The AP reports on the results of removing the feral cats from an isolated island in an effort to get the island back to what it was before man introduced non-native species. It sounded like a good idea, but the results were a disaster because the rabbit population then exploded, and the rabbits ate the vegetation that the birds needed for cover.
This is an illustration of the theory of the second best, the idea that if a system has several things that are less than optimal, then removing one of the defects can actually make the system worse because it is possible that the defects offset each other. An economic system with an industry that is a monopoly and a polluter may be less efficient if only one of the problems is solved because one problem leads the industry to produce more than is economically efficient and the other leads it to produce less.
The idea was also illustrated in a Simpson's episode in which Mr. Burns was told that curing any one of his many diseases would kill thim because all the things wrong with him were in a delicate balance of offsetting one another.