What I'm looking at doing is a small game where you land on a planet, and the atmosphere is not survivable by a human being, but there is local life. Over the course of the game, terraforming has to take place, that will allow humans (and other sentient species that just so happen to have exactly identical requirements for breathing as humans) to live in the wildlife of the planet, without completely destroying the local life.
My initial thought was that the flora and fauna be oxygen and carbon based like on Earth, but with a lower concentration of oxygen (maybe 10%?) and then terraform to Earth levels.
Is this viable (Just partially, or in the short term), or would the local wildlife be affected too much by this? I'd like for the the wildlife to just go from outside the terraformed area, into it, without dying, so that they could serve as viable livestock, and for the plants that are viable as crops, to be able to survive as well.
It's fine if the animals are actually attracted to the terraformed air, or if the plants somehow grow faster or slower. Anything that presents new challenges is fine, I just want to avoid a dome of death, and for the initial colonists to initially have to use shelters and a oxygen supply mechanic, but later be able to breathe normally.
If the oxygen concentration plan isn't viable at all, what are the alternatives?
Notes: For all other intents and purposes, the planet is earth-like enough, that only this one minor thing, whatever that is, prevents full scale colonization. The planets and animals can be eaten, there is water, some of the plant life can be used like wood, but for fuel and for construction, gravity is in the 1g vicinity, pressure at sea level is close to 1 bar, etc...
The terraformed atmosphere is confined inside a energy barrier, that also lessens or magnifies weather effects, but is completely passable by everything else, such that wildlife doesn't even notice the barrier itself.
So the colonization would have to happen like this:
Initial shelters, with small engineering team, that rely on oxygen collectors(Or purifiers) to provide clean air.
They build a initial terraforming shield, that converts a large enough area that people could exploit the natural resources (including flora and fauna) to support a small town or city, at least in the short term, without having to wear oxygen gear all the time.
They expand the initial shield, and then set out to build a few more shields outside of the initial one, to expand the number of potential cities, and create the infrastructure needed to build a machine that converts the remaining atmosphere to one habitable by humans, but without killing of, or otherwise massively destroying the local wildlife.