1. Understanding Food Chains & Webs (revised 7/22)
Notes to faculty
Outcomes: By working with the 4 different representations students should better
understand the energetics of trophic transfer and make connections between metabolic
processes at the organismal and ecosystem scales. The activity of “compare/contrast”
should help them think more carefully about what illustrations in texts are suppose to
show (but may not).
What students do: They compare/contrast 4 different representations of food chains and
describe important concepts they think each image is designed to demonstrate and what
ideas are not well shown in the representations. The comparisons/contrasts should force
students to examine the images and ideas behind them more carefully than usual.
What to pay particular attention to: Most students are familiar with drawings and other
images of food chains in textbooks and therefore believe they understand the core
concepts illustrated. This familiarity may foster “lazy thinking” however (students gloss
over the important ideas) and some representations actually do not illustrate essential
concepts well at all.
o Image I – very simplistic; does not label trophic levels or show energy loss with
trophic transfer. The size of each drawing (including the arrows) is the same.
(Principle not shown – Conservation of Energy).
o
Image II – an abstraction that does depict energy loss with each trophic transfer
and the idea of a web as opposed to a straight chain. Terms such as “primary
producer” are given. However the sizes of the 3 square are the same. The heat
arrows are also very small. (Principle not well shown – Conservation of Energy).
o
Image 3 – Shows less energy in each successive trophic level and gives
representative energy values (kcal) per sq. meter per year “available in the
bodies”. In the discussion be sure to pay attention to these units – that students
know what a Kcal is and what per sq. meter per year means. It is very useful to
help them imagine this in a concrete way. “Available in the bodies” is a critical
phrase. (Principle shown - Conservation of Energy; Process emphasized –
Transformation)
o
Image 4 – Shows the metabolic processes behind the great loss of energy with
each trophic transfer. Help students make the link between Image 3 and 4 – the
atmomic/molecular-organismal-ecosystem link. (Principle well shown –
Conservation of Energy; Scale well shown- atomic up to organism )
These images focus on energy transfer and many students fail to connect energy flow in
ecosystems with metabolic processes involving carbon transformations (Principles Conservation of Matter & Energy; Scale from atomic to ecosystem). You need to keep
emphasizing this point.
Also, putting organisms into trophic levels is problematic for many systems. For
instance, insects may have herbvirous and carnivorous life stage and many animals are
omonivores. Therefore food chains/webs/pyramids can easily get very complicated and
lose their utility.
Units & Quantiative Skills: Energy measurements (Kcal and Joules) – what these mean;
Data as numbers, graphs, etc
Logistics: In smaller classes students can work in small groups on the two sets of
questions in preparation for a general discussion. Each group (or ones you pick
randomly) could “present” each of the 4 images as well, although this would take more
time. For large classes, the “turn-to-your neighbor” method would work if you project
Images II/II together, pause for discussion, and repeat with images III/IV.
Hidden Curriculum addressed
• Principles: Conservation of Energy (“loss” of energy with each trophic transfer
and the metabolic mechanisms behind that loss; Conservation of Matter (Primary
production/photosynthesis, transfer of matter to consumers/decomposers/atmosphere)
• Processes: Generation (photosynthesis); Transformation (building of
biomolecules within an organism, consumption of one organism by another); Oxidation
(autotrophic respiration, heterotrophic respiration, decomposition)
• Scale & Time: Processes across atomic/molecular-organismal-ecosystem
scales
• Forms & Representations: Ecosystems Matter & Energy Flow (Food web and
chain images signify specific ecological processes and concepts to ecologists; Trophic
levels are abstracted collections of organisms performing the same food “function” in an
ecosystem. Box-and-arrow diagrams). Energy (different forms – e.g. sunlight, animal
biomass).
Directions for Students
All biology textbooks have images showing food webs and food chains, but they
can be very different from one another. Some get across important concepts
better than others. Below are 4 figures depicting food webs/chains in different
ways. Address the questions and write down any questions you have or points
of confusion.
I.
http://www.castlefordschools.com/kent
II.
Question 1. Contrast Images I and II above. What important ideas about trophic
levels (food chains) do you think students will “get” from each one. What ideas
might they not lean as well from each one? What questions do you have?
Question 2. Images III and IV below attempt to show several important
ideas/concepts about energy flow through levels of a food chain. List these
concepts and explain how each image shows the ideas in different ways. Write
down ideas/concepts that you think are not shown well with each image.
III.
http://www.edquest.ca/notesimages/pyramid.gif
IV.
http://fig.cox.miami.edu/Faculty/Dana/entropy.jpg
Note ; “J” is Joule, a measure of energy used by physicists and is equivalent to the work
done to produce power of one watt for one second; it is 0.0024 Kilocalorie (the unit often
used by biologist).