A chain is a simplification
A food chain is a line that shows who eats whom. Grass is eaten by a grasshopper, the grasshopper is eaten by a frog, the frog is eaten by a snake, and the snake is eaten by a hawk. Each arrow points in the direction the energy travels, from the eaten to the eater.
Food chains are useful for teaching, but almost no animal eats just one thing. The frog also eats flies and beetles. The hawk eats mice and small birds as well as snakes. When you draw all these overlapping links on one diagram, the straight line turns into a net. That net is a food web, and it is a much more realistic picture of how a community is connected.
The levels of a chain
Ecologists label each step a trophic level, from a Greek-derived word for feeding.
- Producers sit at the bottom. Plants, algae and some bacteria capture energy from sunlight, or in a few unusual places from chemicals, and build it into living tissue.
- Primary consumers eat producers. These are herbivores such as caterpillars, rabbits and zooplankton.
- Secondary consumers eat primary consumers. Small predators such as frogs, spiders and many fish belong here.
- Tertiary consumers and top predators eat other predators. Examples include hawks, orcas and large sharks.
Many animals do not fit a single level neatly. Omnivores such as bears and humans eat from several. A fox may eat berries one day and a mouse the next. This is one reason webs are more accurate than chains.
Decomposers close the loop
Every web also includes organisms that feed on dead material. Detritivores, such as earthworms and woodlice, eat dead leaves and waste. Decomposers, mainly fungi and bacteria, break dead material down further. Their work returns nutrients to the soil or water where producers can take them up again. Without them, nutrients would stay locked in dead matter. In many ecosystems, more energy passes through this decomposer route than through living plant-eaters.
Why energy thins out
Energy does not pass along a chain without loss. When an animal eats, much of the energy in its food is used for moving, staying warm, growing and keeping its body working, and is eventually released as heat. Some of the food is never digested. Only a fraction ends up as new body tissue that the next predator can eat.
A common rule of thumb says that roughly 10 percent of the energy at one level is available to the next. The real figure varies quite a lot, depending on the organisms and the ecosystem, so treat 10 percent as a rough illustration rather than a law. As a simple example with round numbers, if grass in a field captured 10,000 units of energy, about 1,000 might reach the grasshoppers, about 100 the frogs and about 10 the snakes. The exact numbers will differ in real places, but the pattern of steep decline holds.
This is why food chains are usually short, often only three to five links, and why top predators tend to be fewer and rarer than the animals they eat. There is simply less energy available at the top to support large populations. Energy follows a one-way route: it enters as sunlight and leaves as heat. Nutrients such as nitrogen and carbon, on the other hand, are recycled, a distinction that is central to the idea of an ecosystem.
Pyramids of numbers and biomass
Ecologists sometimes draw this thinning as a pyramid, with producers forming a wide base. A pyramid can show the amount of energy at each level, the total mass of living material (biomass), or the number of individuals. Energy pyramids are always wider at the bottom. Number and biomass pyramids can sometimes look odd. One large tree supports thousands of insects, so a count of individuals can be upside down. In some ocean systems, the standing mass of tiny algae at a given moment can be smaller than the mass of the animals eating them, because the algae reproduce so fast that they are replaced very quickly.
Why webs matter
A web shows that species depend on each other in indirect ways. If a disease reduces one prey species, a predator may switch to another, which then declines in turn. If a top predator disappears, the animals it ate may increase and eat more of their own food. Ecologists call such chains of effects trophic cascades. Some well-known examples have been described in lakes, coastal kelp forests and on land, though how strong these effects are varies from place to place and is still debated.
Webs also show that some species are especially important. A keystone species is one whose effect on its community is much larger than its numbers would suggest. Sea otters that eat sea urchins, which in turn graze kelp, are often given as an example. Webs with many alternative links are generally thought to cope better with the loss of one species than webs with few, which is part of why variety in life matters for resilience, a theme taken up in the article on species biodiversity.
Humans in the web
People are part of food webs too. We eat plants, herbivores and fish at several levels. Fishing often targets large predatory fish at the top of marine webs, and over time this can change what is left in the water. Pollutants can also move through webs. Some persistent chemicals build up in animals’ bodies and become more concentrated at higher levels, a process called biomagnification. It is one reason top predators can be affected by substances that are very dilute in the environment.
Understanding webs does not let us predict exactly what will happen when something changes, because real communities are complicated. But it helps explain why wildlife management so often needs to consider several species at once.