Question

In all ecosystems what happens when the keystrokes species is removed?

Answer 1

removal of a keystone species makes the rest of the system more unstable, but the best way to understand how this works is to visualize it, either by drawing a model, or using a diagram. 

 Atlantic Cod is a keystone species.  if the cod were removed, the seal population would decline because of a prey shortage, which would increase the populations of all the other fish species that the seals consume. Those increased fish populations would then overexploit their available prey, causing the population of prey to decline, resulting in a decline in predatory fish, resulting in an increased decline in the seal population, etc.
 
In addition, each increase or decrease in any species will affect every other species it interacts with, creating one hell of an unpredictable mess, which often leads to ecosystem collapse and/or extinction.

This phenomenon is known as trophic cascade.