Use real case studies, including wolves in Yellowstone, to explain how changing one species in an ecosystem can cause a cascade of indirect effects on many others.
Ecosystems can be disrupted by invasive species, habitat loss, pollution, overhunting, or disease, and removing or adding even one species can ripple through an entire food web in what scientists call a trophic cascade. One of the best-documented cases happened in Yellowstone National Park: by 1926, gray wolves had been eliminated from the park. Without their main predator, the elk population grew large and stayed in one place longer, heavily overgrazing young willow and aspen trees along streams.
That overgrazing had effects far beyond the trees themselves: fewer streamside trees meant less shade, weaker riverbanks, and less food and building material for beavers, whose numbers dropped along with the wetlands they normally create. When wolves were reintroduced to Yellowstone in 1995, elk behavior and numbers changed, streamside vegetation began recovering in many areas, and beaver and songbird populations responded over the following years. Ecologists still study exactly how much credit belongs to the wolves alone versus other changing conditions, but the broad pattern, that removing or restoring one predator affected plants, rivers, and several other species, is well documented.
Mastery looks like: They can explain the Yellowstone (or a similar) case as a chain of direct and indirect effects, and apply the same cause-and-effect reasoning to a new scenario.
Common stumbles: Treating the disruption as affecting only the one species directly involved; skipping steps in the cascade instead of tracing the full chain.