Everybody's got to eat: Food Webs and Energy Pyramids

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Objective

Students will be able to explain how the dynamics of food webs in an ecosystem are determined by limitations of energy transfer from one trophic level to another.

Big Idea

Organisms are composed of matter but require energy to function. Ecological feeding relationships can thus be understood as transfers of energy and recycling of matter.

Introduction

This lesson covers the basics of energy transfer in ecosystems, including trophic levels and consideration of the different types of consumers. 

 

The lesson essentially consists of two parts:

1. A pre-class textbook reading and homework assignment focused on close reading techniques, critical-thinking questions, and content vocabulary development.

2. An in class presentation that provides supplementary examples to review the concepts and vocabulary from the chapter along with a class discussion seeking to draw students into more critical examination of the topic at hand and assist in their ability to connect the concepts to their personal experiences. 

The textbook reading comes from Environmental Science: Your World, Your Turn by Jay Withgott.

If you do not have that particular textbook, I would recommend finding a similar chapter or chapters and modifying the lesson accordingly.  

Alternately, the powerpoint attached to the Direct Instruction section covers most of the same concepts and vocabulary as the chapter.  If you have a shorter class period, you may want to skip the reading assignment and assign the discussion questions as homework.  You could then hold the class discussion on the following day.       

In my case, I assign the textbook reading on the meeting previous to this lesson.  In that way, students will have already covered the concepts on their own and the powerpoint presentation will be less of a lecture and more of an opportunity for students to ask questions and clarify their understanding. 

Connection to Standard:

In this lesson, students will prepare for class by reading and determining the central idea of a text, establish familiarity with relevant scientific vocabulary, and then draw evidence from the text to support arguments and opinions presented as part of their participation in a group discussion.  

 

Direct Instruction

30 minutes

Like I mentioned above, I assign the textbook reading as a homework assignment to be completed upon arrival to this class period.  The powerpoint presentation is then more of a review and an opportunity for students to ask questions.  

 Wondering WHY I use lectures as a pedagogical strategy?  Watch this video.

Wondering HOW I use the Powerpoint to differentiate instruction? Watch this video.

Wondering why I choose to have a reading assignment AND a lecture on the same content?  Read this rationale.

Wondering how you might use this lesson's resources if you don't plan on presenting a lecture? Read this reflection.

 

When class begins, I ask students to get their homework out and first give them about 10 minutes to discuss the critical thinking questions with their group members.  During this time, I walk around and put a stamp on completed homework and answer any questions that students bring up.  If students bring up a good question or insightful comment, I ask them to please remember to bring that up in the larger class discussion to follow the presentation.  

Affording this time before the presentation allows students to "field test" their answers with a smaller group, increasing their confidence to participate in the larger discussion.  Also, because the discussion is graded by groups, it allows the ideas of individual members to influence the thinking of their peers which may lead to greater insights or even new questions.  Finally, while I walk around, I listen to the nature of student discussions and get a better sense of what kinds of questions may be floating around the room, allowing me to emphasize certain aspects of the lesson or offer more detailed examples to scaffold the instruction.

After I have stamped all the homework assignments, I distribute the note sheet that accompanies the presentation.  As I've mentioned in previous lessons, offering students a note sheet provides a readymade study guide for later and allows students to focus on their thoughts and the concepts being discussed as opposed to focusing all of their attention on copying down copies amounts of notes.   

 

Please Note: I find it important to really do thorough checks for understanding during this presentation on a few points:  

1. Some students confuse detritivores with decomposers.  This is understandable in that both are organisms that get their sustenance from dead organisms or from organic wastes.  The distinction is more biological than ecological: detritivores are animals that may have ecological roles similar to a decomposer.  However, unlike true decomposers (of kingdom fungi or one of the baceria kingdoms), detritivores do not completely breakdown wastes and dead organisms, although they may play an important role in the initial decomposition of said wastes or organisms. 

 

2. As I discuss in more detail in the discussion section and the reflection for this lesson, students sometimes have trouble conceptualizing the idea of the 10% rule, and just how much of the energy they consume does not get stored.  This article might be a good short read to help students think more about this.  If you don't want to have students spend class time reading the article, you might just go ahead and tell students that the USDA estimates that the average American consumes nearly 2,000 lbs of food per year.  I mentioned this statistic to my students and asked if the average American weighed that much.  They replied that, obviously, the average weight is probably around 10% of that figure.  I then asked, "if we eat that much, how come we don't weigh that much?". 

It then became more clear to most of them that most of the food we eat simply gets used for energy or is expelled as waste.  Once they understand this for humans, it's not too difficult to transfer that understanding to other species.

Discussion

30 minutes

Following the presentation, I let students know that we will wrap up by having a class discussion to review the concepts of the lesson.  Again, depending on your class length, it may be preferable to have this follow-up discussion on the following day.  

The discussion protocol for this lesson:

  1. all groups are required to participate in the discussion and will receive a “participation” grade for the day

  2. groups with more than one member that participate will receive a higher participation grade

  3. groups that participate more frequently will receive a higher grade

 

These criteria make the group collectively responsible for their grade and accountable to each other. If no one in the group participates, the group as a whole will receive a failing grade.  If only one member of the group participates, regardless of how often, the group can’t receive any grade higher than a C.  

 

To keep track of participation, I begin by making a map of the class with the group tables labeled by group name.  Since there are four students at each table, as a student from a particular group participates, I make a tally mark in the position of that student in their group.  In this way, I can tally how often the group participates, which members are participating, and how often.  To determine "average" participation, I add up all tally marks and divide by the number of groups, rounding down.  I then use this rubric to determine their participation grades.

If you'd prefer to not give a grade for participation in discussions, see this reflection where I discuss the conditions that arose that allowed me to not to grade for participation but still have meaningful discussions with broad participation.

 

 

Please note that, as I discuss in this reflection, an early perusal of student homework during the opening of class made me realize that I needed to clarify the phenomenon of energy loss between trophic levels on an energy pyramid before we could delve into the question of the environmental benefits of vegetarianism.  In this case, I drew an energy pyramid on the board and asked students to offer specific environmental costs associated with each trophic level.  In this way, all students were clear on how useful energy is lost at every trophic level (becuase it is used by the organisms on that level) and how additional trophic levels in the human food chain add additional environmental costs.  After that clarification, we were able to have a much more focused discussion of that question and allowed me to achieve the learning objectives outlined in the discussion guide.

In the event that students are having a hard time making these connections, it may be helpful to guide them with questions such as,

  • "Do farm animals need energy?  What do they use it for?"
  • "Where do farm animals get their energy from?"
  • "What resources are used to make animal feed?"
  • "Where do you buy food?", "How did it get there?"

 Questions such as these can go a long way towards propelling students down the cognitive path you've planned them to travel and introduce an element of critical thinking into the discussion.