Pariksha SaathiLesson 1 of 10 · Environment and Ecology
Ecosystems: Food Chains, Trophic Levels, Ecological Pyramids and Nutrient Cycles
Drafted with AI assistance and fact-checked on 28 October 2026; not yet read line by line by a human editor. Spotted a mistake? Tell us. Editorial standards & corrections
What an ecosystem is
An ecosystem is a functional unit of nature in which living organisms interact with one another and with their physical surroundings. The term was coined by the British ecologist A.G. Tansley in 1935. An ecosystem can be as small as a pond or as large as a forest, and the biosphere is the largest of all. Its abiotic components are the non-living factors such as sunlight, temperature, rainfall, soil, air and minerals; its biotic components are the living organisms. Two processes keep every ecosystem running: a one-way flow of energy that starts from the sun, and a cyclic flow of nutrients that are used again and again.
Producers, consumers and decomposers
Producers, or autotrophs, make their own food. Green plants on land and phytoplankton in water use sunlight in photosynthesis, while some bacteria near deep-sea hydrothermal vents make food by chemosynthesis, without sunlight. Consumers, or heterotrophs, depend on others: primary consumers are herbivores such as deer, grasshoppers and zooplankton; secondary consumers are carnivores that eat herbivores, such as frogs; tertiary consumers are top carnivores such as the tiger, eagle and shark. Decomposers, mainly bacteria and fungi, break dead matter down into simple inorganic substances, while detritivores such as earthworms feed on the dead matter (detritus) itself.
Food chains and food webs
A food chain shows who eats whom, for example grass, grasshopper, frog, snake and eagle. A grazing food chain begins with living green plants, while a detritus food chain begins with dead organic matter. In a terrestrial ecosystem such as a forest, much more energy flows through the detritus food chain than through the grazing chain; in aquatic ecosystems the grazing chain is the main route. Food chains seldom have more than four or five links because energy is lost at every step. Interlinked chains form a food web, and a complex web makes an ecosystem more stable because the loss of one species does not break the whole system.
Trophic levels and the 10 per cent law
Each step of a food chain is a trophic level: producers form the first, herbivores the second, carnivores the third and top carnivores the fourth. Less than 50 per cent of the sunlight reaching the Earth is photosynthetically active radiation, and plants capture only 2 to 10 per cent of it. Raymond Lindeman gave the 10 per cent law in 1942: only about 10 per cent of the energy at one level passes to the next, because the rest is used in respiration and lost as heat. If plants store 10,000 joules, herbivores get about 1,000, carnivores about 100 and top carnivores about 10.
Ecological pyramids
Charles Elton introduced the pyramid idea in 1927, so these are also called Eltonian pyramids; producers form the base and each trophic level sits above the one it eats. The pyramid of numbers is upright in a grassland, spindle-shaped in a tree ecosystem where one tree supports thousands of insects, and fully inverted in a parasitic food chain. The pyramid of biomass is upright on land but inverted in the sea or a pond, because a small standing crop of fast-multiplying phytoplankton supports a larger mass of fish. The pyramid of energy is always upright.
Ecological succession
Succession is the gradual and fairly predictable change in the species of an area over time, until a stable climax community in balance with the climate develops. The whole sequence of communities is a sere, and each stage a seral stage. Primary succession starts where there was never any soil, such as bare rock, cooled lava or a new sand dune, and takes hundreds to thousands of years. Its pioneers on rock are lichens, which secrete acids that break the rock and begin soil formation; mosses, grasses, shrubs and finally trees follow. Secondary succession starts where a community was removed but soil remains, as on abandoned farmland or a burnt forest, and is much faster. Succession in water (hydrarch) and on dry rock (xerarch) both move towards a medium-moisture (mesic) climax.
Biogeochemical cycles
Gaseous cycles such as those of carbon and nitrogen have their reservoir in the atmosphere, while sedimentary cycles such as phosphorus and sulphur have it in the Earth's crust. In the carbon cycle, plants fix carbon dioxide by photosynthesis, and it returns to the air through respiration, decomposition, volcanoes and the burning of fossil fuels; by the NCERT textbook figure, about 71 per cent of the Earth's carbon is dissolved in the oceans. Nitrogen forms about 78 per cent of the atmosphere, but plants cannot use the gas directly. It is fixed by bacteria such as Rhizobium and Azotobacter, by cyanobacteria such as Anabaena and Nostoc, by lightning and by the industrial Haber process; decomposers turn dead protein into ammonia (ammonification); Nitrosomonas and Nitrobacter turn ammonia into nitrite and then nitrate (nitrification); plants absorb nitrates (assimilation); and Pseudomonas returns nitrogen gas to the air (denitrification). The water cycle, driven by solar energy, moves water through evaporation, transpiration, condensation, precipitation, infiltration and runoff; about 97 per cent of the Earth's water is saline ocean water.
Key facts
- Ecosystem: term coined by A.G. Tansley in 1935; it has abiotic (non-living) and biotic (living) components, and the biosphere is the largest ecosystem.
- Producers are autotrophs (green plants, phytoplankton); consumers are heterotrophs; decomposers (bacteria, fungi) return nutrients to the soil; earthworms are detritivores.
- Grazing food chain starts from living plants; detritus food chain starts from dead organic matter and carries more energy in a forest.
- 10 per cent law (Raymond Lindeman, 1942): 10,000 J in plants gives about 1,000 J to herbivores, 100 J to carnivores and 10 J to top carnivores.
- Pyramid of numbers: upright in grassland, spindle-shaped in a tree ecosystem, inverted in a parasitic chain. Biomass pyramid: inverted in the sea. Energy pyramid: always upright.
- Ecological pyramids were introduced by Charles Elton in 1927 and are called Eltonian pyramids.
- Primary succession starts on bare rock with lichens as pioneers; secondary succession starts on abandoned farmland or burnt forest where soil remains, and is faster.
- Gaseous cycles: carbon and nitrogen (reservoir in the atmosphere); sedimentary cycles: phosphorus and sulphur (reservoir in the Earth's crust).
- Nitrogen cycle order: fixation (Rhizobium, Azotobacter, Anabaena, lightning), ammonification, nitrification (Nitrosomonas then Nitrobacter), assimilation, denitrification (Pseudomonas).
- About 71 per cent of the Earth's carbon is dissolved in the oceans (NCERT figure); nitrogen forms about 78 per cent of the air; about 97 per cent of the Earth's water is in the oceans.
Quick revision
Ecosystem term: Tansley 1935; producers, consumers, decomposers; grazing vs detritus food chain; 10 per cent law: Lindeman 1942; Eltonian pyramids: Elton 1927; energy pyramid always upright, biomass inverted in sea, numbers inverted in parasitic chain; succession: lichens pioneer on rock, climax community last; carbon and nitrogen gaseous cycles, phosphorus sedimentary; Nitrosomonas to nitrite, Nitrobacter to nitrate, Pseudomonas denitrifies.