Have there been any notable tries in the recent past (think 20th century or later) to purposefully change any ecosystem in order to save it or some particularly species in it, but which have instead produced the opposite effect? I am sure there must have been, but can't bring any to my mind at the moment.
Like, for example, if you have an island that has some large felines, which frequently attack sheep that are becoming scarce, so the felines are starving and becoming scarce, too. So you introduce feral goats, which were supposed to provide meat for felines, however, goats were (a) too nimble for felines to hunt well due to, eg. geography of the island (too many rocks that goats can prance about, but cats not so well), and (b) ate all the grass that sheep still had. So the net effect was that sheep died out, cats died out and there remained a few goats.
I am aware of a number of cases, where these things could have happened due to humans not aware or not caring about the problem (eg. introducing rabbits in Australia), but I am wondering specifically about misplaced efforts to save/stabilize an ecosystem.