( 4). Many examples come from intensively used areas of agriculture and urbanization in the developed world, perhaps presaging such problems elsewhere. Previously common species, including the rocky mountain grasshopper and the passenger pigeon, have been driven extinct through anthropogenic activities. However, typically, the more substantive concern is the extinction of commonness, or the ecological and functional extinction of common species. None of this is to say that rare species are unimportant: There is ample evidence to the contrary. Rather, we need to give weight both to retaining the different kinds of Leopold’s cogs and wheels and to ensuring that we have sufficient of each.