Don't you love that word? There's a new book out called Rewilding the World, about wilderness preservation. Let's hope that's not a hopeless goal, but then again, how much of the world could we reasonably hope to rewild?
This is highly relevant to a certain sort of debate people get into about domesticated animals. Defenders of animal farming sometimes point out that about 25% of earth's land surface is used for grazing. All over the place, billions of animals are living out natural lives the moments of which are mostly pleasant. There would be much less animal happiness in the world if we stopped using these animals for meat, milk, wool, etc. (Note, I am not talking about factory farmed animals here.)
Critics can challenge the basic ethical perspective that takes quantities of happiness as ethically important, but there are lots of reasons to take that perspective seriously. It's certainly true that many people do take it seriously. So it would be a bad idea to dismiss this sort of defender of animal farming by saying total happiness doesn't matter.
Some critics simply point out that animal farming has not, in fact, increased the population of happy animals. For thousands of years, human beings have been clearing land to make room for livestock. There were once wild animals on that land, and they may very well have had richer and more pleasurable lives. The replacement of wild animals continues today, as forest continues to be cut down to make room for grazing animals or their feedcrops.
Looking backwards or forwards, you don't find points in time when you can deem the replacement of forest with pasture a net boon to animal happiness. It's another question though, whether today we can argue for reducing the number of grazing animals on grounds that wild animals will compensate for any resulting drop in total happiness.
If human beings suddenly vanished, and with them all their livestock, wildlife would fill in all the empty spaces. But if humans don't vanish, but merely put an end to animal agriculture, surely grazing land would be put to some other economic purpose. Humankind abhors an economic vacuum.
The choice is not between retaining billions of farm animals and bringing back the same number of wild animals. Not gonna happen! It's between having a world teaming with sheep pastures, cattle ranches, and the like, and a world where land is used (and abused) in some other way. The former pastures would become--what?--paper forests, or housing developments, or sites for mining, or carefully pest-controlled plant farms...or who knows what?
If there were fewer grazing farm animals, that would mean a net decrease in happiness, but also a decrease in killing. For those who think happiness is good, but also believe killing animals is inherently wrong, the whole thing is hard get a grip on. For them, increasing happiness and decreasing killing are incommensurable goods. Would we live in a better world if the pastures were emptied out and put to some other anthropocentric use? I think that's a very hard question to answer.