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Ecosystem structure, food chains and energy flow

Ecology basics · Part 1 / 2 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Ecology and its levels of organisation, habitat and niche; the ecosystem and its abiotic and biotic parts; producers, consumers, decomposers and detritivores; grazing and detritus food chains and food webs; one-way energy flow and the ten per cent law; gross and net primary productivity; pyramids of number, biomass and energy; bioaccumulation and biomagnification.

Environment and Ecology in the Prelims rests on one idea: living things and their surroundings form working units, and what happens to one part of such a unit travels to the rest. That unit is the ecosystem. A pond, a forest, a grassland, a coral reef or a paddy field each counts as one, large or small. This first part of the subject explains how an ecosystem is built and how it runs: its living and non-living parts, who eats whom, how energy passes from the sun through plants and animals, how much each ecosystem produces, how these relations are drawn as pyramids, and why some poisons grow stronger as they climb a food chain. Part 2 turns to how ecosystems change over time and how nutrients cycle through them.

Ecology studies the ties between living things and between them and their surroundings. The term dates from 1866, when Ernst Haeckel, a biologist from Germany, built it on the Greek word 'oikos', meaning a house or home. Ecologists study life at rising levels of organisation. An organism is a single living being; a population is all the members of one species living in an area; a community is all the populations of different species sharing that area; an ecosystem is the community together with its non-living environment; a biome is a large region with a similar climate and type of vegetation, such as the tropical rainforest or the desert; and the biosphere is the thin layer of Earth where life exists. Two further words are easily confused. Habitat answers the question 'where does this species live?', naming the physical surroundings it is found in. Niche answers a different question, 'what does it do there?', covering its whole way of making a living: what it feeds on, at what hour it is active and which conditions it cannot do without.

The British ecologist Arthur Tansley introduced the term ecosystem in 1935 to stress that organisms cannot be studied apart from their physical setting. Every ecosystem has two kinds of components. The abiotic, or non-living, components include sunlight, temperature, water, air, soil and dissolved nutrients such as nitrogen and phosphorus. The biotic, or living, components are sorted by how they get food. Autotrophs, the producers, make their own food: green plants, algae and some bacteria do it by photosynthesis, using sunlight. A few bacteria are chemosynthetic and draw energy from chemical reactions instead; around hydrothermal vents on the deep sea floor, where no light reaches, whole communities depend on them. Heterotrophs cannot make their own food and must eat other organisms. Ecosystems may be natural, such as forests, deserts, lakes and oceans, or made by people, such as crop fields, aquariums and reservoirs; they may also be classed as terrestrial or aquatic, and aquatic ones as freshwater or marine.

Heterotrophs are of several kinds. Herbivores such as deer, cattle, grasshoppers and many zooplankton eat plants and are primary consumers. Carnivores that eat herbivores, such as frogs or small fish, are secondary consumers, and those that eat other carnivores, such as snakes, hawks or large fish, are tertiary consumers. Omnivores, such as bears, crows and humans, take food from more than one level. At the end of every chain stand the organisms that live on dead matter. A decomposer, mainly bacteria and fungi, breaks dead plants, animals and wastes down chemically, releasing simple nutrients back into soil and water for producers to take up again. Detritivores such as earthworms, millipedes, termites and dung beetles feed on dead organic matter and break it into smaller pieces, which speeds up the decomposers' work. Without decomposers, nutrients would stay locked in dead bodies and producers would soon starve.

A food chain is the sequence in which food energy passes from one organism to the next. The grazing food chain starts with living green plants: grass is eaten by a grasshopper, the grasshopper by a frog, the frog by a snake and the snake by an eagle. In a pond the same pattern runs from phytoplankton to zooplankton to small fish to large fish to a fish-eating bird. The detritus food chain starts instead with dead organic matter, such as fallen leaves, dead animals and dung, which is eaten by detritivores and decomposers, which are in turn eaten by small animals and so on. The two chains are linked, because everything in the grazing chain eventually dies and enters the detritus chain. In many forests a large share of the energy fixed by plants never passes through herbivores at all; it reaches the rest of the ecosystem through fallen leaves and wood and the detritus route.

Real ecosystems are not single chains. Most animals eat several kinds of food and are eaten by several predators, so chains cross and join into a food web. Each step in a chain is a trophic level: producers are the first level, herbivores the second, carnivores that eat herbivores the third, and so on. An omnivore may sit on more than one level at once; a person eating rice is a primary consumer, and the same person eating fish is a secondary or tertiary consumer. Webs make ecosystems more stable. If one prey species declines, a predator with several food sources can switch to another, while in a simple chain the loss of one link can bring down everything above it. For this reason species-rich ecosystems such as tropical forests are generally better at absorbing shocks than simple ones such as monoculture crop fields.

Energy enters almost every ecosystem as sunlight, and producers capture only a small part of it, commonly put at around one per cent of the light falling on them. From there energy flows in one direction only: from producers to herbivores to carnivores, never back to the sun. At each step most of it is used for living, in breathing, moving and keeping warm, and leaves the ecosystem as heat, in line with the laws of thermodynamics: energy is not created or destroyed, but every transfer loses some as heat that cannot be reused. In 1942 the American ecologist Raymond Lindeman described this pattern, now taught as the ten per cent law: on average only about a tenth of the energy at one trophic level is passed on to the next. If plants store 10,000 units, herbivores get about 1,000, first carnivores about 100 and second carnivores about 10. That is why food chains seldom have more than four or five levels, and why top predators such as tigers are always few.

