Q.Which of the following is an ecosystem service provided by a natural ecosystem?

Ecological Concepts: Zooming Out From Organism to Biosphere

Point a camera at a single deer grazing in a forest, and you are looking at an organism. Zoom out a little and you see the whole herd of deer sharing that forest — a population. Zoom out further and the tigers, grasses, insects and fungi living alongside that herd come into frame — a community. Pull back once more and you see the community together with the soil, water, sunlight and air it depends on — an ecosystem. Keep pulling back and entire regions of similar ecosystems (all the world's tropical forests, say) form a biome, and every biome on Earth together makes up the biosphere.

This is the idea of levels of ecological organisation — the same living world, examined at successively larger scales. The NCERT Class XII Biology ecology unit (Chapters 11–13) is built around exactly this zoom-out: Chapter 11 (Organisms and Populations) stays at the organism/population scale, Chapter 12 (Ecosystem) zooms out to the community-plus-environment scale, and Chapter 13 (Biodiversity and Conservation) zooms out again to ask how much variety of life exists and how to protect it. "Ecological Concepts" as a bucket covers facts that sit at any of these scales — not one single narrow idea, but the connected vocabulary ecologists use to describe life at every zoom level.

Populations: The First Zoom-Out From the Individual

A population is a group of individuals of the same species living in a defined geographical area at a given time, capable of interbreeding. Because it is a group and not a single organism, a population has properties an individual cannot have on its own — birth rate and death rate (measured per capita, not as raw counts), a sex ratio, an age distribution, and a population density (the size of the population relative to a unit of space). These "group-level" attributes are exactly what distinguishes population ecology from studying one organism's physiology.

Ecosystem Structure: What an Ecosystem Is Made Of

An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical surroundings. Every ecosystem — a pond, a forest, a crop field, even an aquarium — is built from the same two kinds of components:

• Abiotic components — the non-living inorganic and organic materials: air, water, soil, and the nutrients dissolved in them.
• Biotic components — the living organisms, organised by their functional role: producers (autotrophs that fix solar energy), consumers (heterotrophs that eat producers or each other), and decomposers (fungi and bacteria that break down dead matter).

Two things give an ecosystem its physical shape: its species composition (which plant and animal species are present) and its stratification — the vertical layering of species at different heights, such as tall trees forming the top canopy of a forest, shrubs below them, and grasses and herbs at the ground.

Productivity: How Fast an Ecosystem Makes New Matter

Primary productivity is the rate at which producers capture solar energy and convert it into organic (biomass) matter.

• Gross Primary Productivity (GPP) — the total rate of production of organic matter, i.e. the total solar energy fixed by all producers.
• Net Primary Productivity (NPP) — what is left of GPP after the producers themselves use some of it up in their own respiration: NPP = GPP − Respiration losses (R). NPP is the biomass actually available to the consumers (herbivores and decomposers) of the ecosystem.
• Secondary productivity is the rate at which consumers assimilate the food energy they take in from producers or from other consumers.

Decomposition: Breaking Matter Back Down

Decomposers convert the complex organic compounds in dead plant and animal remains ("detritus") back into simple inorganic nutrients, so those nutrients can be reused by producers. This happens in three overlapping steps:

1. Fragmentation — detritivores (like earthworms) physically break detritus into smaller particles.
2. Leaching — water-soluble inorganic nutrients seep down through the soil and are lost from the topsoil layer as precipitates.
3. Catabolism — bacterial and fungal enzymes chemically break down the remaining detritus into simpler inorganic substances.

Partial decomposition leaves behind a dark, colloidal, highly microbe-resistant substance called humus, which acts as a slow-release nutrient reservoir; its further breakdown to release inorganic nutrients is called mineralisation. Decomposition is largely an oxygen-requiring process, is slower when detritus is rich in lignin/chitin, and is faster when it is rich in nitrogen and simple sugars — warm, moist conditions favour it, while cold and waterlogged (anaerobic) conditions suppress it and let organic matter accumulate.

Energy Flow and Ecological Pyramids

Unlike nutrients, energy in an ecosystem flows in one direction only — from the sun, into producers, and then into successive consumers — and is progressively lost as heat at every transfer; it is never recycled back to the producers the way a nutrient is. Because a real ecosystem rarely has a single, unbranched sequence of "who eats whom," feeding relationships form an interconnected food web rather than one isolated food chain.

Ecological pyramids represent this trophic structure visually (producers at the base, successive consumers stacked above):

Nutrient (Biogeochemical) Cycling …