It's a tricky situation, because in most of those scenarios, human manipulation of the ecosystem -- often with good original intent -- is the reason the problem exists in the first place. In Colorado, for example, pike were introduced, and we now have some of the best pike fishing in North America. Problem (for some) is that those pike just love to eat trout. So in some places, there are now mandatory kill regulations -- catch a pike and you are duty-bound to kill it. For certain invasive species, like snakeheads or Asian carp, that's not so much of a dilemma. Asian carp in the Great Lakes is a nightmare scenario that could devastate that fishery. Of course, that fishery depends on Pacific salmon and steelhead, which weren't in the Great Lakes at all until people put them there over 40 years ago.