Organisms that live on Earth are either autotrophs or heterotrophs. Autotrophs obtain energy directly from their physical environment through photosynthesis or chemosynthesis. Heterotrophs must obtain their energy by eating autotrophs or by eating other heterotrophs. Plants are autotrophs, but all animals are heterotrophs. Animals that feed on living organisms or parts of living organisms can be classified into three general groups based on feeding behavior: herbivores, carnivores, and omnivores.
A herbivore is an animal that eats the tissues of green plants and only plants. Herbivores consume many different parts of plants. This group of animals can be further subdivided into such categories as folivores (eaters of leaves), frugivores (eaters of fruit), and granivores (seed eaters). Herbivores have special adaptations that allow them to extract sufficient energy and nutrients from plants. For example, herbivores have large stomachs, allowing them to eat a large amount of material. Some herbivores have two stomachs. Food stored in the first stomach is partially broken down by bacteria through a process similar to fermentation. Then the material is regurgitated, chewed some more, swallowed again, and passed to the second stomach. Herbivores have also evolved special teeth adapted for cutting and grinding plant tissue. Herbivores include such animals as grasshoppers, caterpillars, cattle, and antelope.
Carnivores eat other animals. The animals eaten by carnivores may be herbivores, omnivores, or other carnivores. Since animal tissue is more densely packed with nutrients than plant tissue, carnivores have relatively shorter digestive tracts. They have also evolved special teeth suitable for tearing flesh. Some members of order Carnivora, such as cats, have all fanglike teeth ideally suited for cutting or tearing through animal tissue.
Carnivores are all predators. They are generally larger than their prey (but not always; for example, wolves prey on moose and caribou). Predators pursue and capture their prey, so they require a large amount of calories. They therefore must catch and eat many individual prey items during their lives.
Carnivores are generally very important to the ecosystems they inhabit. Since carnivores eat other animals and must eat a large number of prey individuals, they almost always act as a check on populations of herbivores. Carnivores include animals such as cats, wolves, shrews, and polar bears. Wolves are classified as carnivores but most of the Canids (which also include foxes, coyotes, and domestic dogs) will eat fruit and berries or other plant tissue when they are hungry and the fruit is available.
Omnivores obtain energy by eating both plant and animal tissue. Some omnivores hunt, pursue, and catch other animals, just as predators do. Most omnivores will readily eat the eggs of other animals. Many omnivores are also scavengers. Omnivores eat plants, but not all plants and not all parts of the plants they do eat. Omnivores can eat fruits and tubers. Some grains can be eaten by omnivores.
Humans are omnivores who have expanded their food choices by discovering fire and inventing cooking. Cooking animal or plant tissue tends to break down complex molecules, making more nutrients and calories available. Since omnivores eat both plant and animal tissue, they have evolved several different kinds of teeth, including incisors for shearing plant and animal tissue, canines and bicuspids for tearing and crushing meat, and molars for grinding seeds and other plant tissues. Omnivores include black bears, humans, many apes, cockroaches, chickens, and raccoons.
Detrivores are a group of organisms that get energy from dead or decaying plant and animal matter. Detrivores can be classified as detritus feeders or decomposers. There are no animals that act as decomposers, so animal detrivores are all detritus feeders. Animal detrivores can be considered omnivores that eat the dead remains of other organisms. Earthworms are typical detrivores.
The sun is the source of all energy for life on Earth. Plants harvest sunlight and store the energy in the chemical bonds of carbohydrates, fats, and proteins. Animals obtain their energy from plants. Herbivores obtain solar energy directly by eating plants. Carnivores obtain solar energy indirectly by eating other animals. All animals have evolved feeding behaviors that allow them to obtain sufficient energy and essential nutrients to live, grow, and reproduce. Some animals (herbivores) eat vast quantities of food with low nutritional value. Other animals (carnivores) consume smaller amounts of food with higher nutritional content, but they have to work harder to get it. Still other animals (omnivores) eat a variety of foods allowing them to use whatever food sources are available at any given time.
A trophic level is a group of organisms that all consume the same general types of food in a food web or a food chain. In a typical food web, all producers (autotrophs) belong to the first trophic level and all herbivores (primary consumers) belong to the second trophic level.
The second trophic level in a grassland ecosystem would be all of the herbivores that eat the grass. This group can include a wide variety of organisms. For example, in the original grasslands of the central United States, the second trophic level included grasshoppers, rabbits, voles and other small rodents, prairie dogs, and American bison (Bison bison ). Since all of these creatures eat the same grass, they are all at the same trophic level, despite their differences in size, reproductive habits, or any other factors.
The third trophic level includes primary carnivores, such as wolves and warblers. Primary carnivores prey on herbivores. Secondary carnivores, such as falcons and killer whales, prey on primary carnivores as well as herbivores. Omnivores, such as humans, are able to feed at several different trophic levels.
see also Feeding Strategies; Foraging Strategies.
Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.
Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth, 1990.
Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer, 1987.
the Feeding of the Five Thousand in the Bible (Mark ch. 6), the miracle by which Jesus fed the five thousand who had gathered to hear him on the only food which they had, five loaves and two fishes; when everyone had eaten, the fragments filled twelve baskets.