A food chain is a series of organisms that sequentially feed on one another, showing the path energy takes as it moves through an ecosystem. It starts with organisms that produce their own food, passes through animals that eat them, and continues upward to top predators. Each step in the chain is called a trophic level, and the number of levels an ecosystem can support is limited by how much energy gets lost along the way.
How a Food Chain Is Structured
Every food chain begins with producers, the organisms that convert nonliving resources into food. On land, these are plants. In oceans and lakes, they’re algae and tiny floating organisms called phytoplankton. Most producers use sunlight to build sugars from carbon dioxide and water through photosynthesis. In places sunlight can’t reach, like deep-sea hydrothermal vents, certain bacteria use chemical energy instead, a process called chemosynthesis. Together, these two processes fuel all life on Earth.
From there, energy flows upward through consumers:
- Primary consumers eat only producers. These are herbivores like deer grazing on prairie grass, grasshoppers feeding on leaves, or zooplankton consuming microscopic algae in the water.
- Secondary consumers eat primary consumers. Many are carnivores, like egrets or alligators. Others are omnivores, like raccoons, which eat both plants and smaller animals such as crayfish and frogs.
- Tertiary consumers eat other predators. A bald eagle, for instance, feeds on predatory fish and snakes that themselves eat smaller herbivores. These top-level predators typically have no natural predators of their own.
A simple marine food chain illustrates this neatly: phytoplankton are eaten by zooplankton or krill, which are eaten by small fish, which are eaten by larger predatory fish or marine mammals. Each link in that chain represents one trophic level.
The 10 Percent Rule
Energy doesn’t transfer efficiently between trophic levels. On average, only about 10 percent of the energy available at one level passes to the next. The rest is lost, mostly as heat generated during normal metabolic processes like breathing, moving, and maintaining body temperature. Some organisms also die without being eaten, meaning their stored energy never reaches a consumer at all.
This massive energy loss is the reason food chains rarely exceed four or five levels. By the time you reach a top predator, only a tiny fraction of the original solar energy captured by plants remains. It’s also why producers vastly outnumber herbivores, herbivores outnumber small predators, and top predators are the rarest organisms in any ecosystem. Picture it as a pyramid: a broad base of plant life supporting progressively thinner layers of animal life above it.
Decomposers Close the Loop
Food chains don’t just end at the top predator. When any organism dies, or when animals produce waste, decomposers break that material down and return its nutrients to the soil or water. Fungi, bacteria, and small invertebrates all play this role. Without them, dead matter would pile up and the nutrients locked inside it would never become available to plants again.
Some decomposers do more than just break things down. Burrowing invertebrates like isopods (pill bugs and their relatives) consume dead plant matter, digest it, and excrete nutrients in already mineralized forms that plants can absorb directly. Their burrows act as channels that transport nitrogen and phosphorus deeper into the soil, creating pockets of fertility in otherwise nutrient-poor ground. In desert ecosystems, this burrowing activity can regulate the entire pace of nutrient recycling.
Food Chains vs. Food Webs
A food chain is a useful simplification, but real ecosystems are messier. Most animals eat more than one type of food, and most prey species are eaten by more than one predator. A raccoon eats berries, acorns, crayfish, frogs, and fish. A bald eagle eats both predatory fish and snakes. These overlapping relationships mean that individual food chains connect to each other, forming a food web: a network of interconnected chains that more accurately represents how energy flows through an ecosystem.
Food chains are still valuable as a teaching tool because they isolate a single pathway and make the direction of energy flow easy to follow. But when ecologists study real ecosystems, they work with food webs, because disruptions to one species can ripple outward in ways a single chain wouldn’t predict.
What Happens When a Link Breaks
Removing a species from a food chain can trigger a domino effect called a trophic cascade. The classic pattern works like this: when a top predator disappears, the herbivores it kept in check multiply. Those herbivores then overgraze, stripping vegetation. The result can be a landscape that shifts from green and diverse to barren.
Ecologists have observed this pattern across many ecosystems. The basic insight is straightforward: the world stays green largely because predators keep herbivore populations low enough that plants aren’t eaten faster than they can regrow. When predators are removed, herbivore numbers surge and plant communities can be completely stripped away. This is why apex predators, despite being rare, play an outsized role in maintaining biodiversity. Their presence at the top of the food chain stabilizes every level below them.
Apex predators also tend to regulate their own population sizes through slow reproduction, large territories, and behaviors like infanticide and reproductive suppression. This self-regulation prevents them from overconsuming their prey, which keeps the entire chain in balance.
Terrestrial vs. Aquatic Food Chains
The basic structure of a food chain is the same on land and in water, but the specific organisms filling each role differ. On land, plants are the primary producers, herbivores like insects and deer are primary consumers, and predators like hawks and wolves sit at higher levels. In aquatic systems, the base is built from algae, phytoplankton, and aquatic plants. Herbivorous fish and invertebrates occupy the next level, followed by fish that eat invertebrates, and finally large predatory fish at the top.
Aquatic food chains often have one more trophic level than terrestrial ones. This is partly because aquatic producers like phytoplankton are tiny and consumed very quickly, creating room for longer chains of progressively larger predators. A freshwater or marine chain might run five levels deep: from microscopic algae to zooplankton to small fish to medium fish to a large predatory species like a tuna or shark.