Adding or removing a species from n eco system can lead to an imbalance ecosystem for the reason that a species might be surviving due to another one or some species might be playing the role of check and balance for another, removing that species will cause that check and balance to stop.
A simple hypothetical example is that a lion eats deer, when you remove a lion from that system the number of deer will rise incredibly which will need more vegetation for them, resulting as there will no vegetation be left leading to some other effects and the chain continue…