A food chain is the series of animals, where at each level, called trophic level, energy is transferred from the first to the next organism. Plants take up only 1% of the energy from the sun, and after that only 10% of the energy is transferred at each trophic level.

The food chain give a definite structure to an ecosystem or biosphere (through its trophic levels).Energy and material transfer occurs. A food chain will never have more than 6 trophic levels. Each level looses 90% of energy , that is only 10% is received by successive levels.

The food chain`s length is a continuous variable that provides a measure of the passage of energy and an index of ecological structure that increases in value counting progressively through the linkages in a linear fashion from the lowest to the highest trophic (feeding) levels.[7] Food chains are often used in ecological modeling (such as a three species food chain). They are simplified abstractions of real food webs, but complex in their dynamics and mathematical implications.[2] Ecologists have formulated and tested hypotheses regarding the nature of ecological patterns associated with food chain length, such as increasing length increasing with ecosystem size, reduction of energy at each successive level, or the proposition that long food chain lengths are unstable.[7] Food chain studies have had an important role in ecotoxicology studies tracing the pathways and biomagnification of environmental contaminants.[8] Food chains vary in length from three to six or more levels. A food chain consisting of a flower, a frog, a snake and an owl consists of four levels; whereas a food chain consisting of grass, a grasshopper, a rat, a snake and finally a hawk consists of five levels. Producers, such as plants, are organisms that utilize solar energy or heat energy to synthesize starch. All food chains must start with a producer. In the deep sea, food chains centered around hydrothermal vents exist in the absence of sunlight. Chemosynthetic bacteria and archaea can use hydrogen sulfide from hydrothermal vents as an energy source (just as plants use sunlight) to produce carbohydrates; they form the base of the food chain. Consumers are organisms that eat other organisms: in most food chains, all the organisms in a food chain are consumers. In a deep-sea food chain, tube worms, clams, and mussels harbor the chemosynthetic bacteria and make use of the food they produce. They are all eaten by crabs, which in turn they may be consumed by an octopus.

A food chain is a linear sequence of links in a food web starting from a that are called producers in the web and ends at a species that is called decomposers species in the web. A food chain also shows how the organisms are related with each other by the food they eat.
Food chains vary in length from three to six or more levels.
A food chain consisting of a flower, a frog, a snake and an owl consists of four levels; whereas a food chain consisting of grass, a grasshopper, a rat, a snake and finally a hawk consists of five levels.

A food chain starts with the primary energy source, usually the sun or boiling-hot deep sea vents. The next link in the chain is an organism that make its own food from the primary energy source -- an example is photosynthetic plants that make their own food from sunlight (using a process called photosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs or primary producers.

Next come organisms that eat the autotrophs; these organisms are called herbivores or primary consumers -- an example is a rabbit that eats grass.

The next link in the chain is animals that eat herbivores - these are called secondary consumers -- an example is a snake that eat rabbits.

In turn, these animals are eaten by larger predators -- an example is an owl that eats snakes.

The tertiary consumers are are eaten by quaternary consumers -- an example is a hawk that eats owls. Each food chain end with a top predator, and animal with no natural enemies (like an alligator, hawk, or polar bear).

The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step. A network of many food chains is called a food web.