Week 1: Ecosystem Dynamics & Trophic Cascades

Grade 7 Science | Rosche

MS-LS2-4 · MS-LS2-1 Ecosystems: Interactions, Energy, and Dynamics

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The Phenomenon: Why did removing wolves from Yellowstone change the rivers?

Anchoring Context & Focus Question

Why did removing wolves from Yellowstone change the rivers?

Side-by-side schematic of a Yellowstone valley without wolves (wide eroded river, large elk herd browsing the bank) and with wolves restored (narrower deeper river, willows regrowing, elk displaced uphill)
Yellowstone before and after wolf reintroduction. Wolves were hunted to extinction by 1926 — and when they returned in 1995, scientists watched rivers change course. How can a predator reshape a river?
Animated graph showing predator-prey population cycles: the green solid curve (rabbits/prey) peaks first, then the blue dashed curve (wolves/predator) rises in response, illustrating the Lotka-Volterra cycle where predator and prey populations oscillate out of phase.
Predator-prey population cycles — when rabbit populations peak, wolf populations follow. This Lotka-Volterra pattern helps explain the Yellowstone trophic cascade.
Animated explanation of the week's focus question. Watch then reason about why this happens.
Key Vocabulary (7 terms) — Practice Tool

Cognate Strategy: Many science words look similar in English and Spanish — use your Spanish to learn science!

Term Spanish Definition
trophic level nivel trófico Position in food chain (producer, primary/secondary consumer, apex predator)
trophic cascade cascada trófica When changes at one trophic level affect all other levels
keystone species especie clave Species with disproportionately large ecosystem effects
10% rule regla del 10% Only ~10% of energy transfers between trophic levels
apex predator depredador ápice Predator at top with no natural predators
biodiversity biodiversidad Variety of life in an ecosystem
ecosystem ecosistema All living organisms and their physical environment interacting

Hook – The Wolves and Rivers Mystery

Explore the surprising connection between wolf populations and river behavior in Yellowstone.
Hook concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Hook: Ecosystem Dynamics & Trophic Cascades

The Wolves and Rivers Mystery

In 1926, the last wolf pack in Yellowstone National Park was killed. Over the next 70 years, elk populations exploded, riverbanks eroded, willows and aspens disappeared, and the Lamar River itself changed course. Then in 1995, scientists reintroduced 14 wolves. Within a decade, rivers stabilized and forests returned — even though wolves never touched a single tree. How is that possible?

Cycle 4 Connection

You studied human impact on ecosystems in Cycle 4. Removing a top predator is a form of ecosystem disruption — but the effects ripple in unexpected directions. Use what you know about food webs and energy flow to predict: if wolves eat elk, and elk eat willows, what happens to willow populations when wolves disappear?

What You'll Do

  1. Analyze a Yellowstone before/after data set (wolf population, elk population, willow cover, river bank erosion)
  2. Connect your Cycle 4 knowledge of human impact to predator removal
  3. Predict the chain of effects from wolves → elk → vegetation → rivers
  4. Submit your initial reasoning — no wrong answers yet
COMPLETE THE HOOK FORM

Investigate how removing wolves changed Yellowstone's rivers.

Hook Form — Form pending deployment

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Worked Example and Simulation – Tracing a Trophic Cascade

Worked Example and Simulation concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Worked Example and Simulation: Ecosystem Dynamics & Trophic Cascades
Your Simulation Summary
Your prediction: —
Trial results: —
Average result: —

Worked Example and Simulation: Tracing a Trophic Cascade

Problem: Scientists ask, "If wolves are removed from Yellowstone, what happens to the rivers?" Trace the cascade from apex predator through each trophic level to a physical landscape change.

