Build the Web, Then Break It: A Model of an Oak Woodland

Investigation question: How do matter and energy move among the living and nonliving parts of an oak woodland, and what ripples through the web when one population suddenly changes?

Scientific purpose

Students assemble a card-and-yarn model of a California oak woodland that includes producers, consumers at several levels, decomposers, the air and soil, sunlight and the heat that leaves. They define the system boundary, trace matter around the loop and energy through it, then disturb one population and predict the direct and indirect effects link by link. The model is the evidence for the matter-cycling and energy-flow target, and the disturbance test is practice at predicting population change.

Why this helps students with this chapter

A printed food web is someone else's model; students read it passively and often read the arrows backward. Building the web themselves, card by card and string by string, forces every arrow decision into the open, and adding the air, the soil, the sun and the heat makes the nonliving parts of the ecosystem visible instead of implied. Breaking the web afterward turns direct and indirect effects from definitions into predictions the group has to defend.

Standards connections

MS-LS2-3 (assesses, developing a model of matter cycling and energy flow that includes nonliving components and a stated boundary): Students build a model with living parts, nonliving pools (air and soil), an energy input (sunlight) and an energy output (heat), connect every organism to decomposers, and write what is inside and outside the system. That is the model this PE asks for, without any chemical reactions.

MS-LS2-4 (practice, predicting effects of a change to a biological component): Groups remove or boost one population and reason through direct and indirect effects with sign chains, which builds the cause-and-effect reasoning the PE's arguments require; the evidence here is the model, not field data.

Roles (rotate each session)

• Web builder: Sorts the cards by role, lays them out with producers at the bottom and top predators at the top, and keeps the layout readable as yarn is added.
• Arrow caller: Reads each card's 'what it eats' line aloud and names every arrow that must be strung, always from the food to the eater.
• Recorder: Fills in the card inventory, the boundary statement and the disturbance table; writes the sign chains exactly as the group agrees them.
• Checker and presenter: Walks the finished web checking every arrow points at the eater and every organism connects to the decomposers, then presents the group's disturbance chain to the class.

Materials

Estimated cost: 8 to 12 per class (an estimate; Three skeins of yarn (about $3 each), ten sheets of cardstock, masking tape and sticky notes; poster paper and scissors are classroom stock. The card set is reusable for years if laminated.)

Safety

• Scissors: cut away from fingers and keep them closed on the desk when not in use.
• Yarn is a tangling and tripping hazard; keep it on the desk or poster, never around a neck or wrist, including in the human-web variant.
• Masking tape only; it lifts cleanly. No hazards beyond ordinary classroom materials.

Teacher preparation and setup

1. Print the 19-card set from the recording sheet, one set per group, and cut the cards apart. Label the back of each set with a group number so sets do not mix.
2. Pre-cut yarn into roughly 30 cm lengths, about 25 green, 20 brown and 15 yellow per group, so stringing does not stall on cutting.
3. Write the four disturbance cards on index cards: A. A disease kills about four fifths of the California ground squirrels. B. A wet spring triples the grasshopper population. C. Rat poison set out near a ranch kills most deer mice and ground squirrels; animals that eat poisoned rodents can be poisoned too. D. Trappers remove nearly all the coyotes from the valley.
4. Build one web yourself first so you know the three arrow rules cold: feeding arrows from food to eater, a brown arrow from every organism card to the decomposer card, and yellow arrows for sunlight into producers, air and soil into producers, carbon dioxide from every living card back to air and soil, nutrients from decomposers to air and soil, and heat from every living card to the surroundings.
5. Post the arrow rules and the phrase 'the arrow points to the eater' where every group can see them.

Investigation design

Student procedure

1. Session 1. Sort the 19 cards into four piles: producers, consumers, decomposers, and nonliving. Each card says what it eats or what it is; use that, not your memory, and talk through any card the group disagrees about.
2. Lay the cards out on the poster with the three producers along the bottom, the animals that eat producers in a row above them, animals that eat animals above that, and the decomposer and millipede cards off to one side. Put the sunlight, air and soil, and surroundings cards around the edge. Tape each card down lightly.
3. String the feeding arrows in green. The arrow caller reads each consumer card's 'eats' line, and for every food named you tape one green length from the food card to the eater card, with a small paper arrowhead or a marker arrow at the eater end. Say the rule each time: the arrow points to the eater.
4. String the brown arrows: one from every organism card, producers included, to the decomposer card, because every organism sheds wastes and eventually dies. Add a brown arrow from the millipede to the decomposers too, since its droppings and remains feed bacteria and fungi.
5. String the yellow arrows for the nonliving parts: sunlight to each producer; air and soil to each producer (carbon dioxide, water and nutrients taken in); decomposers to air and soil (carbon dioxide and nutrients released); one arrow from the producer row and one from the consumer rows back to air and soil (carbon dioxide breathed out); and one arrow from each row of living cards to the surroundings card for heat.
6. Fill in the card inventory table: role, arrows in and arrows out for every card. Then write the boundary statement: what is inside your model, what is outside it, and exactly what crosses the boundary in each direction.
7. Find and write two food chains, number their trophic levels, and name the top predators in your model. Sketch the finished web with its three arrow colors on the recording sheet. Leave the web taped down for session 2 if you can.
8. Session 2. Draw one disturbance card. Write the disturbance at the top of the disturbance table. Put a − sticky note on the population that drops (or a + on one that rises).
9. Follow the arrows one link at a time. For each neighboring population, decide whether it rises or falls and place a + or − sticky note on it. Write the row: population, direct or indirect, up or down, and the chain of reasoning in words ('fewer squirrels means less food for hawks, so hawks fall').
10. Keep going to at least two links beyond the first (at least two indirect effects). When two arrows give opposite predictions for the same card, write both and mark the card '?'. That is a real limit of the model, not a mistake.
11. Present your chain to the class in under two minutes, showing the sticky notes. Listen for whether other groups' webs predicted the same directions for the same disturbance.
12. Answer the analysis questions individually. Untape the cards, coil the yarn by color and return the set.

