height/depth levels — e.g., in a forest, trees occupy the top vertical stratum, shrubs the second, and herbs/grasses the bottom layer).
4. Functional processes. The components function together as a unit through four key processes: (i) Productivity (rate of biomass generation by producers), (ii) Decomposition (breakdown of dead organic matter into inorganic nutrients), (iii) Energy flow (unidirectional movement of energy from the sun through producers to successive consumer levels, with dissipation as heat), and (iv) Nutrient cycling (storage and repeated movement of nutrient elements through the ecosystem’s abiotic and biotic components).

Conclusion: an ecosystem’s components comprise the abiotic physico-chemical environment and three biotic functional groups — producers, consumers and decomposers — organised structurally through species composition and stratification, and unified functionally through productivity, decomposition, energy flow and nutrient cycling, exactly as demonstrated by the self-sustaining pond ecosystem example.

An ecological pyramid is a graphical representation of the relationship between organisms at different trophic levels of an ecosystem, expressed in terms of their number, biomass, or energy. In this representation, the base of the pyramid always represents the producers (the first trophic level), while the apex represents the tertiary or top-level consumer — and because the amount available (whether counted as individuals, biomass, or energy) typically decreases at each successive level, the resulting shape usually resembles a true pyramid: broad at the base, narrowing towards the top. There are three types usually studied: (a) pyramid of number, (b) pyramid of biomass, and (c) pyramid of energy.

(a) Pyramid of numbers. This pyramid plots the number of individual organisms present at each trophic level. In a typical grassland ecosystem, this pyramid is dramatically upright: an estimated nearly 6 million plants at the producer level are ultimately needed to support just three top carnivores at the apex — illustrating just how steeply individual numbers fall away at each higher trophic transfer. However, this pyramid is not always upright: in a tree-dominated ecosystem, a single large tree (just one producer ‘individual’) can support a very large number of insects feeding on it, which in turn support fewer birds — giving a pyramid of numbers that is partially or wholly inverted at the base.

(b) Pyramid of biomass. This pyramid plots the total biomass (mass of living material, usually measured as dry weight for accuracy) present at each trophic level, usually expressed per unit area. In most terrestrial ecosystems, this pyramid shows a sharp decrease in biomass at successively higher trophic levels — a large plant biomass supports a much smaller herbivore biomass, which supports a smaller carnivore biomass still, giving the usual upright shape. But in the sea, the pyramid of biomass is typically found to be inverted: the standing crop (biomass present at any given instant) of phytoplankton is small, yet it supports a much larger standing crop of zooplankton. This apparent paradox is explained by the extremely rapid reproduction and turnover rate of phytoplankton — although the instantaneous biomass of phytoplankton is low, they are consumed and replaced so quickly that the total amount of biomass produced and passed on over time is more than sufficient to sustain a much larger standing mass of zooplankton.

Conclusion: ecological pyramids graphically represent the number, biomass or energy relationship across trophic levels with producers at the base; the pyramid of numbers is usually upright (dramatically so in a grassland) but can invert in a tree-dominated ecosystem, while the pyramid of biomass is usually upright on land but is characteristically inverted in the sea, where a small but fast-turnover phytoplankton biomass sustains a larger zooplankton biomass.

Primary productivity is the amount of biomass or organic matter produced per unit area over a given time period by plants during photosynthesis — the very first stage at which solar energy is converted into chemical (food) energy and enters the living world. A constant input of solar energy is the basic requirement for any ecosystem to function and sustain itself at all, which is why primary productivity is considered the foundation of all subsequent energy flow through an ecosystem.

1. Measurement and units. Primary production is expressed either in terms of weight (gm^–2) or energy (kcal m^–2). When this is expressed as a rate — i.e., the biomass produced per unit area per unit time — it is called productivity, expressed as gm^–2yr^–1 or (kcal m^–2)yr^–1, allowing productivity to be compared meaningfully across different ecosystems.
2. Gross vs Net Primary Productivity. Gross Primary Productivity (GPP) is the total rate of production of organic matter during photosynthesis. However, plants themselves use up a considerable portion of this GPP in their own respiration (R). What remains after subtracting these respiratory losses is the Net Primary Productivity (NPP), given by the relationship GPP – R = NPP. NPP is the biologically crucial figure, because it represents the biomass that is actually available for consumption by heterotrophs — herbivores and decomposers — making it the real energy input to the rest of the food chain/web.
3. Global scale. The annual net primary productivity of the entire biosphere is approximately 170 billion tons (dry weight) of organic matter. Strikingly, despite the oceans covering roughly 70% of the Earth’s surface, they contribute only about 55 billion tons of this total — meaning the remaining, much larger share of global NPP is generated on land, which occupies far less surface area.

Factors affecting primary productivity:

1. Plant species composition. Primary productivity depends heavily on which particular plant species inhabit a given area, since different species photosynthesise and allocate biomass at inherently different rates and efficiencies.
2. Environmental factors. A variety of environmental conditions — such as temperature, rainfall/water availability, and climatic regime — directly influence the rate at which photosynthesis and plant growth can proceed in a given area.
3. Nutrient availability. The supply of essential nutrients (nitrogen, phosphorus, and other minerals) in the soil or water strongly constrains how much biomass plants are able to produce, since photosynthesis alone (capturing light and CO₂) is not sufficient without adequate mineral nutrition to build new tissue.
4. Photosynthetic capacity of plants. The inherent biochemical and physiological efficiency with which a plant species is able to convert captured light energy into fixed carbon (its photosynthetic capacity) sets an upper ceiling on its potential productivity, independent of external resource availability.
5. Light availability (particularly relevant in aquatic systems). Because photosynthesis fundamentally requires light, the depth to which sunlight can penetrate is a major limiting factor, especially underwater — this is a central reason why oceans, despite their enormous surface area, show comparatively low overall productivity, since only their thin, sunlit surface layer can support significant photosynthesis.

