All organisms need energy and nutrients to survive. Plants are able to gather water, air, nutrients and sunlight with their leaves and their roots to make the food energy they need to survive. Plants, and organisms like them, are known as producers. Because of their self-sufficiency, it is conceivable for a natural world to exist in which there were only producers. But many organisms do not make their own food and have developed adaptations that allow them to get the things they need by consuming other organisms. These eating organisms are known as consumers. All the animals on the earth, including humans, are considered consumers.

Consumers need space, shelter, water, air, energy and nutrients to survive. Space provides the consumer room to live and shelter provides it protection. Water is necessary for animals because it is the most abundant molecule that makes up their bodies.  It is part of all of the body fluids and is a solvent for the chemical compounds in the body. Oxygen allows an organism to release the energy stored in food compounds. Energy is necessary to give the body the fuel it needs to perform its life functions, such as growth, maintenance and reproduction. Nutrients are necessary to make the chemical compounds that make up the structural components of the body as well as to synthesize enzymes, which help to perform life processes. In consumers, water can be acquired by drinking, and oxygen by respiration, but both nutrients and energy can only be acquired by eating other organisms.

Some consumers eat only plants, some consumers eat only animals and some consumers eat both. Consumers that eat only plants are known as herbivores. Herbivores feed on the leaves, flowers, roots and fruits of plants. Examples are deer, rabbits and grasshoppers. Consumers that eat only animals are known as carnivores. Examples are sharks, alligators and foxes. Most carnivores feed primarily on herbivores, while some will eat other carnivores. Animals that eat both plants and animals are known as omnivores. Examples are raccoons, songbirds and humans.

Because of the different ways they acquire food energy, consumers have very different adaptations than plants. One of them is locomotion. It is hard to catch something to eat if you are standing in one place, though some animals have figured out ways to do that. The ability to move around, to find plants or to capture prey, is an adaptation that the self-sufficient plants have not needed to develop. Animals, though, have developed locomotion, and along with this the nervous and muscular system that allows this to happen. Through locomotion, animals can find the food they need to survive.

Once a herbivore finds a plant it wishes to consume, it does not need adaptations to keep the plant still while it is feeding. With carnivores, this is not the case. Carnivores have developed many different adaptations to help them to hold and/or kill their prey. The sticky, extendable tongue of a frog, the sharp teeth of a shark, the venomous fangs of a rattlesnake, the sucker-covered arms of an octopus, the talons and beak of an eagle and the tool-making and tool-using capacity of humans are all adaptations that allow these carnivores to keep prey still while they are feeding.

A blade of grass or the flesh of a wildebeest will not do a consumer much good if it can not be broken down into usable chemical compounds. Because of this, consumers have developed many adaptations for breaking down food into smaller parts. Teeth are an example of this. The sharp teeth of carnivores are not actually adapted for chewing. They are adapted for holding the prey until it can be swallowed whole, or for tearing the animal into smaller chunks that can then be swallowed. Other animals, such as birds have beaks for holding and tearing. Crustaceans have shredding devices in their mouths. Mammals are the only animals capable of true chewing. Though canine and incisor teeth are adapted for holding and tearing, the molars are adapted for crushing and grinding, and other animals do not have these. To allow them to eat both plants and animals, omnivores have adaptations that allow them to hold, tear and to grind food.

Herbivores have to have special adaptations to help them to tear through the tough cellulose cell walls of plants. The molars of mammal herbivores are developed for this purpose. They have enamel ridges that allow them to more effectively grind plant matter. Their molars are also usually wide and corrugated. Invertebrate herbivores have scraping mouthparts or grinding mandibles that perform the same function as a mammal's molars. Plant material would not be able to be digested without these adaptations, because the enzymes in animals' digestive systems cannot break down the tough cellulose cell walls of plant cells. By tearing through the cell walls with teeth or mandibles, this allows the consumer's digestive enzymes to break apart the cell contents. Because they do not have the correct adaptations, carnivores cannot digest enough plant matter to live on and they depend on eating other animals to get what they need. 

After mechanically breaking down food with teeth or other adaptations, consumers have digestive systems that allow them to chemically break down food. The digestive tracts of consumers contain enzymes, which are biological catalysts that can break down food into absorbable parts. Proteins are broken down into amino acids, carbohydrates are broken down into simple sugars and fats are broken down into fatty acids and glycerol. These simpler compounds can then be absorbed in the blood stream and carried to all of the cells in the body for use.

Because consumers are dependent on producers for survival, there will always be more producers in a community than consumers. If consumers outnumbered the producers in a community, the herbivores would quickly eat up all the producers and then starve because there would be no more food available to them. Once the herbivores were gone, the carnivores would soon die. 

Consumers also have to outnumber producers because energy is always lost in the transfer from one organism to another. Only a small amount of the food energy made by the producer is passed to the consumer who eats it. For this reason the consumer will need to eat more than one producer to get the food energy it needs. If that consumer is eaten, only a small amount of its food energy passes on to the carnivore that ate it. This is why food chains are often drawn as food pyramids, showing that energy and number of organisms are always highest at the bottom of the food chain, and always lowest at the top.

Food chains help illustrate the interdependence of all living things. Consumers could not survive if it were not for the existence of producers. Carnivores could not exist if it were not for the existence of herbivores. Consumers depend on other organisms because they can not get what they need to survive without them.