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…