Ecosystem

An ecosystem is a functional unit in which the living organisms (the biotic community) interact with one another and with their non-living (abiotic) environment. Its structure has two aspects: the species composition and the vertical stratification (layering) of the community. The biotic components are the producers (green plants), the consumers (herbivores and carnivores) and the decomposers (saprotrophs). The four key functions of an ecosystem are productivity, decomposition, energy flow and nutrient cycling.

Productivity is the rate at which biomass (organic matter) is produced. Primary productivity is the amount produced by producers per unit area per unit time, and it has two forms. Gross Primary Productivity (GPP) is the total rate of photosynthesis. Plants use some of this energy in their own respiration (R); what remains is the Net Primary Productivity (NPP): NPP = GPP − R. NPP is the biomass available to the consumers. Secondary productivity is the rate at which consumers form new organic matter.

Decomposition is the breakdown of dead organic matter (detritus) into inorganic substances by decomposers (saprotrophs — bacteria and fungi), helped by detritivores like earthworms. It proceeds in steps: fragmentation (breaking detritus into small pieces), leaching (water-soluble nutrients seep down), catabolism (enzymes break it into simpler compounds), humification (formation of dark, amorphous, resistant humus) and mineralisation (humus is broken down to release inorganic nutrients).

The rate of decomposition depends on the climate and the quality of the detritus. It is faster in a warm and moist environment and slower in cold/dry conditions; it is also faster when the detritus is rich in nitrogen and water-soluble sugars, and slower when rich in lignin and chitin. Oxygen is also required (decomposition is largely aerobic). For NEET, fix the ecosystem components/functions, the GPP/NPP distinction (NPP = GPP − R, available to consumers), and the decomposition steps with humus, mineralisation, and the warm-moist (fast) condition.

The sun is the ultimate source of energy for almost all ecosystems. Of the incident light, only the photosynthetically active radiation (PAR) is useful, and producers capture only a small fraction (about 2–10%) of it. A defining feature is that energy flow is unidirectional — it moves from the sun to producers to consumers and is ultimately lost as heat; it never flows back.

Organisms are arranged in trophic levels according to their source of food: producers (first trophic level), primary consumers/herbivores (second), secondary consumers/carnivores (third), and so on. The sequence of who-eats-whom is a food chain, and interconnected food chains form a food web. There are two types of food chain: the grazing food chain (GFC), which starts from green plants (producers), and the detritus food chain (DFC), which starts from dead organic matter.

The flow of energy obeys the 10% law (proposed by Lindeman): only about 10% of the energy at one trophic level is transferred to and stored at the next level; the other ~90% is lost (mainly as heat) in respiration and life processes. Because of this huge loss at each step, food chains usually have only 3–4 (rarely 5) trophic levels.

The relationship between trophic levels is shown by ecological pyramids — of number, biomass or energy. The pyramid of energy is always upright, because energy always decreases at each higher level. But the pyramids of number and biomass can be inverted: in the sea, a small standing biomass of phytoplankton supports a larger biomass of zooplankton and fish (inverted biomass pyramid), and a single big tree supports a large number of insects and birds (inverted number pyramid). Pyramids also ignore that a species may occupy more than one trophic level. For NEET, fix unidirectional flow, trophic levels, GFC vs DFC, the 10% law (Lindeman), and which pyramids can be inverted (energy never; number/biomass can be).

Nutrients, unlike energy, are not lost but recycled between the living organisms and the environment — this movement is nutrient cycling (the cycles are called biogeochemical cycles). Each cycle has a reservoir, and on this basis there are two types. Gaseous cycles have their reservoir in the atmosphere (e.g. the carbon and nitrogen cycles), while sedimentary cycles have their reservoir in the Earth's crust / rocks (e.g. the phosphorus and sulphur cycles).

The carbon cycle is a gaseous cycle. A huge amount of carbon is held in the oceans, and a large amount circulates through the atmosphere as CO₂. Carbon enters the living world when producers fix CO₂ by photosynthesis; it returns to the atmosphere through the respiration of organisms, the decomposition of dead matter, and the burning of fossil fuels and wood. Human activities (burning fossil fuels, deforestation) have raised atmospheric CO₂, contributing to global warming.

The phosphorus cycle is a sedimentary cycle, and a favourite point of contrast. Its reservoir is the rocks, which contain phosphorus as phosphates. The weathering of rocks releases phosphate into the soil and water; plants absorb the phosphate, animals get it by eating plants, and decomposers return it to the soil.

The key NEET distinction is that the phosphorus cycle has no significant gaseous (atmospheric) phase — phosphorus does not enter the air as carbon does — and there is little or no respiratory release of phosphorus into the atmosphere. So carbon moves rapidly through a gaseous reservoir, while phosphorus moves slowly through a rock/sediment reservoir. For NEET, fix gaseous vs sedimentary cycles, the carbon cycle (atmospheric CO₂; photosynthesis in, respiration/decomposition/burning out) and the phosphorus cycle (reservoir = rocks/phosphates; no gaseous phase).

Ecological succession is the gradual and fairly predictable change in the species composition of a community at a given site over time. It proceeds through a series of stages (each called a seral stage / sere) until a stable, self-perpetuating community — the climax community — is reached, in balance with the environment.

There are two main kinds. Primary succession starts in an area where no living organisms ever existed and there is no soil — such as a bare rock, a new volcanic island, or a cooled lava flow. Because soil has to form first, primary succession is very slow (it may take thousands of years). The first organisms to colonise are the pioneer species; on bare rock these are usually lichens, which secrete acids that weather the rock and help form soil, paving the way for mosses, herbs, shrubs and finally trees.

Secondary succession occurs in areas where a community existed but was removed — for example land cleared by fire, flood or abandoned farmland — but the soil is still present. Because soil (with nutrients and seeds) already exists, secondary succession is much faster than primary succession. Succession that begins in water is called hydrarch (it moves toward a mesic, i.e. moderate, climax) and that beginning on dry land is called xerarch; both tend toward the mesic condition.

Finally, healthy ecosystems give us ecosystem services — the products and processes that nature provides free of cost. These include purifying air and water, pollinating crops, cycling nutrients, forming and conserving soil, regulating climate, controlling floods and droughts, and providing aesthetic, cultural and recreational value. (Economists led by Robert Costanza estimated the value of nature's services at trillions of dollars per year, with soil formation accounting for the largest share.) These services remind us why ecosystems must be conserved — leading into the next chapter on biodiversity. For NEET, fix the definition of succession, primary (bare rock, pioneer = lichens, slow) vs secondary (soil present, faster), hydrarch/xerarch, the climax community, and examples of ecosystem services.