Step 1: Identify the Trophic Levels

  1. Apex predator: Gray wolf (Canis lupus) — eats elk, controls herbivore behavior
  2. Primary consumer (herbivore): Elk (Cervus canadensis) — eats willows, aspens, grasses
  3. Producers: Riparian vegetation — willows, aspens, cottonwoods along riverbanks
  4. Abiotic system: River banks, soil stability, stream channel shape

Step 2: Trace the Cascade (Wolves Removed)

  1. Wolves disappear → elk population grows AND elk stop fearing risky areas (riverbanks)
  2. Elk overgraze willows and aspens along every riverbank in Yellowstone
  3. Tree roots that held soil in place disappear → banks erode, rivers widen and meander
  4. Without shade trees, river water warms → fish populations decline

Step 3: The "Ecology of Fear" — Why It's Not Just Numbers

The wolf's most powerful effect is behavioral, not just numerical. Even when wolf numbers are small, elk avoid risky foraging sites (open riverbanks). This is called the ecology of fear. Plants recover not because elk disappear, but because elk move. Scientists call this a trophic cascade — changes ripple from apex predator all the way to abiotic factors like river shape.
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Station 1 – Trophic Cascade Simulation

Model how changes at one trophic level ripple through an ecosystem.
Station 1 concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Station 1: Ecosystem Dynamics & Trophic Cascades

Station 1 — Trophic Cascade Simulation (20 pts)

Use the Trophic Cascade Simulator to model what happens when you add or remove species from a Yellowstone-style food web. Your job is to run at least three scenarios and record how each change ripples through every trophic level.

Simulation Instructions

  1. Baseline run: Start with a balanced ecosystem (wolf + elk + vegetation). Record population sizes after 20 simulated years.
  2. Wolf removal: Set wolf population to zero. Run 20 years. Record changes to elk and vegetation.
  3. Wolf reintroduction: Reintroduce wolves at 10% of baseline. Run 20 more years. Record what recovers and what doesn't.
  4. Write a CER (Claim–Evidence–Reasoning) response explaining why vegetation recovered faster than expected.

Spiral Connection — Cycle 7 Geologic Time

In Cycle 7 you learned that Earth's ecosystems have changed dramatically over geologic time. Wolves and elk co-evolved over millions of years. When humans removed wolves in <100 years, we disrupted a relationship 10,000+ years old. How does scale and proportion (CCC-3) help explain why ecosystems struggle to recover quickly?
COMPLETE THE STATION 1 FORM

Simulate trophic cascades and observe population effects.

Station 1 Form — Form pending deployment

This form is authored but has not yet been published to Google Forms. Your teacher will share the link once it is live. No action needed from students until then.

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Station 2 – Population Dynamics Analysis

Analyze population data to identify predator-prey relationships.
Station 2 concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Station 2: Ecosystem Dynamics & Trophic Cascades

Station 2 — Population Dynamics Analysis (20 pts)

You are given 100 years of real Yellowstone population data: wolf counts, elk counts, and willow cover percentages measured from aerial surveys. Your job is to find patterns, identify time lags, and explain what the data reveals about trophic cascades.

Data Analysis Tasks

  1. Graph it: Plot wolf population (blue), elk population (orange), and willow cover % (green) on the same time axis from 1920–2020.
  2. Identify the lag: When wolf populations declined (1920–1940), elk numbers did not peak immediately. How many years was the lag? Why might there be a delay?
  3. Find the turning point: Mark on your graph where wolf reintroduction (1995) appears to affect elk behavior. Is it in the population numbers, or something else?
  4. Argue from evidence (MS-LS2-4): Write one sentence: "The data supports/does not support the claim that wolf reintroduction affected vegetation because ___".
COMPLETE THE STATION 2 FORM

Analyze population data and graph predator-prey dynamics.

Station 2 Form — Form pending deployment

This form is authored but has not yet been published to Google Forms. Your teacher will share the link once it is live. No action needed from students until then.

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Station 3 – Design a Conservation Plan

Apply ecosystem knowledge to design an evidence-based conservation strategy.
Station 3 concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Station 3: Ecosystem Dynamics & Trophic Cascades

Station 3 — Design a Conservation Plan (25 pts)

Missouri once had a top predator too: the eastern mountain lion. It was hunted to extinction in the state by the early 1900s. White-tailed deer populations have surged. Forest understory vegetation is being browsed into collapse in many Ozark areas. Your challenge: design a trophic restoration plan for an Ozark stream ecosystem near St. Louis.

