Ecosystem — NEET Notes

Ecosystem — structure and function

An ecosystem can be visualised as a functional unit of nature where living organisms interact among themselves and also with the surrounding physical environment. The term was coined by A. G. Tansley in 1935 — a fact NEET 2016 tested directly. Ecosystems vary enormously in size, from a small pond to a large forest or a sea; many ecologists regard the entire biosphere as one global ecosystem composed of all the local ones. For convenience they are divided into two broad categories: terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary). Crop fields and aquariums are man-made ecosystems.

Every ecosystem has two components that must be considered together. The abiotic component is the non-living surround — sunlight, temperature, water, air, soil, and dissolved inorganic and organic substances. The biotic component is the living surround — producers (autotrophs), consumers (heterotrophs), and decomposers (saprotrophs). The interaction between the two produces a characteristic physical structure for each ecosystem type. Two structural features describe it: species composition — the identification and enumeration of plant and animal species — and stratification, the vertical distribution of species into layers (trees occupy the top stratum of a forest, shrubs the second, herbs and grasses the bottom). NEET 2017 asked which ecosystem shows the clearest stratification — tropical rain forest, with its distinct canopy, understorey, and forest floor.

A pond illustrates all four functions in one frame. The abiotic part is the water, the dissolved gases and salts, and the rich sediment at the bottom. Autotrophs are phytoplankton, algae, and floating, submerged, and marginal plants. Consumers are the zooplankton and bottom-dwelling animals. Decomposers — fungi, bacteria, flagellates — are especially abundant in the sediment, mineralising dead matter into nutrients the autotrophs use again. Energy flows in one direction; nutrients cycle.

Productivity — primary, secondary, GPP and NPP

An ecosystem runs on a constant input of solar energy. 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 g m⁻² or kcal m⁻². The rate of biomass production is called productivity, expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Productivity has two components, separated by a single, NEET-loved equation.

Secondary productivity is the rate at which consumers form new organic matter — herbivores converting plant tissue into animal tissue, carnivores converting prey biomass into predator biomass. Primary productivity depends on the plant species present, on the availability of nutrients, on photosynthetic capacity, and on environmental factors like light, temperature, and water — which is why it varies enormously between ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tonnes (dry weight) of organic matter. Despite occupying about 70% of Earth's surface, the oceans contribute only about 55 billion tonnes; the rest is on land. The limiting factor in most aquatic systems is light and nutrient availability — particularly phosphorus.

Decomposition — five steps from leaf to soil

Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water, and nutrients — the process is called decomposition. The raw material is detritus: dead plant remains such as leaves, bark, and flowers, together with the dead bodies and faecal matter of animals. NEET 2023 made this a five-statement question — knowing the order and the operator of each step is non-negotiable.

Decomposition is largely an oxygen-requiring process. Its rate is controlled by two factors: the chemical composition of the detritus and the surrounding climate. In a given climate, decomposition is slower if detritus is rich in lignin and chitin and faster if detritus is rich in nitrogen and water-soluble substances like sugars. NEET 2022 tested this exact wording in a two-statement question — the misleading half claimed "decomposition is faster if rich in lignin and chitin," and was marked incorrect. Temperature and soil moisture are the most important climatic regulators: warm and moist conditions accelerate decomposition; low temperature and anaerobiosis inhibit it, allowing organic material to accumulate (which is how peat bogs and coal seams formed).

Energy flow — the Sun, the producers, the 10% rule

Except for the deep-sea hydrothermal-vent ecosystem, the Sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR) — the wavelengths plants can actually absorb. Of the PAR that reaches them, plants capture only about 2–10%, and on that thin sliver the entire living world is built. Two laws of thermodynamics constrain what happens next. The first law (energy is conserved) explains why every joule that flows from Sun to producer to consumer must be accounted for. The second law (entropy always increases) explains why ecosystems need a constant supply of energy: every transfer dissipates some as heat, and only an ongoing solar input keeps the order of life from collapsing into disorder.

Energy flow through an ecosystem is therefore unidirectional — from Sun to producers to consumers, with heat losses at each step and no return path. Nutrients, in contrast, cycle. The two patterns sit side by side at the heart of ecosystem function.

