Suppose a species is brought to another region, where it quickly overruns its local rivals and drives the native species to extinction. This is something that has been suggested might happen with the larger grey squirrels that are slowly overwhelming the smaller red squirrels in Europe. Many people would suggest that this is a problem, but I wonder if that is really the case. One way or another, individual red squirrels will end up dying, either because other red squirrels are eating their food, or because grey squirrels are eating it. If more red squirrels die than would otherwise, the flip-side seems to be that there are more grey squirrels flourishing than otherwise. For the starving red squirrels, it doesn't seem to matter who is eating their food; and for the flourishing grey squirrels, it doesn't seem to matter where exactly they are flourishing. Of course, there is the risk of the newcomers ruining the entire local ecology and turning things into a barren wasteland, but that doesn't seem to happen consistently, certainly not in the case of the squirrels. Yet people talk about the invasion of the larger squirrels as though it were some kind of problem, and there are "worries" about the spread of grey squirrels. Why? What, beyond concerns about ecological collapse that appear unfounded, is intrinsically wrong with one species of animal replacing another?