University of Illinois Extension
Environmental Awareness: Knowing Your World

Energy and Ecosystems - Use Some Lose Some

Life on Earth is possible because energy flows one way through ecosystems, while matter cycles endlessly. Water and elements, such as carbon, nitrogen, phosphorous, and sulfur are examples of matter that cycles through ecosystems.

The original source of almost all energy in an ecosystem is the Sun. All of the energy the sun releases does not reach Earth. One one-billionth of the Sun's total energy output actually reaches the Earth. Of all the energy that does reach Earth, slightly less than 34 percent is reflected back to space by clouds. The Earth itself reflects another 66 percent back to space. Less than one percent of the total energy that reaches Earth is used by plants for photosynthesis. Plants are often called producers because of their ability to make their own food from the sun's energy.

When scientists discuss energy, they often refer to the Laws of Thermodynamics. The First Law of Thermodynamics states that energy can neither be created or destroyed. The Second Law states that energy is constantly converted from high quality to low quality. High quality energy is capable of performing a large amount of work, while low quality energy is capable of performing less work. Scientists know that energy always changes from high to low quality when work is performed. During the change, some energy is lost in the form of heat, which cannot do work. The amount of energy lost as heat is often as high as 90 percent of the total energy involved.

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Putting all this together in an example, if 1,000,000 units of solar energy were to reach Earth, one percent or 10,000 units, would be available for plants to use. Of these 10,000 units, plants would lose 90 percent, or 9,000 units, as heat.

If an animal then ate the plants, it would only receive 1,000 units of energy. These animals are called primary consumers because they cannot produce their own food. Cows and sheep are examples of primary consumers. If another animal eats the cow or sheep, it would only receive 100 units of energy, since the cow or sheep would lose 900 units as heat. Animals that eat other animals are called secondary consumers. Scientists believe that four or five of these energy transformations are the most possible before the amount of energy transferred is too small to support life.

Written by By Dr. Nicholas Smith-Sebasto


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