What is the primary cause initiating a classic trophic cascade?

Answer

A significant demographic shift in the apex or keystone predator population.

The primary drivers of classic trophic cascades are almost always related to alteration of populations at the highest feeding levels, such as the removal or addition of a top predator.

What is the primary cause initiating a classic trophic cascade?
EcologyEcosystemPredatortrophic cascadefood web