Sunlight is the primary source of energy in most ecosystems. Producers, such as plants and photosynthetic microorganisms, capture this light energy and convert it to chemical energy stored in sugar molecules that they can then use as food. Producers are at the beginning of all food chains because they are the only organisms that can manufacture their own food from energy that does not come from another organism. In the food chain shown above, the producer is a plant. Organisms that feed on other living organisms are consumers. In a food chain, the second organism is a consumer that eats producers, although it may not necessarily feed on producers exclusively. Organisms that do feed exclusively on producers are called herbivores. Organisms that feed on both producers and other consumers are called omnivores. In the food chain shown above, this level is represented by the grasshopper. The next organisms in a food chain must be consumers that feed on other consumers, although they may feed on producers as well. The frog, snake, and hawk in the food chain above all feed on consumers. The frog is an omnivore, because it also feeds on plant material during at least part of its life cycle. The snake and hawk are carnivores because they eat only other animals. All organisms in the food chain are eventually broken down by decomposers, such as worms, bacteria, and fungi. Decomposers are frequently not shown in food chains, although the wastes and remains of all organisms in an ecosystem are eventually fed on by decomposers.
Energy in Food Chains Producers and consumers both release the energy stored in food molecules through oxidation, which is part of cellular respiration. Some of the released energy is used by the organisms to perform the processes that are necessary for life. The rest of the energy remains in food parts that are excreted as wastes, or it is given off as heat. Because most of the energy in a food chain is used or lost to the environment as it moves up the chain, the bottom level of the food chain contains the most stored energy. In fact, this is where most of the energy in the entire ecosystem can be found. Only about 10% of the energy produced at each level is available to the one above it. For this reason, the higher up in the food chain an organism is, the smaller the size of its population. The diagram below represents the amount of energy available to organisms at four levels in a food chain. Interpreted in terms of the food chain shown above, snakes have access to only 0.1% of the energy present in all the plants in their ecosystem. Because they cannot make use of a larger amount of energy, and therefore there is less energy to sustain each individual snake, there are fewer snakes than plants, grasshoppers, or frogs. The hawks can access even less of the ecosystem's energy, so there are fewer hawks than snakes.
Food Webs Most animals feed on more than one type of organism, and many plants and animals are fed upon by more than one kind of animal. If several food chains that include some of the same organisms are combined, they make a food web. A food web shows many different food chains for a particular ecosystem and how these food chains interrelate. The food web above includes the food chain discussed earlier. However, it also shows four other food chains:
plants  mice hawks
plants mice snakes hawks
plants grasshoppers lizards snakes hawks
plants grasshoppers chickadees
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