Unit 6 · Ecosystems & Our Footprint · Week 3

What happens when one piece of the web changes?

In the schoolyard patch, suppose the spiders suddenly disappeared — all of them. Would anything else change? Predict before you read, then build the model.

SEP: Analyzing & Interpreting Data · Constructing Explanations DCI: LS2.A Interdependent Relationships in Ecosystems · LS2.C Ecosystem Dynamics, Functioning, & Resilience CCC: Cause & Effect · Stability & Change · Energy & Matter

NGSS MS-LS2-1 · MS-LS2-2 · MS-LS2-4 ~50 minutes Learner code: —

Make this lesson work for you:

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How we work this lesson (our expectations)

Follow the connections — changing one thing means tracing every relationship it has
Use data to support predictions — "I think" is fine; "the data shows" is stronger
Systems thinking takes practice — it's okay to feel uncertain; that's part of the work
Supports are for everyone — open any panel you want, any time

30-second check-in social awareness

Before we start, think about a time when one small change cascaded into a bigger change in your life — maybe someone wasn't there for a day and everything felt different. Take one slow breath. Ecosystems work the same way. Today we are going to trace those cascades in living systems. Rate your focus 1–5. If you are at a 1 or 2, start with the simplified reading and the Listen button.

Do-Now Quick recall

Pull from Weeks 1 and 2. No looking back.

W1 spiral Draw a simple three-step food chain from the schoolyard. Label the producer, primary consumer, and secondary consumer. Draw arrows to show the direction of energy flow.
W2 spiral What is the role of decomposers in a food web? Why does removing decomposers eventually hurt producers (plants)?
Make a prediction: if all the spiders in the schoolyard patch disappeared tomorrow, what do you think would happen to the caterpillar population? To the bird population? Write your prediction — we will test it against the model today.

The phenomenon

When spiders are removed from the food web, caterpillar populations explode and overgraze the grass. The bird population shrinks (less prey that can be caught easily). The ecosystem shifts — because every connection in the web is a pathway for energy and matter.

Imagine researchers carefully remove all spiders from our schoolyard patch. What unfolds over the following weeks? With no spiders eating caterpillars, the caterpillar population surges — there is now more caterpillar prey available than before. But caterpillars eat grass. With many more caterpillars, the grass gets eaten faster than it can regrow. The grass population declines. Meanwhile, birds that fed on spiders have less of their preferred prey available, so bird numbers drop too. Decomposers increase because more dead caterpillars are piling up. A single change cascaded in every direction through the web. Every connection in a food web is an energy and matter pathway. When you cut a connection, energy cannot flow along that path, and matter cannot be recycled through it. The whole web shifts — sometimes dramatically. Scientists call an organism whose removal triggers these large cascades a keystone species — not because it is the biggest or strongest, but because removing it destabilizes the system the way pulling a keystone arch-stone collapses a bridge. Our question: how does one change cascade through an entire ecosystem — and what makes some species more critical than others?

Notice & Wonder. Look at the two diagrams — what changed? Write one thing you notice and one thing you wonder about how connected things are.

Build it Interdependence, limiting factors, and cascades

Three connected ideas, one step at a time.

Step 1 — Interdependence: every species depends on others

No organism in an ecosystem lives in isolation. Interdependence means that each species' survival is connected to other species, whether they eat each other, compete for the same resources, or depend on the same habitat. In a food web, those connections are literally the pathways for energy and matter — the same energy flow (W1) and matter cycling (W2) we traced before. When you remove a species, you do not just lose that species; you cut every energy and matter pathway that ran through it. Organisms that depended on it for food lose a food source; organisms that it preyed upon are suddenly released from predation pressure. These ripple effects are called trophic cascades — changes that travel up and down through the food web levels (trophic levels).

Step 2 — Limiting factors: what controls population size?

Populations do not grow forever. A limiting factor is any resource or condition that caps how large a population can grow: food, water, shelter, light, and predation are common examples. When caterpillars had spiders as a limiting factor, their numbers were kept in check. Remove the spider, and the caterpillar population overshoots — until a new limiting factor kicks in. In this case: the grass runs out. Now food scarcity becomes the new limiting factor for caterpillars, and they crash. The ecosystem may swing back and forth before reaching a new balance, or it may shift permanently to a new state. Limiting factors are what keep ecosystems in a dynamic balance.

