Wolves are removed from an ecosystem. Over the following decades, willow and aspen populations decline sharply along riverbanks — even though wolves ate no plants. Which explanation is most consistent with trophic cascade theory?
AWolves competed with elk for water sources; without wolves, elk monopolize water and outcompete riverside plants
BWithout wolf predation, elk populations increase and change their behavior — grazing more heavily and freely in riparian zones — reducing streamside vegetation
CWolves had a direct mutualistic relationship with willows, dispersing their seeds; without wolves, seedling recruitment failed
DWolf removal altered local precipitation patterns through reduced predator-prey landscape dynamics
The wolf-elk-vegetation relationship is the textbook trophic cascade. Wolves affect vegetation not by eating it but through two mechanisms: reducing elk numbers and, critically, changing elk behavior. Elk that no longer fear predation linger in open riparian areas, consuming willows and aspens without limit. This 'landscape of fear' behavioral effect was central to Yellowstone's recovery — vegetation rebounded in areas of high wolf predation risk even before elk numbers dropped significantly.
Question 2 Multiple Choice
In the Yellowstone wolf reintroduction, vegetation recovered along riverbanks not only because elk numbers declined, but also because:
AWolves introduced soil bacteria that accelerated willow regrowth in formerly grazed areas
BWolves changed elk spatial behavior — elk avoided open riparian areas where they were vulnerable, reducing grazing pressure on streamside plants
CWolves competed directly with elk for water access, forcing elk into drier upland habitats away from rivers
DThe reintroduction coincided with a rainfall increase that independently promoted streamside vegetation recovery
The 'ecology of fear' component of trophic cascades is often underappreciated. Even when total predator-caused mortality is modest, prey animals shift habitat use to reduce predation risk. At Yellowstone, elk that previously grazed freely along open riverbanks began avoiding these exposure-prone areas when wolves returned, even at times and locations where wolf density was low. This behavioral shift — not just population reduction — was responsible for much of the vegetation recovery.
Question 3 True / False
Trophic cascades demonstrate that direct predator-prey feeding interactions are the primary meaningful pathway through which top predators structure ecosystems.
TTrue
FFalse
Answer: False
The Yellowstone example shows that behavioral effects — changes in where and how prey animals use habitat in response to predation risk — can be as ecologically significant as direct population reduction. Moreover, cascades can extend beyond the food web into physical ecosystem structure: wolf restoration changed river channel morphology through a series of indirect effects involving vegetation, bank stabilization, and beaver recolonization. Calling this effect 'trophic' is almost an understatement — it altered the physical landscape.
Question 4 True / False
Aquatic ecosystems generally show stronger trophic cascades than terrestrial ecosystems because aquatic primary producers are small-bodied and turn over rapidly, making plant biomass highly responsive to changes in grazing pressure.
TTrue
FFalse
Answer: True
Body size and turnover rate of primary producers are key factors in cascade strength. Phytoplankton can double in hours and are highly susceptible to grazing by zooplankton — a small change in zooplankton abundance causes rapid, large changes in phytoplankton biomass. Terrestrial plants are large, long-lived, and often well-defended; removing herbivores may not immediately or strongly increase plant biomass. This scaling principle explains why lake food web experiments often show dramatic cascades while comparable terrestrial experiments produce weaker or more variable effects.
Question 5 Short Answer
Why does understanding trophic cascades change the conservation rationale for protecting top predators, and what is a keystone species?
Think about your answer, then reveal below.
Model answer: Without cascade theory, protecting a top predator (like wolves or sea otters) is justified mainly for the predator's own sake or for direct effects on prey populations. Cascade theory reveals that top predators structure entire communities through indirect effects — regulating herbivore behavior and abundance, which in turn shapes vegetation, which shapes physical habitat. A 'keystone species' is one with disproportionately large community effects relative to its biomass. Losing a keystone triggers cascading changes far beyond the species directly eaten, making predator conservation a lever for managing entire ecosystems.
The Yellowstone wolf reintroduction became an iconic example because the effects were so broad and unexpected: from vegetation to river morphology to songbird diversity to scavenger communities. If ecologists had only tracked wolf-elk interaction, they would have missed most of the story. Food web thinking — mapping who eats whom and how strong those interactions are — is what allows ecologists to anticipate, rather than be surprised by, these indirect effects.