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When wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, one of the most remarkable trophic cascades occurred.

Trophic cascades occur when predators in a food web suppress the abundance and/or alter traits (e.g., behavior) of their prey, thereby releasing the next lower trophic level from predation (or herbivory if the intermediate trophic level is a herbivore). For example, if the abundance of large piscivorous fish is increased in a lake, the abundance of their prey, zooplanktivorous fish, should decrease, large zooplankton abundance should increase, and phytoplankton biomass should decrease.

This theory has stimulated new research in many areas of ecology. Trophic cascades may also be important for understanding the effects of removing top predators from food webs, as humans have done in many places through hunting and fishing activities.