Food Webs

Food webs show how living things in an ecosystem are connected by feeding relationships and energy flow. They matter because every organism depends directly or indirectly on others for food, survival, and balance. Producers capture energy, consumers transfer it through feeding, and decomposers return matter to the environment. Studying food webs helps explain why changes to one species can affect many others.

Energy enters most ecosystems from the Sun and is stored by producers through photosynthesis. Herbivores eat producers, carnivores eat other animals, omnivores eat both plants and animals, and scavengers feed on dead organisms. Decomposers such as fungi and bacteria break down waste and remains, releasing nutrients back into soil and water. Energy flows in one direction through the web, while nutrients are recycled and reused.

Key Facts

Producers make food using sunlight, usually by photosynthesis: 6CO2 + 6H2O -> C6H12O6 + 6O2
Energy flow often begins with the Sun and moves from producers to primary, secondary, and higher consumers.
Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
Only about 10% of energy is typically passed to the next trophic level.
Decomposers recycle matter by breaking down dead organisms and waste into simpler nutrients.
Food chains are single feeding paths, while food webs show many interconnected feeding relationships.

Vocabulary

Producer: An organism that makes its own food, usually by using sunlight in photosynthesis.
Consumer: An organism that gets energy by eating other organisms.
Decomposer: An organism such as a fungus or bacterium that breaks down dead matter and waste.
Trophic level: A feeding position in a food web, such as producer, primary consumer, or secondary consumer.
Scavenger: An animal that feeds on dead organisms but does not fully break them down into nutrients.

Common Mistakes to Avoid

Confusing energy flow with nutrient cycling, because energy does not get recycled in ecosystems while nutrients like carbon and nitrogen do.
Drawing food web arrows in the wrong direction, because arrows should point from the food source to the organism that receives the energy.
Assuming decomposers are the same as scavengers, because scavengers eat dead organisms but decomposers chemically break them down into simpler substances.
Thinking top predators have the most energy, because energy decreases at each trophic level and the greatest available energy is at the producer level.

Practice Questions

1 A grass plant stores 1000 J of energy from sunlight. If only about 10% passes to a rabbit and then 10% of that passes to a fox, how much energy reaches the fox?
2 In a pond food chain, algae -> insect larvae -> small fish -> heron. Identify the producer, primary consumer, secondary consumer, and tertiary consumer.
3 If a disease greatly reduces the decomposer population in a forest, explain how this would affect nutrient availability and the rest of the food web.