What makes the wildebeest a keystone species in the Serengeti

I don’t understand this Biology question and need help to study.

 

View the YouTube video of a talk by biologist Sean Carroll called The Serengeti Rules and create a thread to answer the following questions:

What is a keystone species?

What makes the wildebeest a keystone species in the Serengeti?

What is a trophic cascade?

Read the threads of other students in the class and reply to at least two of those threads.