Q.(a) Write important features of 'humus' formed during the decomposition cycle in a terrestrial ecosystem.
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🔒 Start your 14-day free trial to unlock the full solution →Part (a)Concept understanding — Decomposition Processes
Let’s begin with something you already know. Think of a fallen leaf in your garden. Over a few weeks, it turns brown, becomes brittle, and eventually crumbles into the soil. You don’t see it “disappear” — it just becomes part of the earth again. That slow, invisible breakdown is the simplest picture of decomposition.
Now, in your NCERT textbook for subjects like Biology or Environmental Studies (which you may encounter in a commerce/humanities context as part of general awareness or ecology), decomposition is defined as the process by which dead organic matter — dead plants, dead animals, fallen leaves, even waste products — is broken down into simpler substances like carbon dioxide, water, and minerals. These simpler substances then return to the soil, air, and water, ready to be used again by living plants.
Decomposition is not destruction. It is recycling. Without it, nutrients would stay locked inside dead bodies and fallen leaves, and new life would have nothing to feed on. The entire cycle of life depends on this breakdown.
The agents that do this work are mostly microorganisms — bacteria and fungi — along with some insects and worms (called detritivores). They are nature’s cleanup crew. When you see a mushroom growing on a rotting log, that mushroom is a fungus actively decomposing the wood.
The process itself happens in a few overlapping stages, though you don’t need to memorise them as a list of formulas. Here is the intuitive sequence:
- Fragmentation: The dead material is physically broken into smaller pieces by earthworms, insects, or even wind and rain. A dead leaf becomes leaf litter.
- Leaching: Water seeps through the fragments, dissolving and carrying away water-soluble nutrients like sugars and amino acids. This is why rain helps speed up decay.
- Catabolism: This is the chemical core. Enzymes from bacteria and fungi break down complex molecules (like cellulose in plant cell walls or proteins in animal tissue) into simpler ones — carbon dioxide, water, and mineral salts.
- Humification: A dark, spongy, nutrient-rich substance called humus is formed. Humus is not fully decomposed; it is a stable, long-lasting organic material that improves soil structure and water-holding capacity. It gives forest soil that rich, earthy smell.
- Mineralisation: The final step where even humus is slowly broken down, releasing inorganic nutrients like nitrogen, phosphorus, and potassium back into the soil for plants to absorb. …
Part (b)Concept understanding — Species Area Relationship
The Species Area Relationship: A First Look
Imagine you are walking through a small park near your home. You might spot a few birds, some insects, and a handful of plant species. Now imagine that same walk through a large forest reserve — hundreds of times bigger. Would you expect to see more kinds of birds, more types of insects, more varieties of trees? Almost certainly yes. That simple, intuitive observation is the seed of the Species Area Relationship.
What It Means
The Species Area Relationship (often abbreviated as SAR) is a pattern ecologists have observed across the natural world: as the area you sample increases, the number of species you find also increases. It is not a vague guess — it is a consistent, well-documented relationship that holds true for most groups of organisms, from plants and birds to insects and mammals.
Why does this happen? A larger area typically contains more habitats — forests, grasslands, wetlands, rocky outcrops — and each habitat supports its own set of species. A bigger area also tends to have more individuals, and with more individuals you are more likely to encounter rare species that might be absent from a small patch. In short, area acts as a rough proxy for ecological diversity and complexity.
Key Points to Remember
- The relationship is positive: bigger area → more species.
- It is not linear — doubling the area does not double the number of species. The increase slows down as area gets very large.
- The pattern holds across scales: from a single leaf (hosting tiny insects and fungi) to an entire continent.
The NCERT textbook for Class 12 Biology (Chapter 15, Biodiversity and Conservation) introduces this concept in the context of biodiversity patterns. It states that the relationship between species richness and area is described by a curve that rises rapidly at first and then flattens. The textbook does not require you to memorise any equation — only to understand the general trend and its implications.
Why It Matters
The Species Area Relationship is not just an academic curiosity. It has real-world consequences, especially for conservation.
- Designing protected areas: If you want to preserve a certain number of species, you need to know how much area is required. A small reserve may protect only a fraction of the region's biodiversity.
- Predicting extinctions: When a habitat is destroyed or fragmented, the remaining area shrinks. Using the SAR, ecologists can estimate how many species are likely to be lost as a result.