Expected observations

• Producers end up with no green arrows in and many green arrows out; needlegrass and the valley oak have the most, five each. Hawks and mountain lions have green arrows in and none out, so they are the model's top predators.
• Every organism card, including the three producers and the millipede, ends with a brown arrow to the decomposers, and the decomposer card has a yellow arrow out to the air and soil but no arrow to any organism.
• Disturbance predictions agree across groups at the first link (direct effects) and begin to differ or get marked '?' two or three links out, especially for omnivores like the deer mouse and the coyote that have arrows pulling in opposite directions.
• The boundary statements list sunlight as crossing inward and heat as crossing outward; strong groups also note that carbon dioxide can cross in and out because the air pool extends beyond the woodland.

Likely difficulties

• Arrows strung backward, from eater to food. The checker role exists for this; have the checker say 'points to the eater' at every arrow in the final walk.
• Groups forget the brown arrow from producers to decomposers, because 'plants do not die' feels true. Ask what happens to fallen leaves.
• The yellow arrows multiply and the web becomes a tangle. Allow one yellow arrow per row of cards for carbon dioxide and heat rather than one per card; the table still records that every living card contributes.
• Students stop the disturbance after one link. Require at least two indirect rows in the table before the group presents.
• Omnivores (deer mouse, coyote, woodpecker) confuse the level numbering. Say that a level belongs to the chain being traced, so the same card can be level 2 in one chain and level 3 in another.

Structured debrief

1. Put two groups' webs side by side and ask the class to find one arrow they strung differently. Trace what that single difference does to their disturbance predictions. The point: a prediction is only as good as the arrows, and real ecologists spend years getting the arrows right.
2. Ask every group that drew card A (squirrel disease) to report the predicted direction for hawks, coyotes, gopher snakes, acorns and deer mice. Hawks and coyotes drop (direct), snakes drop a little (they eat young squirrels), acorns and grass seeds rise (indirect), deer mice may rise because more seeds are left and because hawks and coyotes, with fewer squirrels to eat, may switch to mice, which pushes the other way. The '?' is honest; a model with no numbers cannot settle it.
3. Walk the carbon atom around the loop as a class, then ask whether any atom ever leaves the model for good. No; the air and soil card is a pool, not an exit. Then ask whether energy ever leaves for good. Yes, as heat, every day. Write 'matter cycles, energy flows' above the webs.
4. Close on disturbance C: poison moving up a food chain is real; California has documented anticoagulant rat poison in hawks, owls, bobcats and mountain lions that ate poisoned rodents. The web shows exactly which arrows carry it. Ask what a rancher could do instead (owl boxes, removing food sources, snap traps) and which arrows those options use.

Recording sheet: what students fill in

Lab 1 recording sheet: Build the web, then break it

Cut the 19 cards apart. Each card tells you what the organism eats or, for the nonliving cards, what it supplies or receives. Build every arrow from what the cards say. Remember the rule: the arrow points to the eater.

Session 2: break the web

Analysis questions: answer key

Individual evidence of learning

Each student submits their own card inventory, boundary statement, two food chains, web sketch, disturbance table and written answers to q1 through q11. The sketch with its three arrow types and the boundary statement are the primary evidence for MS-LS2-3; the disturbance table and q4 are the evidence for direct and indirect effect reasoning.

Assessment criteria

Accessibility adaptations

• Provide a partly built web (producers and the grasshopper, squirrel and deer already connected) for groups that need a head start, and large-print cards with a raised tape strip on producers for students with low vision.
• Let a student who cannot easily handle yarn take the arrow caller or checker role and direct a partner's hands; accept a labeled photo of the web in place of the sketch.
• Offer the sentence frames 'More ___ means fewer ___ because ___ eats ___' and 'That means more ___ even though ___ never touches ___' for the disturbance table and q4.
• Run the human-web variant for students who need movement: each student holds one card, yarn passes from food to eater across the room, and the disturbed student sits down so everyone holding a slack string can raise a hand.

Low-cost alternative

Skip the printed cards and have students write the 19 card names on index cards from the list on the board, using pencil lines on butcher paper instead of yarn (solid for feeding, dashed for decomposers, dotted for nonliving). The same tables and questions apply.

Model limitations

The card set is a tiny slice of a real woodland: it leaves out most insects, birds, plants and soil animals, so 'top predator' and the number of arrows are properties of the model, not of the ecosystem. The model shows direction of change only; it has no numbers, no timing, no seasons and no behavior changes such as a predator switching prey, which is why some predictions must be marked '?'. Diet lines on the cards are simplified from typical diets of these California species. This activity has not been physically tested by the authors; the two-session timing is an estimate.

Teacher notes

Check arrow direction before anything else; a web with reversed arrows makes every later answer wrong for the wrong reason. For q3, a boundary statement that puts the sun outside and names light in and heat out is the heart of MS-LS2-3. Keep one well-built web taped up for slides 10 and 17; walking a carbon atom around real yarn beats any diagram. For disturbance C, the hawk poisoning is documented in California; keep the discussion on the web's arrows rather than on brand names of poisons.