|
|
 |
|
|
|
|
|
|
|
|
|
|
|
|
 |
|
|
|
Background
|
|
|
|
Have you ever wondered why scientists say that the energy used by all living things on earth can be traced back to sunlight as its original source? Have you ever wondered why there are so many more plants and insects on earth than there are large meat-eating animals like eagles, bears, and wolves? All forms of life on earth-whether they are one-celled protozoans, insects, plants, reptiles, or human beings-depend on a flow of energy. This flow of energy begins 93 million miles away from earth iii the form of solar energy. The amount of energy the earth receives from the sun each thy is 1.5 billion times the amount of electrical energy generated in the United States each year and only a small fraction of this energy-less than one percent-is used by the cells of earth’s living things.
|
|
|
|
All living things need energy for metabolism (the total of all activities of an organism that enable it to stay alive, grow, and reproduce). Living things get the energy they need to survive from the chemical energy (food) they consume and break down. Living things are classified into two major groups based on how they obtain their food.
|
|
|
|
 |
|
|
|
Producer
|
|
|
|
Producers make their own food by changing light energy (sunlight) into chemical (food) energy through a process called photosynthesis. Green plants, the primary producers, are able to trap light energy because their leaves contain a green pigment called chlorophyll in small structures called chloroplasts. Photosynthesis takes place in chloroplasts. Producers like green plants store the starches and sugars they produce and use this stored food energy for metabolism.
|
|
|
 |
|
|
|
|
Consumer
|
|
|
|
Consumers are organisms that can’t make their own food. They get the energy they need by eating plants or other animals and breaking down the chemical energy stored in their tissues or by living in or on plants and animals as parasites. All animals are consumers. Some special types of consumers called decomposers get energy they need by breaking down the tissues of dead organisms. Many types of bacteria and fungi are decomposers.
|
|
|
|
When many different kinds of producers and consumers live in the same ecosystem they form a community of organisms. Energy flows through a community of organisms via food chains. All food chains begin with energy from the sun. Green plants (producers) then form the base of all food chains. Animals that eat green plants (primary consumers) form the second link, followed by animals that eat the animals that eat green plants (secondary consumers). The animals at the top of a food chain (tertiary consumers) are not eaten by any other animals. Each level of a food chain or web is called a trophic (feeding) level. A sample of a food chain with four trophic levels would be grass (producer) — field mouse (primary consumer) — snake (secondary consumer) — hawk (tertiary consumer).
|
|
|
|
 |
|
|
|
Sometimes a community contains many different, interconnected food chains called a food web. In a food web, some consumers may eat many different kinds of plants or animals or they may even eat both plants and animals. Herbivores are consumers that only eat plants; carnivores are consumers that only eat other animals, and omnivores are consumers that derive their energy from both plants and animals.
|
|
|
|
As energy flows from the sun to producers to consumers in a food chain or food web, an energy pyramid is formed. Like any other pyramid, it is larger at the base and smaller at the top. Plants are only able to convert a small amount of the light energy they receive into stored food energy. Only about 10% of the total food energy produced by plants is passed on to the herbivores that eat them. Then again, only about 10% of the food energy contained in herbivores is transferred to the carnivores that consume them. In fact about 90% of the total food energy available is lost as you move from one trophic level to another. In other words, less energy is available at each level of a food chain as you move toward the top, and so, animals provide the food energy needed to support 1,000 pounds of primary consumers that support 100 pounds of secondary consumers that support one 10-pound tertiary consumer!
Although the amount of food energy available significantly decreases as you move up a food chain, it is important to remember that the total amount of energy in the entire ecosystem still remains the same. At each trophic level, some of the available food energy is converted to kinetic energy for movement and basic metabolism such as breathing, some energy is given off by animals as heat, etc. Although the total amount of energy in all its forms remains the same, only about 10% of the original food energy is transferred from one trophic level to another.
|
|
