Mars needs 'em, or at least, their end product! 8-)
This is all about one of my many, many, mad obsessions, terraforming. The martian regolith is full of peroxides and short on organics, so, as I read in a sidebar in GURPS Mars a long, long time ago, Mars needs s#!t... mixed one to one with the harsh soil, the peroxides sterilize the manure and it all becomes the basis for soil. People are one source, but I maintain that Mars needs elephants!
Of course, building a dome and providing enough fodder for elephants is a whole other challenge, but 'I want to live forever and terraform Mars!'
In fact, I don't think we can, nor should we, terraform Mars. It's not ideal starting material; the gravity is too low, and other things are wrong with it. A good martian habitat would probably spin like an old record player, tilted inward slightly, to add a little spin gravity to the four tenths we'd get on the surface of Mars. So, build your dome on the Red Planet and spin some moldy oldies, or green and pleasant hills!