Ecosystem Productivity, Decomposition, Succession

A recurring trap in this chapter is treating productivity as one single number. NCERT draws a sharp line between gross primary productivity, the total rate of photosynthesis, and net primary productivity, what is actually left over for the rest of the ecosystem to consume, and a UPSC-style question tests exactly that gap rather than the general idea that plants produce biomass. A second trap hides inside a throwaway textbook figure: oceans cover about 70 per cent of the Earth's surface yet produce well under half the planet's net primary productivity, the reverse of what surface area alone would suggest. This note works through that productivity arithmetic, the actual mechanics of decomposition, and how a bare patch of rock or a disturbed patch of forest rebuilds into a stable community, three genuinely separate ecosystem processes sitting alongside the food chains, cycles and symbiosis already covered elsewhere. NCERT treats productivity, decomposition, energy flow and nutrient cycling as the four functions that make an ecosystem work as a unit; this note takes the two of those four that get tested on their own precise terms, productivity and decomposition, and adds ecological succession, the process by which an ecosystem's community composition itself changes over time.

Food Webs, Nutrient Cycles and Pollinators covers trophic levels, food chains and webs, ecological pyramids, and the carbon, nitrogen and phosphorus cycles in depth; Plant and Animal Classification Basics covers basic symbiosis, including legume-Rhizobium nitrogen fixation and fungus-farming ants. This note stays on ground neither one covers: how ecosystem productivity is actually measured, how dead matter is broken down step by step, and how a community rebuilds itself after disturbance.

Ecosystem productivity: GPP, NPP and secondary productivity

Primary production is defined as the amount of biomass or organic matter produced per unit area over a period of time by plants during photosynthesis, expressed either as weight (g m⁻²) or energy (kcal m⁻²). The rate of that production, expressed per year (g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹), is what NCERT calls productivity, and it splits into two figures that get tested against each other directly. Gross primary productivity (GPP) is the total rate of organic matter production during photosynthesis. Plants use a considerable share of that GPP in their own respiration (R), so what remains, GPP minus R, is net primary productivity (NPP), the biomass actually available for heterotrophs, herbivores and decomposers alike, to consume. A third figure sits on the consumer side: secondary productivity is the rate at which consumers form new organic matter, and it is, by definition, dependent on the NPP handed up from producers rather than a separate input of its own. The conversion efficiency at the base of this chain is strikingly small: NCERT's own worked example of an energy pyramid shows producers converting only about 1 per cent of the sunlight actually available to them into net primary productivity, everything else being lost before it ever becomes usable biomass.

Before any of this happens, only a fraction of incoming sunlight is even usable: less than 50 per cent of incident solar radiation is photosynthetically active radiation (PAR), and plants capture only about 2 to 10 per cent of that PAR, a figure NCERT tests directly as a standalone definitional question, not just as background. Productivity is also not uniform. It depends on the plant species inhabiting a particular area and on a set of environmental factors together, availability of nutrients and a plant's own photosynthetic capacity chief among them, which is why different ecosystem types report very different figures. The annual net primary productivity of the whole biosphere is estimated at approximately 170 billion tonnes of organic matter (dry weight). Oceans cover about 70 per cent of the Earth's surface, yet contribute only about 55 billion tonnes of that total, meaning land, on well under a third of the surface, still accounts for the larger share, a genuinely counter-intuitive figure worth holding onto exactly as stated rather than by instinct.

Decomposition

Decomposition is the process by which decomposers break complex organic matter down into inorganic substances, carbon dioxide, water and nutrients. Its raw material is detritus: dead plant remains such as leaves, bark and flowers, together with dead animal remains, including faecal matter. NCERT sets out five steps that act on detritus simultaneously rather than in a strict sequence, illustrated with a running example worth remembering as a single mental picture: a leaf falls from a tree, is partly eaten by insects and other animals as it enters the food web, is partly broken down where it lies by fungi and bacteria until it loses form and becomes litter, has some of its nutrients leach into the soil by chemical action, and is further broken down by earthworms, bacteria, soil mites and fungi until it becomes part of an organic-rich soil that feeds the next generation of the same tree. This is also why NCERT calls the earthworm the farmer's friend: its fragmentation work both breaks down organic matter and loosens the soil it works through.

Fragmentation is a physical step carried out by detritivores, earthworms being the standard example, which break detritus down into smaller particles, distinct from the chemical work decomposers do. Leaching moves water-soluble inorganic nutrients down into the soil horizon, where they precipitate as unavailable salts. Catabolism is the chemical step proper: bacterial and fungal enzymes degrade detritus into simpler inorganic substances. Two further steps happen specifically in the soil: humification, the accumulation of a dark-coloured, amorphous, colloidal substance called humus, which is highly resistant to further microbial attack and decomposes at an extremely slow rate, acting as a standing reservoir of nutrients; and mineralisation, in which microbes further degrade that humus and release its inorganic nutrients back for reuse by plants.

