Food Chains and Webs GLOSSARY autotrophs: organisms, such as plants, that are capable of making their own food. biological magnification: the process by which small amounts of a poisonous chemical become increasingly concentrated as they are consumed by organisms which occupy higher levels in an energy pyramid. carnivore: a flesh-eating animal, such as a mountain lion or wolf. community: a group of different organisms living together in a given area. consumers: organisms within a community that eat producers or other consumers. decomposers: organisms whose function is to clean up a community by digesting the dead bodies of producers and/or consumers. energy pyramid: a model which illustrates the loss of energy that occurs throughout food chains and webs. first-order consumers: organisms that eat the producers in a food chain. food chain: describes the order in which organisms in a community feed on each other. food web: describes all the interconnected food chains within a community. fourth-order consumers: organisms that eat the third-order consumers in a food chain. herbivore: an animal that only eats plants. heterotrophs: organisms that cannot produce their own food, and must rely on other organisms for energy. niche: the role an organism plays within a community. omnivore: an animal that eats both plants and animals. photosynthesis: the process by which plants convert light energy from the sun into chemical energy in the form of food. producers: organisms within a community that provide food for other organisms incapable of producing their own food. Copyright 1998 CyberEd Inc. 18
second-order consumers: organisms that eat the first-order consumers in a food chain. third-order consumers: organisms that eat the second-order consumers in a food chain. Copyright 1998 CyberEd Inc. 19
Study Guide #1 ENERGY TRANSFER All of the activities of life require a source of energy. This energy is obtained directly through feeding or indirectly from the sun. Plants are able to absorb light energy from the sun and convert it into chemical energy through a process called photosynthesis. The potential energy created is stored as food in the plants tissues. Organisms, such as plants, which are able to manufacture their own food, are called autotrophs. Animals and plants that are incapable of obtaining energy from light directly must obtain it indirectly by consuming other plants or animals. Organisms that cannot produce their own food and must obtain it from their environment are called heterotrophs. Copyright 1998 CyberEd Inc. 3
Study Guide #2 BIOLOGICAL COMMUNITIES Environmental conditions in a given area support a specific group of plants and animals. This group is called a community. For example, a tropical rain forest may be composed of assorted insects, birds, reptiles and a canopy of trees. Depending on their source of nourishment, members of a community are categorized into three niches or trophic levels. Green plants, algae or bacteria that are capable of converting light into chemical energy via photosynthesis are called producers. The producers provide energy to the heterotrophs that cannot produce their own food. Consumers are organisms in the community that feed on producers or other consumers. Mice that eat plants are consumers. Owls that eat mice are also consumers. The third category, decomposers, feed on the dead bodies of producers and consumers. The decomposers recycle the dead remains into nutrients, which can then be used by other organisms. Members of this group include bacteria and fungi. Copyright 1998 CyberEd Inc. 4
Study Guide #3 FOOD CHAINS On the basis of what they eat, animals are categorized into herbivores, carnivores and omnivores. Herbivores are vegetarians that eat only plants. Carnivores are flesh-eaters and omnivores eat both plants and animals. The term food chain describes the sequence in which organisms in a community feed on each other. Food chains always start with autotrophic producers. The producers are eaten by herbivores or omnivores, which are, called first-order consumers. Animals that feed on the firstorder consumers are usually carnivores. They are called second-order consumers. Animals that devour second-order consumers are called thirdorder consumers. An herbivorous insect eating a plant is an example of a first-order consumer eating a producer. A frog that eats the insect is a second-order consumer. A snake that eats the frog is a third-order consumer. If yet another carnivore eats the snake it is a fourth-order consumer. As you can see, there may be one or several carnivores in a food chain. A member of a community may feed at different levels in various food chains. Some omnivores, such as humans, routinely feed at all levels. Copyright 1998 CyberEd Inc. 5
Study Guide #4 FOOD WEBS AND ENERGY PYRAMIDS The food chains within a community are intricately related. Organisms may participate in more than one chain and often fill different levels in each chain. The interconnected food chains of a community and the flow of energy between them make up a food web. An energy pyramid shows the amount of energy captured and stored at each trophic (nutritional) level of the food chain. Every level captures about ten percent of the energy contained in the level below it. The other ninety percent is used to fuel life activities or is unassimilated. Moving up the levels of the pyramid, the number of organisms decreases with the amount of available energy. Third-order consumers, such as humans, must eat large amounts of food to maintain their energy requirements. As the world population explodes, humans place a growing burden on the levels below us in the food chain. Because of the interdependence of all members of the food web, stress on one group will be detrimental to those that depend on it as a source of energy. Copyright 1998 CyberEd Inc. 6
Another problem can arise when small amounts of poisonous chemicals consumed by organisms at one level of the energy pyramid become increasingly concentrated when ingested by organisms farther up the pyramid. This potentially deadly process is called biological magnification. Copyright 1998 CyberEd Inc. 7