[!]STUDY GUIDE: Big IdeasUse to answer form questions
1.Ecosystems are dynamic; populations fluctuate and respond to changes. Students often think ecosystems are stable and unchanging, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.
2.Food webs are complex networks; removing one species affects many others. Students often think energy flows in simple linear food chains, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.
3.Biodiversity provides ecosystem services, resilience, and stability. Students often think biodiversity is 'nice to have' but not essential, but evidence from investigations shows otherwise. Use data from today's stations to identify which observations best support this understanding.
DRAWKEY TERMSGoogle each → Images tab → sketch what you see
The 10% Energy Rule
Only about 10% of energy transfers between trophic levels
Trophic level
Position in food chain (producer, primary/secondary consumer, apex predator)
Real Data from Yellowstone
Scientists have tracked population changes since wolf reintroduction in 1995
Trophic Cascade
When changes at one trophic level affect all other levels
OTHER VOCAB: keystone species · 10% rule · apex predator · biodiversity · ecosystem
WRITEFOCUS QUESTIONYour initial thinking
After wolves were hunted out of Yellowstone in the 1920s, elk herds exploded and stripped riverbanks bare. When wolves were reintroduced in 1995, trees grew back, roots stabilized the soil, and rivers literally changed course.
Why did removing wolves from Yellowstone change the rivers?
I think...wolves don't drink the river or push the dirt, but they control the elk. Without wolves, elk overgrazed willows and aspens along the riverbanks. Without those tree roots holding the soil, the banks crumbled and the rivers literally changed shape. That's a TROPHIC CASCADE — top predators control plants by controlling herbivores.
WRITEAFTER HOOK FORMReflect before continuing
I learned that...ecosystems are connected webs, not separate parts. A change at the TOP of the food chain (wolves) cascaded all the way down: more elk → fewer willows → less beaver habitat → eroding riverbanks → different river paths. Real data from 1995 reintroduction confirms this — willow heights tripled in some areas.
Station 1 — Trophic Cascade Simulation
I observed that...in the trophic-cascade simulation, removing the top predator (wolves) caused the herbivore (elk) population to spike. Within a few simulated years, plant biomass crashed, then secondary consumers like songbirds dropped because their habitat was gone.
This shows that...predators don't just eat prey — they also control prey BEHAVIOR (where the elk feel safe to graze) and prey NUMBERS. Either way, removing a top predator releases the herbivores, and the plants get hammered.
The key concept is...A trophic cascade is a chain reaction where changes at one trophic level ripple through every other level. Top-down control means predators shape plant communities indirectly, through the herbivores they eat or scare.
CHECKDAY 1 CHECK-INBefore you leave
Q1:Based on today's investigation of ecosystem dynamics & trophic
cascades, explain one key finding using evidence from your data.
Connect your answer to this week's focus question: Why did removing
wolves from Yellowstone change the rivers?
Removing wolves changed the rivers through a trophic cascade. Without wolves, elk grazed freely along riverbanks and stripped the willows and aspens. Without tree roots, the soil eroded and the river channels widened. When wolves returned in 1995, elk avoided open riverbanks, willows recovered, beavers came back to build dams, and the rivers stabilized. Wolves are a keystone species — their effect cascades through every level of the ecosystem, all the way down to the physical environment.
The data shows...in a real food web, energy flows in MANY directions, not a single line. A coyote might eat rabbits, mice, AND berries; a hawk might eat the coyote's prey too. Removing one species rarely affects just 'one level' — it shakes the whole web.
This connects to...the 10% Rule: only about 10% of energy at one trophic level transfers to the next. That's why there are LOTS of grass plants, fewer elk, even fewer wolves. The 90% lost as heat (cellular respiration) is why food chains rarely have more than 4–5 levels.
Station 3 — Design a Conservation Plan
The main idea is...wolves are a KEYSTONE species — small in number but huge in impact. Yellowstone's data (1995–today) shows that when wolves returned, elk avoided riverbanks, willows recovered, beavers came back to build dams, and river channels stabilized.
This connects to what we learned because...this directly answers our focus question: wolves changed the rivers by controlling where elk grazed. It also shows that ecosystems aren't 'naturally balanced' — they DEPEND on key species. Removing one keystone unravels the whole web.
I can apply this by...when I hear about a species being threatened, I can ask: is this a keystone species? What CASCADES would happen if it disappeared? (Example: sea otters control sea urchins → urchins eat kelp → kelp forests are the nursery for hundreds of species.)
DRAWCONCEPT MODELShow your understanding visually
Draw a Yellowstone food web showing the trophic cascade after wolves were removed (1920s) vs. after reintroduction (1995). Show 5 levels: wolves → elk → willows → beavers → river morphology. Use arrow thickness for energy flow (10% rule); circle the keystone species; label one apex predator.
Two side-by-side food web diagrams. LEFT (1920–1995, no wolves): elk drawn LARGE with many arrows down to bare ground; willows tiny; songbirds and beavers crossed out; river drawn wide and eroded. RIGHT (1995–today, wolves reintroduced): wolves at top → arrow to elk (medium size); elk → willows (now tall); willows → beavers + songbirds; beaver dams → narrow stable river. Arrows labeled '10% energy passes up'. Label: 'Trophic Cascade: top predator → herbivore → plants → physical environment.'
Labels included
Relationships shown
Key terms used
CHECKTEST YOUR THINKINGRate each: FALSE ← 1-2-3-4-5 → TRUE
1. Ecosystems are naturally stable and stay the same unless humans disturb them.
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2. Energy in a food web flows in a simple straight line from producer to top predator.
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3. Biodiversity is nice to have but not essential for an ecosystem to function.
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4. A keystone species matters only when its population is large enough to dominate other species in the ecosystem.
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5. [Last week · C7W4] Mass extinctions in Earth's past were random events that didn't reshape today's ecosystems.
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6. [Spiral · C4W3] More nutrients flowing into a stream or pond are always good — fertilizer runoff boosts ecosystem biodiversity.
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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:
Statement #___ -- I rated it ___ because:
Statement #___ -- I rated it ___ because:
THINKSCIENTIST CIRCLE DEBRIEFReflect after your Scientist Circle discussion
AHA! Moment
The one I changed my thinking most about is...predators control plants by controlling herbivores — and they even change the SHAPE of rivers. Wolves didn't touch the rivers; they just changed elk behavior, and the rivers followed.
Remaining Question
I still wonder...If trophic cascades are this powerful, what happens to ecosystems where humans have removed the top predators (sharks in the ocean, big cats in Africa)? Can we predict what will fall apart next?
WRITEFOCUS QUESTION REVISITFinal written response
Why did removing wolves from Yellowstone change the rivers?
Word Bank: trophic level · trophic cascade · keystone species · 10% rule · apex predator · biodiversity · ecosystem · The 10% Energy Rule
CLAIM: Removing wolves changed Yellowstone's rivers because wolves are a keystone species whose absence triggered a trophic cascade — elk overgrazed riverside vegetation, riverbanks lost their root systems, and the rivers eroded and changed course.
EVIDENCE: After wolves were hunted out by 1926, elk populations climbed and willow heights along streams crashed. After reintroduction in 1995, willows tripled in height in some areas, beavers returned (from 1 colony to 9+), and stream channels narrowed and stabilized.
REASONING: A trophic cascade means impacts at the top of the food web ripple all the way down. Wolves controlled where elk grazed; elk controlled willow survival; willows controlled bank stability and beaver habitat — and beavers controlled the river itself. Remove one keystone species and the entire physical landscape can change.