Step 3 — Competition and resource sharing

When two species need the same resource, they compete. Competition can be between individuals of the same species (intraspecific) — caterpillars competing with each other for the best leaves — or between different species (interspecific) — caterpillars and grasshoppers both eating grass. Competition limits all populations simultaneously. When a species is added or removed, it changes the competitive landscape for every other species sharing its resources. This is why trophic cascades can be so far-reaching: removing the spider doesn't just affect caterpillars and birds; it can shift which plant species dominate (if some plants are eaten more than others by the freed caterpillars), which in turn changes the habitat available to every other organism.

Worked example — tracing the cascade from spider removal (watch how an expert reasons)

Step 1 — Identify the direct effect: Spiders eat caterpillars. Remove spiders → caterpillars lose a predator → caterpillar population increases rapidly (no longer limited by spider predation).
Step 2 — Trace up the web: Birds ate spiders. No spiders → birds have fewer prey → bird numbers gradually decline (food scarcity becomes their limiting factor).
Step 3 — Trace down the web: Caterpillars eat grass. More caterpillars → more grass consumed → grass population declines.
Step 4 — Note the energy and matter link: This is not just about numbers. The energy that flowed through spiders (W1) now cannot flow that way. The carbon in spider bodies now goes directly to other decomposers (W2) rather than up to birds. Every cascade is a change in energy flow and matter cycling — not just a population number.
The web is not a list of species; it is a network of energy and matter pathways. Pull one thread and the whole net shifts.

Show me a cascade table I can fill in Tier 2 · organizer

Fill in the cause and effect for each step of the cascade:

Step 1 — Spiders removed → Caterpillar population ________ (increases / decreases) because ________.

Step 2 — More caterpillars → Grass population ________ because ________.

Step 3 — No spiders → Bird population ________ because ________.

Step 4 — More dead caterpillars → Decomposer population ________ because ________.

Word bank: increases · decreases · predator · prey · food · competition · limiting factor · trophic cascade · energy · matter

Your turn Predict a cascade

Apply the interdependence model to a new scenario. Choose how you respond — all are equally valid.

✎️ Draw a before/after food web diagram with arrows 📝 Write a Claim–Evidence–Reasoning explanation of the cascade 📣 Record yourself narrating the cascade step by step

The task: A new scenario — in a local pond ecosystem, the heron (a large wading bird) is the top predator. It eats frogs; frogs eat dragonfly larvae; dragonfly larvae eat algae. If herons are removed from this pond due to habitat loss, predict and explain: (1) what happens to the frog population; (2) what happens to the dragonfly larvae population; (3) what happens to the algae; (4) how does this relate to the energy and matter connections you learned in W1 and W2?

I want sentence frames and a step-by-step structure Tier 2 · frames

When herons are removed, frogs ________ because ________.

More/fewer frogs means dragonfly larvae ________ because ________.

More/fewer dragonfly larvae means algae ________ because ________.

This relates to energy flow (W1) because ________.

This relates to matter cycling (W2) because ________.

Word bank: increase · decrease · predator · prey · limiting factor · trophic cascade · energy pathway · matter cycle · population · competition · food · algae · dragonfly · frog · heron

Let's make it concrete first Tier 3 · re-teach

Analogy: Imagine a school schedule. The teacher (heron) keeps students (frogs) on task. Without the teacher, students run wild. With wild students, the classroom materials (dragonfly larvae) get used up fast. With no materials, students slow down (algae recovers). One removal cascades through every role in the room. Each "role" in an ecosystem is like a job in the food web. Removing one job changes what every other job does.

Guided choices: When herons (top predator) are removed, frog populations will ________ (increase / decrease). With ________ (more / fewer) frogs eating dragonfly larvae, the dragonfly population will ________ (increase / decrease). With ________ dragonfly larvae eating algae, the algae population will ________ (increase / decrease).