- Understanding island biology: The relationship was first studied on islands, where area is clearly defined and isolation limits immigration. The same logic applies to "habitat islands" — patches of forest surrounded by farmland, or national parks surrounded by cities. …
Part (a)
Humus is the dark-coloured, amorphous, colloidal substance formed during decomposition (humification) in the soil. Its important features:
- Dark coloured and amorphous (structureless).
- Highly resistant to microbial action, so it decomposes very slowly (long turnover).
- Colloidal in nature, giving high water- and nutrient-holding capacity. …
Part (a): Humus is the dark, amorphous, colloidal, decomposition-resistant end product of humification that stores nutrients and improves soil fertility.
Part (b): Species richness vs area is a straight rising line on log–log axes for bats and fishes, described by log S = log C + Z log A.
Part (a)
Concept-first idea: Decomposition converts detritus into inorganic nutrients; a stable, dark, resistant fraction accumulates as humus during humification.
Important features of humus:
- Dark coloured and amorphous — structureless, brown-to-black material.
- Highly resistant to microbial action — it undergoes decomposition at an extremely slow rate, so it persists in the soil for a long time.
- Colloidal in nature — very fine particles with a large surface area, giving high water-holding and nutrient-holding (cation-exchange) capacity.
- Nutrient reservoir — it undergoes slow mineralisation, releasing inorganic nutrients gradually and steadily to plants.
- Improves soil structure, aeration and fertility. …
Showing the 12 most recent of 21 on this concept.
- CBSE 2026Set V11 markMCQQ.Dead plant remains such as leaves, bark and flowers constitute(a) Debris(b) Humus(c) Detritus(d) Fossil
›Reveal solutionSolution
Dead plant material (leaves, bark, flowers) plus dead animal remains form the detritus.
In an ecosystem, dead plant remains such as leaves, bark and flowers, along with dead animal remains and faecal matter, constitute the detritus. Detritus is the raw material for the process of decomposition, in which detritivores and decomposers break it down. Humus is the dark, amorphous, partially decomposed and highly resistant organic matter that accumulates i …
- CBSE 2026Set A1 markMCQQ.What is the raw material for decomposition called?(a) Humus(b) Inorganic substance(c) Detritus(d) Organic substance
›Reveal solutionSolution
Decomposition acts on detritus — dead organic remains such as fallen leaves, dead bodies and faecal matter.
Decomposition is the process by which decomposers break down complex organic matter into inorganic substances. The raw material for this process is detritus, which consists of dead plant remains (leaves, bark, flowers) and dead animal bodies and their faecal matter. Thro …
- CBSE 2026Set A1 markMCQQ.What are those called which obtain energy and nutrition from breaking dead organic matter or detritus?(a) Saprotrophs(b) Heterotrophs(c) Detritivores(d) Parasite
›Reveal solutionSolution
Detritivores feed on and break down detritus (dead organic matter) to obtain energy and nutrients.
Detritivores, such as earthworms and many soil arthropods, feed on detritus — dead organic matter — and fragment it, obtaining their energy and nutrition from it. This fragmentation of detritus is an important early …
- CBSE 2025Set X11 markMCQQ.In which of the processes given, the humus is degraded by some microbes and release of inorganic nutrients occur?(a) Leaching(b) Mineralisation(c) Catabolism(d) Humification
›Reveal solutionSolution
The microbial breakdown of humus that releases inorganic nutrients is mineralisation.
During decomposition, humus is further degraded by some microbes and this releases inorganic nutrients (like ammonium, nitrate, phosphate) back into the soil. This process is called mineralisation.
- (a) Leaching is the movement of water-soluble inorganic nutrients down into the soil horizons where they get precipitated. …
- CBSE 2025Set A1 markQ.Match the correct pair and write the match for 'Detritivorous'. Column I:(i) Apomixis(ii) Darwin(iii) Toddy(iv) Agarose(v) Detritivorous. Column II:(a) Galapagos Islands(b) Southern India(c) Sea weeds(d) Earthworm(e) Grass family.
›Reveal solutionSolution
Detritivorous matches with (d) Earthworm, a well-known soil detritivore/decomposer.
Detritivores are organisms that feed on detritus — the partially decomposed remains of dead plants and animals, and faecal matter. Earthworms are one of the most familiar detritivores, ingesting soil rich in organic detritus, digesting the organic fraction, and excreting nutrient-rich castings; in doing so, they also help b …
- CBSE 2025Set ANNUAL1 markMCQQ.Decomposition would be fast if the detritus contains _____.(a) Cellulose(b) Lignin and chitin(c) Nitrogen and water soluble sugars(d) Cellulose and pectin
›Reveal solutionSolution
The rate of decomposition of detritus is controlled by its chemical composition: detritus rich in nitrogen and simple, water-soluble substances like sugars decomposes rapidly, whereas detritus rich in lignin and chitin decomposes very slowly.
