## Definition A **trophic cascade** is an indirect ecological effect in which a change at one trophic level propagates downward (or upward) through a food web, altering the abundance and behaviour of species two or more levels away from the original perturbation. The classic pattern is a three-level cascade: removing (or adding) an apex predator releases the herbivore tier, which then overexploits the producer tier — a chain of indirect effects that bypasses any direct interaction between predator and plant. ## Trophic Levels and the Food Web Charles Elton mapped animal communities as layered hierarchies — producers (plants), primary consumers (herbivores), secondary and tertiary consumers (carnivores) — connected into food chains and, more realistically, food webs. Elton noted an *ecological pyramid*: abundant individuals at the base, progressively fewer at each level above. The stability of that pyramid depends on the regulatory relationships between levels. The Hairston-Smith-Slobodkin (HSS) hypothesis, also called the "green world hypothesis" (1960), proposed that ecosystems are green because herbivore populations are kept below the level at which they would consume all vegetation — held in check by predators from above. This top-down regulation implies that predators are the key stabilising force, not bottom-up food supply. ## Mechanism A trophic cascade unfolds through two linked negative feedback loops being disrupted: 1. **Top-down cascade (predator removal):** Apex predator is removed → prey (herbivore) population rises → herbivores over-consume producers → vegetation collapses. 2. **Top-down cascade (predator addition):** Apex predator is reintroduced → prey suppressed → producers recover → dependent species cascade upward in recovery. The effect is not limited to direct prey; through competition and habitat modification it radiates laterally to non-prey species. Carroll calls this the **Second Serengeti Law**: *Certain species generate strong indirect effects through trophic cascades.* ## Canonical Examples **Sea otter → sea urchin → kelp (Pacific coast)** Sea otters prey on sea urchins; urchins graze kelp. Where otters are absent (hunted to near extinction in the 19th century), urchins multiply and reduce kelp forests to "urchin barrens." Re-establishment of otters restores kelp canopy, which supports fish, bald eagles, and other species. The cascade later reversed when killer whales, deprived of harbour seals, switched to eating otters — a higher-order cascade. **Wolf → elk → riparian vegetation (Yellowstone)** Grey wolves were extirpated from Yellowstone by the 1920s. Without wolf predation, elk populations grew and grazed down aspen, willow, and cottonwood along rivers. After wolf reintroduction in 1995, elk avoided valley bottoms (a behavioural cascade, not just numerical suppression), allowing woody riparian vegetation to recover. Beaver colonies in the Lamar Valley grew from one to twelve within a decade, altering stream hydrology. **Rinderpest eradication → wildebeest → grass → fire (Serengeti)** When the rinderpest pathogen (imported via cattle) was eliminated, wildebeest populations grew from ~370,000 to ~1.4 million. More wildebeest meant shorter grass, less fuel for fires, fewer fires, and more surviving tree seedlings — benefiting giraffe and a cascade of dependent species. ## Limits and Complexity Carroll cautions that real food webs are not linear chains. Cascades are attenuated by: - **Omnivory** — consumers feeding at multiple levels dilute the cascade signal. - **Compensatory prey switching** — predators shift to alternative prey when the primary prey is scarce. - **Bottom-up constraints** — nutrient or light limitation can override top-down effects. - **Irreversibility** — some cascades (e.g. river-bank erosion after decades of elk grazing) cannot be reversed merely by restoring the predator. ## Related - [[Keystone Species]] - [[Biological Regulation]] - [[Carrying Capacity]] - [[The Serengeti Rules (Ecological Laws)]] - [[Negative Feedback Regulation]] ## Sources - [[The Serengeti Rules (Carroll 2016)]]