Now say your answers out loud and explain each step in one sentence.

Push my thinking Extension

The cascade model assumes a simple food chain, but real ecosystems have food webs with many connections. Research how food web redundancy affects resilience: what happens when multiple predators can eat the same prey? Does the cascade still occur? Then design a thought experiment: compare two ecosystems, one with high food-web redundancy (many connections) and one with low redundancy (mostly linear food chains). Predict which ecosystem is more resilient to the loss of a single species and explain your reasoning using energy flow and limiting factors. Finally, connect this to what you know about biodiversity — why do ecologists say "biodiversity = resilience"?

Check yourself Did it click?

Success criteria — I can:

explain that all species in an ecosystem are connected through energy and matter pathways;
predict and trace how removing one species cascades through a food web;
identify limiting factors and explain how they control population size.

A student says: "The heron is gone, but it only ate frogs — so everything else in the pond should stay the same." What is wrong with this reasoning?

Re-teach: species are not independent — every connection is a pathway Tier 3

The core idea: In a food web, every species is connected to every other species — directly or indirectly — through chains of energy and matter transfer. Removing one species does not just affect its immediate prey or predator; the effect travels up and down through all the connections that ran through that species.

Use the cascade model: when herons are removed — (1) frogs lose their predator and increase; (2) dragonfly larvae are eaten more and decrease; (3) algae is eaten less and increases. The heron "controls" algae indirectly, through two links. This is called a trophic cascade.

Energy and matter link (W1 + W2): the energy that used to flow from frogs to herons now stays in frog bodies. The matter in herons no longer cycles the same way. Every cascade is also a rerouting of energy flow and matter cycling. This is why changes to one species can alter the entire ecosystem's productivity, nutrient cycling, and habitat structure.

Re-read option B. Try to say the cascade aloud in three steps before moving on.

Reflect, then show what you know

Reflection responsible decision-making

If you were advising a city that wanted to develop a meadow, what information about the food web would you want before you decided? How does knowing about trophic cascades change the way you think about that decision? Write 2–3 sentences. There is no wrong answer.

Exit ticket — answer on paper or in your notebook. (2 new · 2 spiral · 1 transfer)

New Define "limiting factor" in your own words and give one example from today's lesson.
New Explain why removing one species from a food web can affect species that are two or three levels away from it in the web.
Spiral — W1 In today's cascade, when caterpillars overpopulate and the bird population drops, what happens to the energy flow that used to go from caterpillars to birds? Where does that energy go instead?
Spiral — W2 After the spider population crashes, many caterpillars eventually die from food scarcity. What role do decomposers play in what happens next — and does that help or hurt the ecosystem's recovery?
Transfer → W4 Humans are removing species from ecosystems at an accelerating rate — habitat loss, overhunting, and climate change all reduce populations. Based on what you know about trophic cascades, predict what large-scale human impact on biodiversity could do to ecosystems. What would the cascade look like if many species were removed at once?

Educator panel — facilitation, accommodations, answer key

At a glance

Standards: NGSS MS-LS2-1 (factors that affect the growth of organisms in an ecosystem), MS-LS2-2 (patterns of interactions among organisms), MS-LS2-4 (construct an argument for the effects of resource availability on organisms). 3D: SEP Analyzing & Interpreting Data + Constructing Explanations · DCI LS2.A Interdependent Relationships + LS2.C Ecosystem Dynamics · CCC Cause & Effect + Stability & Change + Energy & Matter. Key throughline: Every cascade is explicitly framed as a rerouting of energy (W1) and matter (W2) pathways — not just a change in population numbers. This keeps the year-long CCC thread alive and prepares students for W4 (humans redirect energy and matter flows at large scale) and W5 (the design solution must account for cascades).

Keystone-species note: The concept of a species whose removal triggers a disproportionate cascade is described generically here — do not name specific branded programs or published curricula. The ecological phenomenon is real and well-documented in the scientific literature; students may explore specific scientific examples (sea otters, wolves in Yellowstone, etc.) as Extension.