Only about 10 per cent of the energy is transferred to each trophic level from the lower trophic level.

Lindeman's 10 % law (1942) — NCERT Class 12, Chapter 12

Each organism occupies a place in the food chain known as its trophic level. Producers (green plants in terrestrial systems; phytoplankton, algae, and higher plants in aquatic systems) sit at trophic level 1. Primary consumers — the herbivores — are at level 2. Carnivores that eat herbivores (primary carnivores) are at level 3; carnivores that eat primary carnivores are at level 4. NEET 2020 tested this directly with a grassland match-up: grass (level 1), rabbit (level 2), crow (level 3), vulture (level 4). One organism can occupy more than one trophic level simultaneously — a sparrow is a primary consumer when eating seeds and a secondary consumer when eating insects. The trophic level represents a function, not a species.

At each trophic level there exists a certain mass of living material at a given moment, called the standing crop. It is measured as biomass (fresh or dry weight; dry weight is more accurate) or as number of individuals per unit area. The amount of inorganic nutrients — carbon, nitrogen, phosphorus, calcium — present in the soil at a given time is called the standing state. NEET 2021 tested this distinction directly: standing crop is the living material, standing state is the nutrient pool. Do not confuse them.

Grazing food chain vs detritus food chain

Two parallel pathways carry energy through every ecosystem. The grazing food chain (GFC) starts with the living producer; the detritus food chain (DFC) starts with the dead. In most aquatic systems the GFC dominates. In most terrestrial systems, by contrast, a much larger fraction of energy flows through the DFC than through the GFC — because most plant material is not eaten alive; it dies and falls to the forest floor before any herbivore reaches it. The two chains interconnect: detritivores are eaten by GFC predators, and omnivores like crows and cockroaches blur the line entirely. These interconnections turn linear chains into food webs.

The decomposers at the base of the DFC — mainly fungi and bacteria — are also called saprotrophs (Greek sapro, to decompose). They secrete digestive enzymes that break dead and waste materials into simple inorganic substances, which they then absorb. NEET 2023 tested this exact fact through a five-statement question: only the options identifying detritivores as agents of fragmentation, microbes as agents of mineralisation, and leaching as the precipitation of soluble nutrients were correct. Statements claiming "the detritus food chain begins with living organisms" or "earthworms perform catabolism" were wrong.

Ecological pyramids — number, biomass, energy

The relationship between trophic levels — in terms of number, biomass, or energy — is conventionally expressed as a pyramid. The base of every pyramid represents the producers; the apex represents the top consumer. The pyramid was first developed by Charles Elton in 1927, and the energy variant by Lindeman in 1942. In most ecosystems, all three pyramids are upright. But two of them can be inverted under specific conditions, and NEET is fond of that exception.

Pyramids carry three structural limitations NEET sometimes probes. They do not accommodate a species that occupies two trophic levels at once. They assume a simple linear food chain, not the web that actually exists in nature. And they leave out the saprophytes entirely, even though decomposition is one of the four functions of the ecosystem.

Ecological succession — hydrarch, xerarch, climax

Biotic communities are not fixed. The process by which the species composition of a community changes over time, with one community replacing another in an orderly sequence, is called ecological succession. The first community to colonise a bare area is the pioneer community — the species in it are the pioneer species. Each transitional community is called a seral community; the whole sequence is a sere. The final, stable, mature stage is the climax community, in dynamic equilibrium with the prevailing climate.

Succession comes in two flavours, distinguished by where it starts. Primary succession begins on a surface where no community existed before — bare rock, newly cooled lava, glacial moraines, sand dunes. Secondary succession begins where a community used to exist but was destroyed — by fire, flood, tilling, or harvesting. Secondary succession is far faster than primary, because the soil is already in place and a seed bank may still be present.

NEET 2016 made this concrete: the pioneers on a bare rock are lichens, which secrete carbonic acid that weathers the rock and creates a thin soil. Once even a sliver of soil exists, mosses move in, followed by small annual herbs and grasses, then perennial herbs, shrubs, shade-intolerant trees, and finally a stable climax forest of shade-tolerant trees. In a pond, the corresponding hydrosere runs from phytoplankton to submerged rooted plants to floating plants to reed-swamp to sedge-meadow to woodland to climax. Notice the convergence: both successions head towards a moist, moderate, mesic climax — because the climax is determined by climate, not by where the sere began. NEET 2021 asked for the term for that final stable community: climax community.

