Lesson 1.03 The Energy Cycle

The sun is the main source of energy for Earth. Its energy travels through space to Earth in the form of electromagnetic waves. The visible light within these waves provides the energy for all living organisms on earth. However, not all life on earth can directly use the sun's energy to make chemical energy in the form of food. This means there has to be a "bridge" that connects the sun's energy to the rest of the organisms on earth.

The kingdom Plantae provides this "bridge". Plants have adaptations that allow them to convert the sun's light energy into chemical energy that can be used by all other living organisms. Plants have special structures in their leaves called chloroplasts. These chloroplasts contain a molecule called chlorophyll that captures the light and transforms it in to chemical energy known as sugar, or food. This process of plants absorbing light energy and changing it into chemical energy is known as photosynthesis.

Because plants are able to generate their own energy through photosynthesis, they are known as producers or autotrophs. Most of the other organisms on earth cannot make their own food and depend on plants and other autotrophs in order to get the chemical energy they need to live. These organisms, such as animals, fungi, bacteria, and some protists, are often referred to as consumers or heterotrophs.

The energy cycle is divided into different trophic levels.

The 1st trophic level of the energy cycle is often called producers because it is made up of all the plants that convert light energy into chemical energy. Plants are essential to the energy cycle because they take light energy and convert it to the form of energy that the rest of the organisms in the ecosystem can use.

The 2nd trophic level of the energy cycle is the primary consumers. The organisms in this trophic level are heterotrophs that feed on the producers, or plant-eating animals. The animals in this level are also known as herbivoresconsumes plants.

The 3rd trophic level in the energy cycle is the secondary consumers. These organisms are typically animals that primarily feed on primary consumers, or animals that eat plant-eating animals. These organisms may also feed on organism from the producer level. The animals that are secondary consumers can be omnivoresconsumes both plants and animals and carnivoresconsumes animals.

The 4th trophic level of the energy cycle are the tertiary consumers. These are animals that feed on organisms from the primary and secondary consumers. Because most of these animals eat mainly consumers, they are carnivores.

The 5th trophic level of the energy cycle are the detritivores. These organisms are responsible for breaking down dead organic material as well as waste products from living organisms. Organisms in this level include bacteria, fungi, worms, insects, and buzzards. These organisms return nutrients to the soil that can be used by the plants to continue the energy cycle.

Energy transfer through an ecosystem can be represented in a food chain or food web.

View the following video clip to hear an explanation of food chains and food webs. Login Instructions Food Chains and Food Webss (2:23)

Food chains show only one path of energy transfer in an ecosystem. A food chain allows you to see the predator/prey relationships within a single path of energy flow.

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Food webs show all the possible paths of energy transfer within an ecosystem. A food web allows you to see all the possible predator/prey relationships within a ecosystem as well as competition relationships within an ecosystem. A competition relationship is where two or more animals or species are vying for the same resource.

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Humans are a part of the environment. Therefore, they impact the energy cycle of ecosystems. People are omnivorous for the most part, meaning that they depend on and eat both plant and animal material for food source. This means that they naturally have an effect on the flow of energy within the ecosystem in which they live.

Humans can have a negative impact on the energy flow in ecosystems. Negative human impacts include the following:

• poaching of animals;
• over fishing of rivers, lakes, and oceans;
• introducing non-native species to an ecosystem (ex. Kudzoo);
• destroying habitat (clear-cutting land);
• polluting all areas of the environment.