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.
Understanding Food Chains, Food Webs, and Energy Pyramids
Energy moves through an ecosystem in one direction, but matter does not. A rabbit uses much of the chemical energy in grass to stay warm, move, grow, breathe, and reproduce. That energy eventually leaves the living system as heat.
The atoms in the grass follow a different path. Carbon may leave as carbon dioxide during respiration. Nitrogen and minerals can become part of rabbit waste or its body.
When the rabbit dies, these materials remain available for other organisms after decay. This difference between energy flow and nutrient cycling is important. Ecosystems need a continuing energy source, while matter is repeatedly reused.
The amount of plant material made in an area is called primary productivity. It depends on light, water, temperature, carbon dioxide, and nutrients. A sunny grassland can support many grazing animals because grasses grow quickly.
A desert has less available water, so it produces less plant tissue and supports fewer large consumers. In lakes and oceans, tiny algae may supply most of the productivity. They are easy to miss because each organism is small, yet they can feed huge numbers of animals.
When studying a diagram, notice that the widest level represents the greatest available energy or biomass. It does not always represent the largest individual organisms.
Many feeding relationships begin with dead material rather than a living plant. Fallen leaves, animal droppings, and dead bodies form detritus. Earthworms, woodlice, fungi, bacteria, and many insects use this material.
Fungi release enzymes outside their bodies, then absorb small molecules from the material they break down. Bacteria perform similar work at a microscopic scale. This pathway is essential because nutrients locked in dead tissue would otherwise become unavailable.
Soil fertility, forest leaf litter, compost heaps, and rotting logs all show decomposition in everyday life. Decomposition usually happens faster in warm, moist conditions because decomposers are more active there.
Food webs can change when one population rises or falls. If a predator becomes rare, its prey may increase and consume more vegetation. This can affect insects, birds, soil conditions, and water quality.
Such connected effects are called trophic cascades. People can alter webs through hunting, habitat loss, invasive species, pesticides, and pollution. Some pollutants break down very slowly and collect in body tissues.
A small amount in water can become a much higher concentration in animals near the top of a web because each predator consumes many prey. When reading food web questions, follow the arrow direction carefully.
In most school diagrams, an arrow shows energy moving from the organism being eaten to the eater. Trace several possible paths, not just one, and remember that an organism can occupy different feeding levels when its diet changes.
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.