Nutrient cycling — gaseous vs sedimentary

The storage and movement of nutrient elements through the various components of an ecosystem is called nutrient cycling or, equivalently, the biogeochemical cycle. The word itself is informative — bio (living), geo (rock), chemical (element). Nutrients are repeatedly used: the Earth gets no fresh delivery from space, and what the decomposers liberate from dead matter is what the producers take up again. There are more than forty elements that organisms need; each has its own cycle.

NCERT divides nutrient cycles into two structural types, distinguished by where the main reservoir sits.

Two functions are common to every cycle. A reservoir meets the deficit caused by an imbalance between the rate of influx and the rate of efflux — it stabilises the system. A series of biotic and abiotic processes moves the nutrient between living and non-living compartments. The two most heavily NEET-tested cycles are carbon (gaseous) and phosphorus (sedimentary).

Carbon cycle

Atmospheric CO₂ is the source of all carbon in living organisms and in the fossil deposits that power our civilisation. Carbon dioxide is highly soluble in water, so oceans hold huge stores of dissolved CO₂ and bicarbonate. The carbon cycle operates through four major processes.

Photosynthesis draws CO₂ from the atmosphere into plant tissue, converting inorganic carbon into organic biomass with the help of sunlight and chlorophyll. Some of that biomass is consumed by plants for their own metabolism; the rest is stored as biomass, available to heterotrophs as food. Respiration — by both plants and animals — oxidises that organic matter and returns CO₂ to the atmosphere. Decomposition by microbes acts on dead bodies and waste, releasing the left-over carbon back as CO₂. Combustion of fossil fuels — coal, crude oil, natural gas — releases CO₂ (and CO) that had been locked underground for millions of years. Forests also act as carbon reservoirs because the carbon they fix cycles very slowly through long-lived woody tissue; forest fires release that stored carbon abruptly.

Human activities — industrialisation, urbanisation, and the burning of fossil fuels — have raised atmospheric CO₂ steadily for two centuries, contributing the dominant share to the greenhouse effect and to global warming. NEET 2019 asked which pair of gases is mainly responsible for the greenhouse effect; the answer is carbon dioxide and methane, with CO₂ contributing roughly 60% and CH₄ another 20% of global warming.

Phosphorus cycle

Phosphorus is a necessary constituent of the protoplasm — every cell carries it in DNA, RNA, ATP, NADP, and phospholipid membranes. Unlike carbon, phosphorus has no gaseous phase: it is a textbook example of a sedimentary cycle. The reservoir is the lithosphere — phosphate rocks and other deposits laid down in past geological ages.

The cycle runs as follows. Weathering of rocks releases phosphate into soil and water — NEET 2022 asked which process accelerates the phosphorus cycle, and the answer is weathering. Plants absorb inorganic phosphate as orthophosphate ions; herbivores get phosphorus by eating the plants; carnivores get it by eating the herbivores. When organisms die, decomposers release phosphate back into the soil in soluble form. Excreta return some phosphorus to the cycle along the way. Much of the phosphorus released eventually escapes into the sea, where part is lost to deep sediments and part to shallow marine sediments. Bones, teeth, and shells resist weathering and lock phosphorus away for long periods. Sea birds play a singular role: through their guano deposits they bring marine phosphorus back to land. Despite all these returns, the cycle's return rate is inadequate to compensate for losses, and human mining of phosphate rock for fertiliser has accelerated the depletion.

Ecosystem services — Costanza's four categories

The products of ecosystem processes — clean air, fresh water, fertile soil, pollination, climate regulation — are collectively called ecosystem services. They are mostly invisible until they fail. In 1997, Robert Costanza and colleagues attempted to put a monetary value on these services and arrived at a global figure of roughly US$33 trillion per year — nearly twice the global GNP at the time. The Millennium Ecosystem Assessment later grouped these services into four standard categories, which NEET expects students to recognise.