Decomposition is the process by which decomposers (bacteria and fungi) break down complex organic matter in detritus into simpler inorganic substances (humus and minerals). The rate at which this happens is not the same for all detritus — it depends heavily on the quality of the detritus:
- Nitrogen content and water-soluble substances (e.g. sugars): these are easy for microbes to metabolise quickly, so decomposition is rapid when they are abundant.
- Lignin and chitin: these are structurally tough, recalcitrant biopolymers (lignin in woody/cellulosic material, chitin in fungal cell walls/insect exoskeletons) that resist microbial enzymatic breakdown, so detritus rich in them decomposes very slowly. …
- CBSE 2025Set ANNUAL1 markMCQQ.The relation between species richness and area for a wide variety of taxa on a logarithmic scale is a(a) Rectangular hyperbola(b) Straight line(c) Sigmoid curve(d) Sine curve
›Reveal solutionSolution
On a log-log plot, species richness rises linearly with area (log S = log C + Z log A); on a normal (non-log) scale it is a rectangular hyperbola.
Ecologists have found that, within a region, species richness (S) increases with explored area (A), but only up to a certain limit; beyond this, the addition of new species with increasing area is minimal. On a normal (arithmetic) scale, this species-area relationship for a wide variety of taxa (plants, birds, fish) turns out to be a curve — specifically, a rectangular hyperbola.
…
- CBSE 2024Set GG1 markMCQQ.Which one of the following is a detritivore?(a) Earthworm(b) Bird(c) Frog(d) Elephant
›Reveal solutionSolution
A detritivore eats dead/decaying organic matter; the earthworm does exactly this, so the answer is (a).
Concept. In an ecosystem, detritivores are organisms that feed on detritus — fragments of dead plants and animals and their waste. They fragment the detritus during feeding, aiding decomposition and nutrient recycling.
…
- CBSE 2024Set E1 markMCQQ.Going down of inorganic water soluble nutrients into the soil nutrients is called as(a) Fragmentation(b) Leaching(c) Catabolism(d) Humification
›Reveal solutionSolution
Leaching = the process in decomposition where water-soluble inorganic nutrients percolate down into the soil.
Decomposition of detritus involves several steps: fragmentation (breaking litter into pieces), leaching, catabolism, humification and mineralisation. During leaching, water-soluble inorganic nutrients dissolve in percolating water and move down (get precipitated) into the deeper soil layers, becoming unavailable to plants at the …
- CBSE 2024Set E1 markMCQQ.Which of the following are decomposers?(a) Fungi and algae(b) Fungi and virus(c) Fungi and bacteria(d) Fungi, bacteria and virus
›Reveal solutionSolution
Fungi and bacteria are the principal decomposers of an ecosystem.
Decomposers (saprotrophs) break down the complex organic remains of dead plants and animals (detritus) into simple inorganic substances, recycling nutrients back to the soil:
- FUNGI and BACTERIA secrete digestive enzymes onto the detritus and absorb the products — they are the main decomposers. …
- CBSE 2024Set BOTANY1 markQ.Correct the statement, if necessary, by changing the underlined word(s) only: The process of accumulation of a dark-coloured amorphous substance in soil is called mineralization.
›Reveal solutionSolution
The process described is humification, not mineralization.
Decomposition of detritus proceeds through fragmentation, leaching, catabolism, humification, and mineralization. Humification leads to the accumulation of a dark-coloured, amorphous substance called humus, which is highly resistant to microbial action and decomposes very slowly, acting as a nutrient reservoir. Mineralization, on the other hand, is a distinct, later step in which microbes further degrade humus and release inorganic nutrients (like CO2, water, and m …
- CBSE 2024Set ANNUAL1 markMCQQ.Bacteria and fungi are(a) producers(b) primary consumers(c) scavengers(d) decomposers
›Reveal solutionSolution
Bacteria and fungi are the principal decomposers of an ecosystem, breaking down dead organic matter and recycling nutrients.
Bacteria and fungi are saprotrophic organisms that obtain their nutrition by secreting digestive enzymes onto dead organic matter (detritus) and absorbing the resulting simpler breakdown products. Through this decomposition activity, they release inorganic nutrients back into the soil/environment for reuse by producers, making them the decomposers of an ecosystem. They are not producers (which are autotrophic …
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