Trophic cascade

Trophic cascades occur when predators in a food web suppress the abundance or alter the 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, smaller fish that eat zooplankton, should decrease. The resulting increase in zooplankton should, in turn, cause the biomass of its prey, phytoplankton, to 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.

A Top Down Cascade is a trophic cascade where the food chain or food web is disrupted by the removal of a top predator, or a third or fourth level consumer. On the other hand, a bottom up cascade occurs when a primary producer, or primary consumer is removed, and there is a reduction of population size through the community. An example could include Paine's study from the University of Washington, where he removed several species in different plots along the North-Western United States coast line, and realized that Pisaster, a common starfish, when removed, created a top-down cascade which involved a surge in barnacle and mussel populations, but a decrease in the populations of chitons, limpets, and whelks. This led to the conclusion that Pisaster was a keystone species in that food web.