Food chain

1. Meaning, components and trophic levels

A food chain traces a particular feeding pathway in an ecosystem. In grass → grasshopper → frog → snake, grass supplies organic matter to the grasshopper, which is eaten by the frog, which may be eaten by the snake. Arrows indicate the direction in which food-derived energy and matter move, not the direction in which a predator attacks its prey. A chain is therefore a simplified model of biological interactions.

Each feeding position is called a trophic level. Autotrophic producers occupy the first trophic level. They manufacture organic compounds from inorganic substances, usually through photosynthesis. Terrestrial producers include grasses and trees; aquatic producers include phytoplankton, algae and aquatic plants. In ecosystems supported by chemosynthesis, certain bacteria and archaea use energy from chemical reactions rather than sunlight.

Primary consumers feed on producers and usually occupy the second trophic level. Secondary consumers eat primary consumers, while tertiary consumers feed on secondary consumers. These labels describe feeding roles rather than permanent ranks assigned to species. An omnivore such as a human occupies a lower trophic level when eating plants than when eating herbivorous animals. A top predator is not necessarily a tertiary consumer in every feeding pathway.

• Producer: grass, phytoplankton or a chemosynthetic microorganism.
• Primary consumer: grasshopper, deer or herbivorous zooplankton.
• Secondary consumer: a frog eating herbivorous insects or a fish eating herbivorous zooplankton.

2. Grazing, detritus and other feeding pathways

A grazing food chain begins with living producers, followed by herbivores and their consumers. Examples include grass → deer → tiger and phytoplankton → herbivorous zooplankton → small fish → larger fish. In many aquatic ecosystems, grazing transfers a substantial share of primary production to consumers. However, the relative importance of grazing varies with ecosystem conditions and the palatability of producers.

A detritus food chain begins with dead organic matter, including fallen leaves, carcasses and faecal material. This material supports decomposer microorganisms and detritivores, which in turn support other consumers. Leaf litter → earthworm → bird illustrates one detrital pathway. In many terrestrial ecosystems, much of plant production enters the detrital pool rather than being directly consumed by herbivores.

Detritivores, such as earthworms, ingest detritus and help fragment it. Decomposer bacteria and fungi secrete enzymes and absorb the products of external digestion. Mineralisation releases inorganic nutrients that producers can reuse. Decomposers act on material originating from all trophic levels; placing them only at the end of a single chain conceals their ecosystem-wide role.

Parasitic pathways transfer resources from hosts to parasites and sometimes to hyperparasites. They demonstrate why a food chain need not progress from smaller organisms to larger ones. Grazing and detritus pathways are also interconnected: consumers produce waste and eventually die, while organisms supported by detritus become prey in wider food webs.

3. Energy transfer, productivity and ecological pyramids

Energy enters most ecosystems as sunlight captured by producers. Gross primary productivity is the total rate at which producers fix energy into organic matter. Net primary productivity equals gross primary productivity minus producer respiration: NPP = GPP − R. NPP represents new producer biomass potentially available to consumers and decomposers, although not all of it is immediately eaten.

At each trophic transfer, some material remains uneaten, some ingested food is not assimilated, and much assimilated energy supports metabolism and is eventually dissipated as heat. Only part becomes new consumer biomass. The ten per cent law, associated with Raymond Lindeman's trophic-dynamic framework, approximates transfer to the next trophic level. Actual ecological efficiencies vary with organisms, food quality and environmental conditions.

Under the simplified ten per cent assumption, 10,000 units of producer energy support about 1,000 units at the primary-consumer level and 100 at the secondary-consumer level. Progressive energy limitation helps explain why food chains generally have relatively few trophic levels. Energy is not destroyed; it becomes dispersed as heat and cannot be recycled through the ecosystem in the same way as nutrients.

An energy pyramid is always upright when energy flow is expressed per unit area per unit time. Biomass pyramids can be inverted in aquatic systems because rapidly reproducing phytoplankton may maintain a small standing biomass while supporting a larger consumer biomass. A numbers pyramid may be inverted when one tree supports many insects. Neither inversion contradicts declining energy availability across trophic levels.

4. Food webs, regulation and ecosystem change

Natural communities usually contain food webs: networks of interconnected food chains. A frog may consume several insect species and be eaten by snakes, birds or mammals. These overlapping relationships provide a more realistic picture than one linear chain. Feeding relationships may also change with season, habitat and an organism's life stage.

Bottom-up regulation occurs when resource availability, such as nutrients or primary production, influences higher trophic levels. Top-down regulation occurs when consumers influence organisms at lower levels. A trophic cascade is an indirect effect transmitted across feeding levels; for example, predator decline may increase herbivores and consequently reduce vegetation.

Alternative prey and feeding pathways can buffer some disturbances, but a complex food web is not automatically stable. Outcomes depend on interaction strengths, species identities, habitat conditions and the nature of disturbance. Overfishing, pesticides, invasive species and habitat fragmentation can disrupt feeding links. Conservation therefore requires protection of prey, producers and habitats, not merely conspicuous predators.

5. Pollutants and examination-oriented distinctions

Bioaccumulation is the build-up of a substance within an organism through uptake from food and the surrounding environment when elimination is insufficient. Biomagnification is an increase in concentration across successive trophic levels. Persistent substances that are readily taken up and poorly eliminated, such as methylmercury and certain organochlorine compounds, can biomagnify.

DDT-related compounds provide a classic example. Their movement through food webs affected fish-eating and predatory birds; DDE, a breakdown product of DDT, is strongly associated with eggshell thinning in susceptible species. Methylmercury can reach high concentrations in long-lived predatory fish. However, not every pollutant or heavy metal biomagnifies: chemical form, bioavailability and biological processing matter.

For Prelims, distinguish concentration from total quantity. A top predator can have a high pollutant concentration even though the total biomass at its trophic level is small. Also remember that nutrients cycle between organisms and the physical environment, whereas usable energy requires continuing input. Food chains describe feeding pathways, not necessarily increasing body size or the number of organisms consumed.

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