9.6 Flow of Energy

Learning Objectives

Explain where all the energy in an ecosystem comes from.
Classify organisms on the basis of how they obtain energy and describe examples of each.
Be able to draw and interpret a food web.
Explain the flow of energy through an ecosystem using an energy pyramid.

Introduction

Marine iguanas live on the Galapagos Islands. These islands used to be covered with hot lava, but are now covered with volcanic rock. The Galapagos Islands are also are home to many different plants and animals. How can an island that used to be covered in lava now be home to new plants and animals? You may also notice that the algae are growing on rocks. How can algae grow without soil? What would happen to the iguana population if the algae could not grow on the rocks?

This chapter discusses how energy flows in an ecosystem and also how new ecosystems form. Think about all these questions as you read this chapter.

Guided Practice

Energy and Producers

Energy is the ability to do work. In organisms, this work can be physical work, like walking or jumping. It can also be the work used to carry out the chemical processes in their bodies. All organisms need a supply of energy to stay alive.

Some organisms can get the energy from the sun. Other organisms get energy from other organisms. Through predator-prey relationships, the energy of one organism is passed on to another. Energy is constantly flowing through a community. With just a few exceptions, all life on Earth depends on the sun’s energy for survival.

Producers

The energy of the sun is first captured by producers (Figure below), organisms that can make their own food. Many producers make their own food through the process of photosynthesis. The "food" the producers make is glucose. Producers make, or produce, food for the rest of the ecosystem.

Recall that the only required ingredients needed for photosynthesis are sunlight, carbon dioxide (CO2), and water (H2O). From these simple inorganic ingredients, photosynthetic organisms can produce glucose (C6H12O6) and other complex organic compounds. The formula for photosynthesis is below:

carbon dioxide + water + sunlight --> glucose + oxygen

The survival of every ecosystem is dependent on the producers. Without producers capturing the energy from the sun and turning it into glucose, an ecosystem could not exist. On land, plants are the dominant producers. Algae called phytoplankton are the most common producers in the oceans.

There are also bacteria that use chemical processes to produce food. They get their energy from sources other than the sun, but they are still called producers.

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Producers include plants (a), algae (b), and diatoms, which are one-celled algae (c).

Consumers and Decomposers

Many organisms are not producers and cannot make their own food from sunlight. The organisms that must consume other organisms to get food for energy are called consumers. All animals are consumers.

The consumers can be placed into different groups:

Herbivores are animals that eat producers to get energy. For example, rabbits and deer are herbivores that eat plants. The caterpillar in Figure below is an herbivore. Animals that eat phytoplankton in aquatic environments are also herbivores.
Carnivores feed on animals, either herbivores or other carnivores. Snakes that eat mice are carnivores. Hawks that eat snakes are also carnivores.
Omnivores eat both producers and consumers. Most people are omnivores, since they eat fruits, vegetables, and grains from plants, and also meat and dairy products from animals. Dogs, bears, and raccoons are also omnivores.

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Examples of consumers are caterpillars (herbivores) and hawks (carnivore).

Decomposers (Figure below) get nutrients and energy by breaking down dead organisms and animal wastes. Through this process, decomposers release nutrients, such as carbon and nitrogen. These nutrients are recycled back into the ecosystem so that the producers can use them.

The stability of an ecosystem also depends on the actions of the decomposers. Examples of decomposers include mushrooms on a decaying log. Bacteria in the soil are also decomposers. Imagine what would happen if there were no decomposers. Wastes and the remains of dead organisms would pile up and the nutrients within the waste and dead organisms would never be released back into the ecosystem.

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Examples of decomposers are bacteria (a) and fungi (b).

Food Chains and Food Webs

Food chains (Figure below) show the eating patterns in an ecosystem. Food energy flows from one organism to another. Arrows are used to show the feeding relationship between the animals. The arrow points from the organism being eaten to the organism that eats it. For example, an arrow from flower to a caterpillar shows that the caterpillar eats the flower. Energy and nutrients are moving from the leaves to the caterpillar. Next, a frog might prey on the caterpillar, a snake may eat the frog, and an owl may eat the snake.

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The producers are always at the beginning of the food chain. The herbivores come next, and are called primary consumers. A primary consumer is an organism that eats the producer. A secondary consumer will eat the primary consumer. These are carnivores. In this example, the producer is the flower, the primary consumer is the caterpillar, and the frog is the secondary consumer. The snake eats the frog, and is called a tertiary consumer. The owl will eat the snake, and is called a quaternary consumer.

Each organism can eat and be eaten by many different types of organisms, so simple food chains are rare in nature. There are also many different species of fish and sharks. In ecosystems, there are many food chains.

Since feeding relationships are so complicated, we can combine food chains together to create a more accurate flow of energy within an ecosystem. A food web (Figure below) shows the feeding relationships between many organisms in an ecosystem. A food web shows many more arrows, but still shows the flow of energy. A complete food web may show hundreds of different feeding relationships. Can you name one producer in the food web below? Now try to find a primary, secondary, and tertiary consumer.

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Energy Pyramids

When a herbivore eats a plant, the energy in the plant tissues is used by the herbivore. The herbivore uses this energy to power its own life processes and to build more body tissues. However, only about 10 percent of the total energy from the plant gets stored in the herbivore’s body as extra body tissue. The rest of the energy is used by the herbivore and released as heat. The next consumer on the food chain that eats the herbivore will only store about 10 percent of the total energy from the herbivore in its own body. This means the carnivore will store only about 1 percent of the total energy that was originally in the plant. In other words, only about 10 percent of energy of one step in a food chain is stored in the next step in the food chain. The majority of the energy is used or released to the environment.

Every time energy is transferred from one organism to another, there is a loss of energy. This loss of energy can be shown in an energy pyramid. An example of an energy pyramid is shown in Figure below. Since there is energy loss in food chains, it takes many producers to support just a few carnivores in a community.

Each step of the food chain in the energy pyramid is called a trophic level. Plants or other photosynthetic organisms are found on the first trophic level, at the bottom of the pyramid. The next level will be the herbivores, then the carnivores that eat the herbivores. The energy pyramid in Figure below shows four levels of a food chain, from producers to carnivores. Because of the high rate of energy loss in food chains, there are usually only four to five trophic levels in the food chain or energy pyramid.

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Review

Producers, such as plants and algae, can make their own food from the sun and other simple sources.
Consumers must obtain their nutrients and energy by eating other organisms.
Decomposers break down animal remains and wastes to get energy.
Food chains and food webs show the feeding patterns in an ecosystem.
As energy is transferred along a food chain, energy is lost as heat.