2. Contrasting Energy Pyramids
Notes to faculty
Outcomes: Students work in groups with two different energy pyramids. By examining,
comparing/contrasting, and questioning these diagrams they should better understand
the ecological concepts energy pyramids are meant to demonstrate and how these ideas
can be applied to more sustainable food production.
What students do: answer a series of questions about the two pyramids. They address
basic questions about pyramids – energy loss with each each trophic transfer, the
process (cellular respiration) behind this loss, why pyramids have different shapes.
There are 4 sets of questions here; you will need to figure out if you want them to
address all 4, how, etc. Terms and concepts you introduce/explain include: food chain
(web), herbivore/predator/top predator/etc., trophic, levels, energy loss through trophic
transfer. Some of these terms are not emphasized in the questions.
What to pay particular attention to: “Energy” is a confusing and vague topic for many
students (understandably so). What does it mean for the “energy” in all plant eaters to be
shown as one level in an energy pyramid? Where does the energy come from and
where does it go? In addition, to really understand the concepts behind these diagrams,
students need to apply what they’ve learned about the energetics of cellular respiration
to both the organismal level (individual organisms’ metabolism) and to the whole
ecosystem level (the combined result of individual organisms’ metabolism clumped
together into trophic levels). Finally, students focus so much on energy that they can
easily forget that energy flow is necessarily linked to transformations in carbon
molecules (energy-matter link). This is a lot!
Logistics: This activity can be done in a class of any size. The time allocation depends,
as usual, on how much time you want to spend on this, your students’ understanding of
these ideas, and their own questions.
Units and Quantitative Skills: Energy measurement (Kcal) – what it means -
Hidden Curriculum addressed
• Principles: Conservation of Energy (why energy is “lost” with each trophic
transfer, where the “energy comes from and where it goes)
• Processes: Generation (photosynthesis); Transformation (building of
biomolecules within an organism, consumption of one organism by another);
Oxidation (autotrophic respiration, heterotrophic respiration, decomposition)
• Scale & Time: molecular/cellular/organismal processes collectively
contribute to ecosystem processes; this a “snapshot” in time.
• Forms & Representations: Ecosystems Matter & Energy Flow - energy
pyramids signify specific ecological processes and concepts to ecosystems
ecologists; they are tools that organize thinking and ideas about ecosystems
ecology. Energy comes in different forms (sunlight, animal and plant biomass).
Student Directions
Below are two “energy pyramids”: one for a grassland ecosystem and other a
marine (ocean) ecosystem. Please address the questions below as best you can.
Write down any questions you have, such as parts of the diagrams you don’t
understand (e.g. what they are suppose to show) and the ecology concepts the
diagrams are suppose to help you understand. Your comments and questions
will be key for the all-class discussion about this topic.
PYRAMID I: Grassland ecosystem
(Organisms include - Primary Producers (such as grasses), Herbivores (e.g.
grasshoppers, bison), Primary Predators (they eat herbivores – e.g. spiders),
Secondary Predators (eat primary predators – e.g. some birds), Tertiary Predators (eat
secondary predators – e.g. hawks).
PYRAMID 2. Marine Ecosystem
(Organisms include – Primary Producers (phytoplankton – small floating algae),
Herbivores (they eat the primary producers - zooplankton – small floating animals,
Predators (eat zooplankton – e.g. some fish)
Questions: One person in your group should read the paragraph immediately
below. Make sure everybody in the group understands what is being said here
before you move on.
“Energy pyramids” are shown in all biology and ecology textbooks to
demonstrate some core ideas in ecosystems ecology. These concepts are key to
sustainable agriculture and energy production as well as basic biology and
ecology. An energy pyramid describes the amount of energy in different parts of
a particular ecosystem at one point in time – for instance, for a corn farm field on
an average summer day. So the concentration of energy contained in the corn,
insects eating the corn, predators eating these herbivorous insects, other
predators eating them, and so on – is depicted in each level of the energy
pyramid.
1. Describe each pyramid above and compare/contrast them. What are the
horizontal boxes meant to represent? What are the arrows? Why so
much emphasis on the arrows? There are no units shown here. What
could be units be? (what is being measured here?). What are the main
differences between Pyramids 1 and 2? Why are the horizontal boxes
smaller from bottom to top? Where does the energy come from for the
whole ecosystem? Write you answers below along with any comments or
questions.
2. A vegetarian would point to the difference between the two pyramids as
evidence for why being a vegetarian is a good thing for environmentally
conscious people. Why?
3. The dotted arrows show energy as being “lost” to the ecosystem. Is it
“lost”? Where does the energy go? Where – what key biological process
– does it come from? Where does that process take place? What
organisms in an ecosystem “do” this process? Why do they do it?
4. Imagine that you are given the task of actually making an energy pyramid
for an acre of corn. How would you go about doing this? What would be
the first step? Hint: imagine piles of organisms group by category – the
plants in one pile, the herbivores in a pile next to the plants, and so on.
Draw the piles in a cartoon below.