Facilitation arc (Rosenshine)

Do-Now prediction (item 3) should be written before any instruction — collect on sticky notes or mini-whiteboards for a quick class gallery (3 min).
Phenomenon SVG + notice/wonder (3 min) — cold-call two students to share from different parts of the web change.
Build Steps 1–3 (12 min) — pause after Step 1 to ask: "Can someone name a W1 energy pathway that gets broken in this cascade? A W2 matter-cycling pathway?" This enforces the throughline.
Differentiated practice — pond scenario (15 min) — circulate and listen for students who stop the cascade at frogs; redirect them to trace through one more level.
Self-check + re-teach (5 min) — classroom cascade game is excellent here if time allows.
Brain break (2 min), reflection + exit (10 min).

IEP / 504 accommodation cards

Common accommodations mapped to this lesson

Accommodation	How it's already built in / what to do
Text read aloud	Every passage has a Listen button (TTS); simplified toggle shortens it.
Reduced reading load	"Reading: Simplified" swaps in shorter plain-language text throughout.
Chunking / extended time	Cascade table in Tier-2 breaks the task into four guided steps; progress saves automatically.
Sentence frames / word bank	Tier-2 panel provides cause-and-effect frames for each cascade step and a 10-term word bank.
Concrete analogy / reduced load	Tier-3 provides the classroom-schedule analogy and guided-choice fill-in frames.
Alternative response mode	Draw a before/after diagram, write a CER, or record a narrated cascade — all accepted.
Behavior / focus (504, ADHD)	Named expectations, 30-sec check-in, the physical classroom cascade game brain break.
Multilingual / EL	Simplified text, side-by-side SVG with labeled before/after nodes, word bank, home-language reflection allowed.

Exit ticket answer key

A limiting factor is anything that sets a cap on how large a population can grow. Example: spiders are a limiting factor for caterpillars — when spiders are removed, caterpillar numbers grow because that cap is gone, until grass scarcity becomes the new limiting factor.
Because every species in a food web is connected through chains of energy and matter transfer. When a species is removed, the effect travels along those chains — up to its predators (who lose a food source) and down to its prey (who are released from predation). Species two or three links away are still connected through intermediate species; the change propagates along every pathway that ran through the removed species.
The energy that used to flow from caterpillars to birds can no longer take that pathway (since birds are declining). That energy is still being transferred — caterpillars use it in their own metabolism and it is lost as heat, or it goes to decomposers when caterpillars die. The energy does not disappear, but the pathway changed. Less energy reaches the bird level of the web.
Decomposers break down the dead caterpillars and release nutrients back into the soil. This helps the ecosystem recover because those nutrients feed new plant (grass) growth. With more grass available, caterpillar populations can eventually stabilize again. Decomposers are helping the matter cycle continue even during the disruption — without them, the dead caterpillar matter would accumulate and nutrients would not be released for plant regrowth.
(Transfer preview for W4.) If many species are removed simultaneously (as is happening due to habitat loss, overhunting, climate change), multiple trophic cascades occur at once and the ecosystem may not be able to recover. Some species may have no remaining predators and overpopulate; others may lose all their food sources and crash. With fewer redundant connections in the web, cascades are more severe. This is why biodiversity loss is an urgent ecological problem — ecosystems become less resilient as connections are cut, and at some threshold they may reach a "tipping point" from which recovery is very difficult.

Why it's designed this way

Interdependence at the food-web level is a central disciplinary core idea for MS-LS2, and the trophic-cascade scenario is the ideal vehicle for three-dimensional reasoning: students must use SEP (constructing explanations from data), DCI (interconnected relationships in ecosystems), and CCC (cause and effect, stability and change) simultaneously to predict and explain the cascade. The cascade table in Tier-2 scaffolds the cause-and-effect structure explicitly, which aligns with Rosenshine's principle of small steps with checks. The classroom-cascade brain break provides an embodied, kinesthetic encoding of the abstract cascade concept — making it memorable and cross-modal (Mayer, 2009). The exit ticket item 5 is designed as a bridge to W4: the transfer item that asks students to scale the cascade model up to human impact at a population level, previewing the habitat-change data set they will analyze next week.

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