Food Chains, Food Webs, and Energy Pyramids

Producers, Consumers, Decomposers, and Trophic Levels

Related Labs

A food chain is a linear sequence showing how energy and matter pass from one organism to another through feeding relationships. Producers (plants, algae, cyanobacteria) capture solar energy via photosynthesis and form the base of every food chain. Primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; and so on up the chain. Each feeding position is called a trophic level.

Real ecosystems are rarely linear — organisms eat and are eaten by many species, forming food webs. Energy pyramids illustrate a critical constraint: only about 10% of energy stored in one trophic level is transferred to the next. The other 90% is lost as heat, used for the organism's own metabolism, or incorporated into non-digestible biomass. This inefficiency is why food chains rarely exceed four or five links, and why it takes far more plant matter to support a population of carnivores than herbivores.

Key Facts

Producers (autotrophs) convert sunlight or chemicals into organic matter; all energy enters here
10% rule: roughly 10% of energy transfers between trophic levels; 90% is lost as heat
Trophic levels: Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer
Decomposers (bacteria, fungi) break down dead organic matter and return nutrients to soil
Food webs show realistic many-to-many feeding relationships; food chains are simplifications
Biomagnification: concentration of toxins (e.g. DDT, mercury) increases at higher trophic levels

Vocabulary

Producer: An organism that makes its own food from inorganic sources (photosynthesis or chemosynthesis); the first trophic level.
Consumer: An organism that obtains energy by eating other organisms; classified as primary, secondary, or tertiary based on trophic level.
Decomposer: An organism (bacteria or fungi) that breaks down dead organic matter, recycling nutrients back into the ecosystem.
Trophic level: The feeding position of an organism in a food chain, numbered from producers (level 1) upward.
Biomagnification: The increasing concentration of a persistent toxin in organisms at successively higher trophic levels.

Common Mistakes to Avoid

Thinking energy is recycled like matter. Matter (carbon, nitrogen) cycles through ecosystems, but energy flows one-way: it enters as sunlight and exits as heat and cannot be reused.
Assuming decomposers are consumers. Decomposers form their own category; they break down non-living organic matter and do not prey on living organisms.
Confusing food chains with food webs. A food chain is one pathway; a food web shows all the interconnected feeding relationships in an ecosystem.
Forgetting the direction of arrows in food chains. Arrows point in the direction of energy flow (from prey to predator), not from predator to prey.

Practice Questions

1 If a meadow ecosystem contains 10,000 kJ at the producer level, how much energy is theoretically available to a secondary consumer? Show your reasoning.
2 Explain why a human who eats beef uses land and energy far less efficiently than one who eats grain directly.
3 A pesticide that does not break down is found in lake water at 0.0002 ppm. Predict what concentration might be found in a fish-eating bird at the top of the food web, using the 10× biomagnification rule per level.