Conclusion: primary productivity is the rate at which producers generate new biomass through photosynthesis, split into GPP and the heterotroph-available NPP (GPP–R), and it is shaped by plant species composition, environmental conditions, nutrient availability, inherent photosynthetic capacity, and — especially in water — by how deep sunlight can penetrate.

Decomposition is the process by which decomposers break down complex organic matter into inorganic substances such as carbon dioxide, water and mineral nutrients. The raw material for this process is called detritus — dead plant remains (leaves, bark, flowers) and dead remains of animals, including faecal matter. Decomposition is largely an oxygen-requiring (aerobic) process, and it is through decomposition that nutrients locked up in dead organic matter are made available again for producers to absorb and reuse, completing the ecosystem’s nutrient cycle.

The five processes of decomposition (NCERT notes that all of these operate simultaneously on detritus, not strictly one after another):

1. Fragmentation. Detritivores (e.g., the earthworm) physically break down detritus into smaller particles, increasing the surface area available for further microbial attack.
2. Leaching. Water-soluble inorganic nutrients contained in the detritus move down into the soil horizon and get precipitated there as unavailable salts — a process driven simply by water percolating through the soil.
3. Catabolism. Bacterial and fungal enzymes degrade detritus into progressively simpler inorganic substances through direct enzymatic breakdown.
4. Humification. This leads to the accumulation of a dark-coloured, amorphous substance called humus, which is highly resistant to further microbial action and therefore decomposes at an extremely slow rate; being colloidal in nature, humus serves as an important reservoir of nutrients in the soil.
5. Mineralisation. Humus is further degraded by certain microbes, and this final breakdown releases inorganic nutrients back into the soil in a usable form — the process known as mineralisation.

Products of decomposition: the end products are simple inorganic substances — principally carbon dioxide, water, and mineral nutrients — which are released back into the soil (or water) and become available once again for uptake and reuse by producers, closing the loop of nutrient cycling.

Factors controlling the rate of decomposition: the rate is controlled jointly by the chemical composition of the detritus and by climatic factors. Decomposition proceeds slowly if the detritus is rich in tough compounds like lignin and chitin, but quickly if it is rich in nitrogen and water-soluble substances like sugars. Among climatic factors, temperature and soil moisture are the most important, acting through their effect on the activity of soil microbes: a warm and moist environment favours rapid decomposition, whereas low temperature and anaerobic (oxygen-poor) conditions inhibit decomposition, leading to a build-up of undecomposed organic material.

Conclusion: decomposition is the oxygen-requiring breakdown of detritus into inorganic nutrients through the simultaneous processes of fragmentation, leaching and catabolism, followed by humification and mineralisation in the soil, with its rate governed by the detritus’s chemical composition and by temperature/moisture conditions.

Except for rare deep-sea hydrothermal ecosystems, the sun is the only source of energy for every ecosystem on Earth, and the flow of this energy through an ecosystem is strictly unidirectional — it passes from the sun to producers and then on to consumers, never flowing backward, which is consistent with the First Law of Thermodynamics (energy is transformed, not destroyed). Ecosystems are equally bound by the Second Law of Thermodynamics: they require a constant supply of incoming energy to synthesise the complex molecules they need and to counteract the universal tendency towards increasing disorder.

1. Capture of solar energy. Of the total incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR) — the only portion plants and photosynthetic bacteria can actually use. Of this already-reduced PAR, producers (plants, photosynthetic bacteria) capture only 2–10% during photosynthesis. Despite being such a tiny fraction of the sun’s total output, this captured energy is enough to sustain the entire living world.
2. Movement through trophic levels. All organisms depend, directly or indirectly, on producers for their food, giving rise to an unbroken, unidirectional chain of energy transfer. Primary consumers (herbivores) feed directly on producers; secondary consumers (primary carnivores) feed on primary consumers; and tertiary consumers (secondary carnivores) feed on secondary consumers — each occupying a successive trophic level based strictly on its source of nutrition.
3. The 10% law. At each step of this transfer, only about 10% of the energy present at a given trophic level is actually passed on to the next trophic level; the remaining roughly 90% is lost, chiefly as heat through respiration. This progressive loss is precisely why the number of trophic levels in a grazing food chain is limited — there usually isn’t enough energy left to meaningfully sustain more than four or five levels (producer → herbivore → primary carnivore → secondary carnivore).
4. Two parallel pathways — GFC and DFC. Energy does not flow through only one channel. The grazing food chain (GFC) begins with living producers and moves energy through herbivores and carnivores, and is the major conduit in aquatic ecosystems. The detritus food chain (DFC) begins instead with dead organic matter: when any organism dies (at any trophic level), it becomes detritus, which serves as an energy source for decomposers (saprotrophs); in terrestrial ecosystems, a much larger share of total energy flow passes through the DFC than through the GFC.
5. Standing crop and its measurement. At any given time, each trophic level has a certain mass of living material present, called its standing crop, measured either as the number of organisms or as biomass (fresh or, more accurately, dry weight) in a unit area.

Conclusion: energy flows unidirectionally from the sun through producers (capturing only 2–10% of the already-limited PAR) and onward through successive consumer trophic levels, losing roughly 90% of the available energy as heat at every transfer under the 10% law, moving in parallel through both the living-plant-based grazing food chain and the dead-matter-based detritus food chain, which together account for all energy flow in an ecosystem.