Design Constraints

  1. Predict the cascade: Draw or describe a food web showing at least 4 trophic levels. Identify where the biggest gap is now (missing predator).
  2. Propose a keystone species strategy: Should Missouri reintroduce mountain lions, or is there another species that could fill the same role? Justify with evidence from the Yellowstone case study.
  3. Identify stakeholders: Farmers, hunters, and conservation biologists all have different opinions. Write one objection a farmer might have and one response from a conservation scientist.
  4. SEP-7 Argument: Write a one-paragraph evidence-based argument: "Reintroducing a top predator to Ozark ecosystems would / would not benefit Missouri streams because ___".

Missouri Connection

Forest Park in St. Louis sits along the Meramec watershed. Deer overgrazing near urban forest edges is a real documented problem — some St. Louis parks have installed deer exclosures (fences) to let native plants recover, essentially mimicking the ecology of fear effect artificially.
COMPLETE THE STATION 3 FORM

Design a conservation plan using trophic cascade principles.

Station 3 Form — Form pending deployment

This form is authored but has not yet been published to Google Forms. Your teacher will share the link once it is live. No action needed from students until then.

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Exit Ticket – Ecosystem Dynamics Integration

Synthesize learning about trophic cascades and ecosystem dynamics.
Exit Ticket concept anchor for Ecosystem Dynamics & Trophic Cascades. Vocab cards: trophic level, trophic cascade, keystone species, 10% rule. Why did removing wolves from Yellowstone change the rivers?
Exit Ticket: Ecosystem Dynamics & Trophic Cascades

Exit Ticket — Ecosystem Dynamics Integration (23 pts)

Demonstrate your understanding through 2 new-content questions (trophic cascades and MS-LS2-4), 2 spiral questions (food webs from Cycle 4 + human impact), and 1 integration question [INTEGRATION + SEP-7] that asks you to construct an evidence-based argument about how a biological change affects ecosystem populations.

COMPLETE THE EXIT TICKET

Demonstrate mastery of ecosystem dynamics and trophic cascade concepts.

Exit Ticket — Form pending deployment

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Enrichment & Extension
Optional content if you finish early or want to go deeper.

Scientist Spotlight: Aldo Leopold

Aldo Leopold (1887–1948) grew up in Burlington, Iowa, just across the Mississippi River from Missouri. As a young forester, he believed that wolves were the enemy of deer hunters and farmers — so he helped eliminate them from the American Southwest. Then, watching a dying wolf's eyes, he had a transformation. Leopold went on to write A Sand County Almanac (1949), one of the most influential environmental books ever written, and invented the field of wildlife ecology. He was the first scientist to describe the concept that would later be called a trophic cascade: removing wolves caused deer to overgraze and mountains to erode. His "thinking like a mountain" essay, written about the Missouri/Iowa border region, is the direct intellectual ancestor of the Yellowstone wolf reintroduction. Leopold's work connects directly to St. Louis — the Missouri River he knew as a child still shapes the ecology of our region today.

Environmental Justice: Missouri's Lost Prairie

Before European settlement, Missouri was covered by over 15 million acres of tallgrass prairie — one of the most biodiverse ecosystems in North America, teeming with bison, prairie chickens, and hundreds of wildflower species. Today, less than 0.1% of that original prairie remains. The rest was converted to row crops like corn and soybeans, eliminating the native food web that supported it. Indigenous nations — including the Osage, who managed Missouri's prairie with controlled burns for thousands of years — were forcibly removed, taking their ecological knowledge with them. Now, in neighborhoods like Baden and Riverview in north St. Louis, communities lack access to green space partly because the native prairie buffer zones that once filtered stormwater and cooled temperatures were never restored. Restoring even small prairie patches in urban St. Louis is an environmental justice issue: the communities who lost the most are often the ones who benefit least from restoration funding.

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Week 1 Complete!

Next Week: Biodiversity & Ecosystem Resilience — how can one small change collapse an entire ecosystem?