Energy Flow in Ecosystems
Lesson 2 of 8
Objective: Students will be able to explain how energy flows through an ecosystem, and how the number of organisms present depends on the amount of energy available.
To start this lesson I present the following video, which is embedded in the presentation I use in the lesson:
Following the video, students pick up a set of guided notes and talk about/write down two big ideas they take away from watching the video (SP8: engage in discussions with scientific peers).
Note to teachers: At this level, students have already been exposed to food chains and food webs, so this video acts to activate prior knowledge.
Note to teachers: Guided notes are given to students as a scaffold to help them summarize content from a lecture or assigned reading. The notes contain blanks where key facts or concepts should be inserted. As students attend to the content being presented, they write the missing information into the blanks to complete the notes.
Once we are done "explaining", I have pairs of students gather one laptop and direct them to the Energy Flow interactive. In the following video, I present a quick walkthrough of the interactive and identify the information I expect the students to gather.
Once students have gathered their data, they are ready for some analysis questions (SP4). Students first are asked what they believe are the limitations of this particular model (SP2, CCC Systems and System Models Models are limited in that they only represent certain aspects of the system under study), and then move on to identifying the relationship between population numbers and energy availability (CCC Systems and System Models Models can be used to represent systems and their interactions—such as inputs, processes and outputs—and energy, matter, and information flows within systems). These observations and conclusions can be seen in the samples of student work (EF SW1, EF SW2, EF SW3, EF SW4), which reveal that the students understood how little energy is passed on as you go higher in trophic levels. Watch as the students attempt to make sense of the information presented by the interactive.
Note to teachers: One of the students had an interesting question "If producers can pass on more energy, why is it that you get hungry sooner after eating a salad than after eating a piece of chicken?" This lead to a discussion centering about the number of calories per gram, and how much energy each of the organisms at the different trophic levels needs to survive. I mention this to illustrate how counterintuitive the concept is, and what students might be thinking as they work through the lesson.
To close this lesson, I post the following question on Edmodo:
"Why do you think that there are rarely more than five links on a food chain?"
In their answers I am looking for references to the amount of energy available as you move to the higher trophic levels in an energy pyramid, such as what you see here.