Productivity is the rate at which an ecosystem builds new organic matter. Gross primary productivity is the total rate at which producers fix energy by photosynthesis. Producers use part of this for their own respiration, and what remains, net primary productivity (gross minus respiration), is the food available to the rest of the ecosystem. The rate at which consumers build their own tissue is secondary productivity. Productivity depends mainly on light, warmth, water and nutrients. Per unit of area, tropical rainforests, coral reefs, estuaries, swamps and marshes are among the most productive ecosystems on Earth, while deserts, tundra and the open ocean are among the least. The open ocean is a useful trap: each square kilometre produces little, because nutrients are scarce far from land, but because it is so vast its total output is very large.

An ecological pyramid shows the trophic levels of an ecosystem as stacked bars, producers at the base and top carnivores at the apex; the British ecologist Charles Elton described the idea in 1927. There are three kinds. A pyramid of numbers counts individuals at each level. It is usually upright, as in a grassland with countless grass plants, fewer grasshoppers and very few hawks, but it is inverted where one large producer feeds many small consumers, as when a single banyan tree supports thousands of insects and the birds that eat them. A pyramid of biomass weighs the living matter at each level. It is upright on land but often inverted in the open sea, where the phytoplankton present at any moment weigh less than the zooplankton and fish they feed, because they grow and are eaten so quickly. A pyramid of energy measures the energy flowing through each level over time. Because energy is lost at every step, it can never be inverted.

Energy thins out as it climbs a food chain, but some poisons do the opposite. Bioaccumulation is the build-up of a chemical inside one organism over its lifetime, because the body takes it in faster than it can break it down or excrete it. biomagnification is the rise in concentration of such a chemical from one trophic level to the next, because each predator eats many contaminated prey over its life. Only substances that last long, dissolve in fat rather than water and are not easily broken down behave this way: DDT and other organochlorine pesticides, methylmercury, and some industrial compounds such as PCBs. In the mid-twentieth century DDT moving up aquatic food chains made the eggshells of fish-eating birds such as pelicans, ospreys and eagles thin enough to crack, a harm made famous by Rachel Carson's book Silent Spring in 1962. In Minamata, Japan, in the 1950s, mercury released by a factory concentrated in fish and shellfish and poisoned the people who ate them.

Putting the pieces together: an ecosystem is a community of producers, consumers and decomposers joined to its non-living surroundings, and its working can be read through two flows. Energy enters as sunlight, is fixed by producers, passes up the trophic levels with about nine-tenths lost at each step, and leaves as heat; it flows once and does not return. Matter behaves differently: the same atoms of carbon, nitrogen and phosphorus are used again and again, released by decomposers and taken up by producers. Food webs give ecosystems resilience, productivity sets how much life they can carry, and pyramids summarise the whole structure, with the energy pyramid alone always upright. Biomagnification shows the darker side of the same links: a pollutant released at the bottom of a chain ends up most concentrated in the animals at the top, including people. Part 2 follows the matter side of this story through succession and the biogeochemical cycles.

Exam traps

• HABITAT is where a species lives (its address); NICHE is its role there (its profession). They are not synonyms.
• Haeckel coined 'ecology' (1866); Tansley coined 'ecosystem' (1935); Lindeman gave the ten per cent law (1942); Elton described ecological pyramids (1927).
• Not all producers use sunlight: chemosynthetic bacteria at deep-sea hydrothermal vents make food from chemical energy.
• Decomposers (bacteria, fungi) break matter down chemically; detritivores (earthworms, termites) eat dead matter and break it into pieces. Earthworms are detritivores, not decomposers.
• ENERGY flows one way and is not recycled; NUTRIENTS cycle. Only about 10% of energy passes from one trophic level to the next.
• Net primary productivity = gross primary productivity minus the producers' respiration.
• The pyramid of ENERGY is always upright; pyramids of NUMBER (a single tree) and BIOMASS (open sea) can be inverted.
• Biomagnification affects persistent, fat-soluble chemicals (DDT, methylmercury, PCBs), not water-soluble ones; concentration is highest at the TOP of the chain.

Recap

• Ecology (Haeckel, 1866); levels: organism, population, community, ecosystem, biome, biosphere; habitat = address, niche = role.
• Ecosystem (Tansley, 1935) = biotic community + abiotic environment; natural or artificial, terrestrial or aquatic.
• Producers: photosynthetic plants and algae, chemosynthetic bacteria; consumers: herbivores, carnivores, omnivores; decomposers (bacteria, fungi) and detritivores (earthworms).
• Grazing chain starts with living plants; detritus chain with dead organic matter; chains interlock into food webs, which add stability.
• Energy flows one way; about 10% passes up each trophic level (Lindeman, 1942); chains rarely exceed 4–5 levels.
• NPP = GPP − respiration; most productive: rainforests, reefs, estuaries, wetlands; least: deserts, tundra, open ocean (large total).
• Pyramids (Elton, 1927): number and biomass can be inverted; energy never.
• Biomagnification of persistent fat-soluble chemicals: DDT and thin eggshells (Silent Spring, 1962); mercury at Minamata (1950s).

Practice questions

Sources

• NASA Earth Observatory: net primary productivity, photosynthesis and the global carbon budget — https://earthobservatory.nasa.gov
• NOAA Ocean Service and Ocean Exploration: marine food webs, phytoplankton, and chemosynthetic communities at hydrothermal vents — https://oceanservice.noaa.gov
• Ministry of Environment, Forest and Climate Change (ENVIS resource material): ecosystems, food chains, energy flow, productivity and ecological pyramids — https://moef.gov.in
• US Geological Survey: mercury and other contaminants in food webs, bioaccumulation and biomagnification — https://www.usgs.gov
• US Environmental Protection Agency: DDT, a brief history and status; persistent bioaccumulative toxic chemicals — https://www.epa.gov