Decomposition is largely an oxygen-requiring process, and its rate is governed by two things: the chemical composition of the detritus itself, slower when detritus is rich in lignin and chitin, quicker when it is rich in nitrogen and water-soluble substances like sugars, and climatic factors, chiefly temperature and soil moisture, which act on decomposition through their effect on soil microbe activity. A warm, moist environment favours decomposition; low temperature and anaerobic (oxygen-poor) conditions inhibit it, causing organic material to accumulate rather than break down.

Ecological succession

Succession is the gradual, broadly predictable change in an area's species composition over time, as one community is progressively replaced by another. It begins in one of two ways. Primary succession starts on land or water that has never previously supported a community at all, bare rock exposed by a retreating glacier or freshly cooled volcanic lava being the standard examples; new land from Hawaiian lava flows, for instance, has been estimated to form at a rate of roughly 32 acres a year, and the same process is documented on retreating glacier forefields in the Andes and on freshly deposited sand dunes such as Canada's Athabasca Dunes. Because there is no soil to begin with, primary succession opens with pioneer species, hardy organisms with minimal soil requirements, commonly lichens and mosses, that colonise the bare surface first. Their presence changes the habitat: weathering and the slow accumulation of decomposing organic matter from these pioneers gradually build a thin soil layer, which in turn lets less hardy species establish, a self-reinforcing mechanism ecologists call facilitation.

Secondary succession starts from a very different baseline: an existing community that has been disturbed or destroyed, by a wildfire, a flood or the abandonment of farmland, but where soil, nutrients and often a seed bank survive intact. The classic documented case is farmland abandoned in the American Midwest, which passes back through grasses and shrubs toward a hardwood forest, without ever needing to build soil from scratch, which is exactly why secondary succession proceeds markedly faster than primary succession on an equivalent site. Both pathways move through a sequence of intermediate communities before reaching a climax community, a comparatively stable end point, specific to the local climate and geography, where species composition stops changing significantly and which persists until the next disturbance resets the process; a documented case of oak-hickory forest recovery after wildfire took more than 150 years to reach that stable point.

Exam angle

This chapter rewards reading a figure or a definition exactly as stated rather than trusting the general shape of the idea. On productivity, the standard trap swaps GPP for NPP, or the reverse, inside a statement, when the tested distinction is specifically that NPP is what remains for the rest of the ecosystem after a plant's own respiration is subtracted, not the total amount a plant photosynthesises. The oceans-versus-land productivity figure is a second reliable trap: a statement assuming oceans, given their much larger surface area, must also produce most of the biosphere's biomass is simply the reverse of the actual figure. On decomposition, questions like to blur detritivores, the physical fragmentation done by organisms like earthworms, with decomposers proper, the chemical breakdown done by fungi and bacteria, or assert decomposition proceeds fastest in cold, waterlogged conditions when the opposite is true, warm and moist conditions favour it, while low temperature and low oxygen actively slow it down. On succession, the standard trap swaps primary and secondary succession's starting conditions, or implies both proceed at the same pace, when the presence or absence of existing soil is exactly what separates a fast recovery from a slow, decades-long one. A final thread worth holding across all three processes: NCERT frames productivity, decomposition and nutrient cycling together as what generates ecosystem services, naming forests purifying air and water as its own example, which is the kind of one-line definitional link a match-the-following question can build an entire statement around.

Quick revision points

• GPP is the total rate of organic matter production during photosynthesis; NPP equals GPP minus respiration losses (R), the biomass actually available to consumers; secondary productivity is the consumer-side rate of new organic matter formation.
• Less than 50 per cent of incident solar radiation is PAR; plants capture only about 2 to 10 per cent of that PAR.
• The biosphere's annual NPP is about 170 billion tonnes (dry weight); oceans, about 70 per cent of Earth's surface, contribute only about 55 billion tonnes of it, land accounts for the larger share.
• Decomposition's five steps operate simultaneously: fragmentation (detritivores, e.g. earthworms), leaching, catabolism (enzymatic breakdown by fungi and bacteria), humification (forms humus, a slow-to-decompose nutrient reservoir), mineralisation (releases humus's inorganic nutrients).
• Decomposition is largely aerobic; it is faster with nitrogen- and sugar-rich detritus and in warm, moist conditions, slower with lignin- and chitin-rich detritus and in cold, anaerobic conditions.
• Primary succession starts where no community existed before (bare rock, fresh lava) and opens with pioneer species such as lichens; secondary succession starts where an existing community was disturbed but soil survives, so it proceeds faster.
• Both pathways move through intermediate communities toward a climax community, a relatively stable end point that persists until the next disturbance.

Try the linked practice questions to see how these productivity figures, decomposition steps and succession stages get tested against each other.