Questions: Trophic Cascades and Top-Down Food Web Control

Question 1

Sea otters are hunted to near extinction in a kelp forest ecosystem. Which of the following best describes the subsequent cascade of effects?

• A. Kelp increases, because otters competed with kelp-eating species for nutrients
• B. Sea urchins decrease, then kelp increases, because removing a predator releases resources for its prey
• C. Sea urchins increase, kelp decreases dramatically, because urchin populations are no longer suppressed
• D. The ecosystem is unaffected at the plant level, because otter predation only directly impacts urchins

Answer: (C)

Otters suppress urchin populations, which in turn suppresses urchin grazing on kelp. Remove the otters → urchin populations explode → urchins devour kelp → 'urchin barrens' replace kelp forests. This is the classic trophic cascade: the predator's effect propagates two levels down through indirect control. Option D captures the intuitive but wrong answer — that a predator only affects what it directly eats. Option B is a common confusion about which direction energy and control flow.

Question 2

In a simple three-level food chain (plants → herbivores → predators), what happens to plant abundance when the top predator population is greatly reduced?

• A. Plant abundance increases, because more herbivores means more nutrient cycling that fertilizes plants
• B. Plant abundance is unchanged, because plants and predators are separated by a trophic level
• C. Plant abundance decreases, because herbivore populations increase and overgraze vegetation
• D. Plant abundance increases initially, then decreases as the food web reaches a new equilibrium

Answer: (C)

Removing predators → herbivore populations increase (released from predation pressure) → herbivores consume more plants → plant abundance decreases. Each trophic level has the opposite effect on the level two below it: predators decrease herbivores, which increases plants; removing predators therefore indirectly decreases plants. Option A reflects a real but much weaker secondary effect (nutrient cycling) that does not override the dominant grazing effect in cascade-prone ecosystems.

Question 3

Trophic cascades are equally strong and predictable in most types of ecosystems, regardless of food web complexity.

• T. True
• F. False

Answer: False

Trophic cascades are strongest in ecosystems with simple food chains, strong predator-prey links, and fast producer turnover — conditions most common in aquatic habitats. In diverse terrestrial food webs with many alternative prey species and omnivores, the cascade signal gets diffused: removing one predator may simply cause prey to shift to other prey, or other predators to compensate. Most ecosystems show elements of both top-down and bottom-up control, and the relative strength of trophic cascades is one of ecology's central empirical questions, with considerable variability across systems.

Question 4

The reintroduction of wolves to Yellowstone National Park led to recovery of riparian vegetation (willows, aspens) along stream banks, demonstrating that predators can indirectly control plant community composition.

• T. True
• F. False

Answer: True

The Yellowstone wolf reintroduction is a landmark trophic cascade study. Wolves suppress elk populations and change elk foraging behavior (elk avoid exposed riparian areas where wolf predation risk is high). With reduced elk browsing, willows and aspens along streams recovered, stabilizing stream banks, providing nesting habitat for songbirds, and creating conditions for beaver recolonization. This 'landscape of fear' effect — where predators modify prey behavior as well as prey numbers — is an important extension of simple trophic cascade theory.

Question 5

In a trophic cascade, why does each trophic level have the opposite sign of effect on the level two below it — predators increase plants, removing predators decreases plants — rather than the same direction of effect?

Model answer: Each trophic level acts as a consumer that suppresses the level below it. Predators suppress herbivores (−), and herbivores suppress plants (−). A predator's effect on plants is therefore the product of two negatives: predators reduce herbivores (−), which reduces the suppression of plants (+). The double negative yields a positive: more predators means more plants. Removing predators inverts this: fewer predators → more herbivores → more plant suppression → fewer plants. In a four-level chain, the logic extends: top predators suppress mesopredators (−), which suppresses herbivores (−), which suppresses plants (−); three negatives give a net negative, so top predators decrease plants.

This alternating sign structure is the mathematical signature of trophic cascades and distinguishes top-down control from bottom-up control. Bottom-up control predicts positive correlations across trophic levels (more nutrients → more plants → more herbivores → more predators). Top-down control predicts alternating positive and negative correlations. Real ecosystems typically show both, which is why ecologists use correlational field data, removal experiments, and food web models together to diagnose the dominant control mechanism in a given system.