Unit D: Changes in Living Systems

Lesson 1.3: The Web of Life    
Figure 1:  Leaf.  Courtesy Wikimedia

In this lesson you will study the role of organisms in the flow of energy, and you will also study the recycling of matter within an ecosystem.

1.  Identify examples of primary producers, consumers, secondary consumers, tertiary consumers and decomposers
2.  Identify examples of herbivores, carnivores and omnivores
3.  Use the term trophic level
4.  Explain that only 10% of energy is effectively passed to the next trophic level
5.  Describe ecosystems using energy pyramid, biomass pyramid and pyramid of numbers models.
6.  Give the advantages of describing the flow of matter and energy in ecosystems as food chains or food webs.

    Organisms can be grouped according to what they eat. Some organisms eat plants, whereas other organisms eat the organisms that eat the plants.

    Tropic Levels and the Roles of Organisms

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    • Read pages 423 to 425 of the textbook up to “Energy Flow in an Ecosystem”.  View the videos above.  Answer Practice question 21.  Check your answers below.

     

    Science 20 Textbook.  Albert Education

    Plants get their energy from the sun.  Other organisms obtain chemical energy by eating other organisms.  Energy flow in an ecosystem is described using an energy pyramid.   The energy pyramid uses trophic levels, which are divisions of species within an ecosystem based on their energy source.

    This video review trophic levels and role or organisms.  It introduces energy pyramids 

    • Read page 425 of the textbook, starting at “Energy Flow in an Ecosystem”

      Matter also cycles through ecosystems.  Two models are used to describe the amount of matter found in trophic levels; the biomass pyramid and the pyramid of numbers.  Please view:  Ecological Pyramids
    • Then, read pages 426 to 428 up to “Food Chains and Food Webs”. Answer the questions as you encounter them. Check your answers below

     

    Science 20 Textbook.  Albert Education

    Food Webs very accurately describe the flows of energy and matter in an ecosystem.

    • Starting at “Food Chains and Food Webs,” read pages 428 to 431 of the textbook. Then, answer Practice questions 27 to 30 on page 432 and check your answers below

     

    Science 20 Textbook.  Albert Education


    1. Primary producers are plants.  They use photosynthesis to produce their own energy.  Primary consumers are living things that eat plants.  Secondary consumers eat primary consumers and tertiary consumers eat secondary consumers.  Decomposers convert dead plants and animals into nutrients used by plants.
    2. Herbivores eat plants, carnivores eat meat and omnivores eat plants and meat.
    3. Plants get their energy from the Sun.  All other organisms obtain their energy as chemical energy by eating other organisms.  Each energy transfer is called a trophic level.
    4. An Energy Pyramid becomes smaller with each trophic level because only about 10% of chemical energy obtained becomes part of the organism.  The rest is used by the organism.
    5. Biomass pyramids show the flow of matter through an ecosystem as dry biomass. A Pyramid of Numbers shows the same thing but takes the size of organisms into account.
    6. Food chains are useful to illustrate how energy moves in an ecosystem, but food webs much more  accurately describe energy flow.

        • Read “1.3 Summary” on page 432 of the textbook. Check the lesson targets.  Then, complete “1.3 Questions”. Check your answers below.

           

          Science 20 Textbook.  Albert Education

        Go to Assignment 1.3: The Web of Life.