A food chain and a food web are both models of feeding relationships in an ecosystem. A food chain traces a single, straight-line path of energy from one organism to the next, while a food web links many food chains together to show the fuller picture of who eats whom. Ecological pyramids offer a third type of model, stacking organisms by their role to visualize how energy or numbers shrink at each level.
Food Chains: The Simplest Model
A food chain is a linear sequence showing how energy moves from one organism to another. Arrows connect each step, pointing in the direction energy flows. So an arrow from grass to a rabbit means the rabbit eats the grass and gains energy from it. A food chain typically has three to five links, starting with a producer (a plant or algae) and ending with a top predator.
A simple terrestrial example: grass is eaten by a rabbit, the rabbit is eaten by a fox, and the fox is eaten by an eagle. In a marine environment, the chain might run from phytoplankton to krill to herring to cod to a seal. Each link represents one feeding relationship, making food chains easy to follow but oversimplified. In real ecosystems, most animals eat more than one type of food.
Food Webs: A More Realistic Picture
Because animals rarely depend on a single food source, ecologists use food webs to map the full network of feeding relationships. A food web is made up of many interconnected food chains. A fox doesn’t just eat rabbits. It also eats mice, birds, and berries. Meanwhile, rabbits are eaten by foxes, hawks, and snakes. A food web captures all of these overlapping connections.
Food webs better reflect what actually happens in nature. They show why removing one species can ripple through an entire ecosystem. When sea otter populations declined in the Pacific Ocean, for example, sea urchin populations exploded because otters were no longer eating them. The urchins then devoured kelp forests, collapsing habitat for dozens of other species. This kind of chain reaction, called a trophic cascade, is only visible when you look at the web of connections rather than a single chain.
Trophic Levels: The Hierarchy Within the Model
Both food chains and food webs organize organisms into trophic levels based on how they get their energy. The first level contains producers, primarily green plants and algae, which make their own food from sunlight. These organisms are also called autotrophs because they build energy-storing molecules themselves rather than consuming other organisms.
The second level holds primary consumers: herbivores like rabbits, deer, krill, and grasshoppers that eat plants directly. The third level contains secondary consumers, carnivores that eat herbivores. Think frogs eating insects, or cod eating herring. At the fourth level sit tertiary consumers, often apex predators like wolves, sharks, large birds of prey, and big cats that feed on other carnivores.
These categories aren’t rigid. Many animals feed across multiple levels. Bears eat both salmon and berries. Humans eat plants and meat. Organisms that mix plant and animal diets are omnivores, and they blur the neat boundaries between levels.
Decomposers: Closing the Loop
No feeding model is complete without decomposers. Organisms like fungi, bacteria, and detritivores (earthworms, dung beetles, vultures) break down dead plants and animals, recycling nutrients back into the soil and water. Without them, dead matter would pile up and critical elements like nitrogen would be locked away permanently.
Decomposers sit at the bottom of the food chain in one sense, feeding on waste and decay, but they serve every level by making nutrients available again to plants. They close the loop that keeps the entire system running.
The Ten Percent Rule
One of the most important patterns in any feeding model is how much energy is lost at each step. Roughly 10% of the energy consumed at one trophic level gets passed to the next. The rest is burned through cellular processes, lost as body heat, or excreted as waste. If rabbits consume 1,000 calories of plant energy, only about 100 calories become new rabbit tissue available to a fox.
This is why food chains rarely have more than four or five levels. By the time energy reaches a top predator, so little remains that there isn’t enough to support another level above it. It also explains why top predators are relatively rare compared to the herbivores and plants below them.
Ecological Pyramids: Visualizing the Pattern
Ecological pyramids are a third model of feeding relationships, designed to show the shrinking energy, biomass, or number of organisms at each level. An energy pyramid is wide at the base (producers) and narrows sharply at each step up, reflecting that 10% transfer rate. Trophic efficiency varies from about 5% to 20% depending on the ecosystem, but the pyramid shape holds consistently for energy.
Biomass pyramids work similarly, showing that the total weight of all producers in an ecosystem far exceeds the total weight of all herbivores, which exceeds the total weight of all carnivores. A pyramid of numbers simply counts organisms at each level. These pyramids make it immediately obvious why an ecosystem needs a massive base of plants to support even a small number of apex predators.
Why Feeding Models Matter Beyond the Classroom
Feeding models aren’t just textbook diagrams. They reveal how toxins travel through ecosystems. Certain chemicals, like DDT and PCBs, don’t break down easily and are stored in fatty tissue. Tiny concentrations in water get absorbed by phytoplankton, then concentrated further in the small fish that eat vast quantities of phytoplankton, and concentrated again in the bigger fish that eat those small fish. This process, called biomagnification, can push toxin levels in top predators to concentrations millions of times higher than in the surrounding water. Large salmon, lake trout, and fish-eating birds at the top of marine food webs have suffered serious deformities and population declines because of it.
Feeding models also help ecologists predict what happens when species are added or removed. A trophic cascade can work from the top down, as with the sea otters and kelp, or from the bottom up when nutrient supplies to producers change. When fertilizer runoff floods a lake with nutrients, algae populations explode, which shifts everything above them. Understanding the model means understanding the consequences.