A trophic cascade happens when a change at one feeding level affects many other levels in an ecosystem. It often begins with a top predator, such as a wolf, shark, or sea otter, whose presence changes the numbers or behavior of herbivores and smaller predators. These changes can ripple downward to plants, algae, soils, rivers, and other habitats.
Trophic cascades matter because they show that predators can help maintain ecosystem balance, not just reduce prey numbers.
In Yellowstone National Park, the return of wolves in the 1990s became a famous example of a possible trophic cascade. Wolves reduced elk numbers in some areas and changed where elk spent time, which allowed young willow, aspen, and cottonwood trees to recover in certain streamside habitats. More vegetation supported beavers, birds, insects, and stronger riverbank structure.
Scientists still study the details, but the Yellowstone case shows how food webs are connected through both direct effects, such as predation, and indirect effects, such as changes in prey behavior.
Key Facts
- A trophic cascade is an indirect chain reaction across feeding levels caused by a change in one trophic level.
- Top predator increases can cause herbivore decreases, which can cause plant biomass increases.
- Top predator removal can cause herbivore increases, which can cause plant biomass decreases.
- A simple biomass relationship can be written as ΔPlants ≈ -kΔHerbivores, where k is a positive effect strength.
- In a three-level food chain, Predator → Herbivore → Plant, the predator and plant often change in the same direction.
- Trophic cascades can be caused by changes in population size or by behavior changes, such as prey avoiding risky feeding areas.
Vocabulary
- Trophic level
- A trophic level is a feeding position in an ecosystem, such as producer, herbivore, or predator.
- Top predator
- A top predator is an animal at or near the top of a food web that has few or no natural predators as an adult.
- Herbivore
- A herbivore is an organism that gets energy by eating plants or algae.
- Indirect effect
- An indirect effect occurs when one species affects another species through a third species or through habitat changes.
- Keystone species
- A keystone species is a species that has a much larger effect on an ecosystem than its abundance alone would suggest.
Common Mistakes to Avoid
- Thinking predators only affect the animals they eat. This is wrong because predators can also change prey behavior, plant growth, habitat structure, and the abundance of many other species.
- Assuming every ecosystem has a strong trophic cascade. This is wrong because cascade strength depends on food web complexity, climate, habitat, prey defenses, and human impacts.
- Confusing correlation with causation in ecosystem data. This is wrong because two populations changing at the same time does not prove one caused the other without evidence of a mechanism or controlled comparison.
- Treating food webs as simple straight chains. This is wrong because most species have multiple food sources and predators, so effects can spread through many pathways.
Practice Questions
- 1 A park has 50 wolves and 8,000 elk. After wolf numbers rise to 75, elk numbers fall to 6,500. What is the percent increase in wolves and the percent decrease in elk?
- 2 In a streamside habitat, willow cover increases from 12 hectares to 30 hectares after elk browsing pressure decreases. What is the absolute increase in willow cover, and what is the percent increase?
- 3 Explain why adding a top predator can increase plant growth even though the predator does not eat plants.