These topics are hotly debated and important in conservation.  For instance, the ruddy duck, a pretty American species imported as an ornamental waterfowl to ponds in Britain, has now gone wild, and is hybridizing with its endangered European relative, the white-headed duck.  This is already causing significant "genetic pollution" in the remaining populations of white-headed duck.  The white-headed duck is an endangered species native to Spain, and Spanish conservationists want the ruddy duck eradicated (by shooting) from Europe.  British bird conservationists are faced with a problem; on the one hand it is nice to have a new wild species on our shores, and it seems wrong to kill any wild species; on the other hand, we don't want to lose a native European species to hybridization.

Another important topic which is big in the news at the moment is the safety of genetically modified (GM) organisms: can genes for herbicide resistance in a crop like oilseed rape "leak out" and become established in wild mustard relatives?  The answer is, of course, YES!  Given species are often not totally reproductively isolated, a gene like herbicide resistance that would be highly advantageous to weed species certainly IS likely to leak out and become selected for.  I am not sure whether this means we shouldn't release GM oilseed rape (this would be deleterious, but not obviously worse than allowing resistance to pesticides to evolve when we try to protect our crops), but we should certainly take the possibility of gene transfer between species into account when we make our decisions.