If we ever do "terraform" a planet it will most likely be far after we have colonized space. After we perfect space habitats, cylinder, torus, or however we do it, our future lies in the direction of space but at some point some agency or group might as a nostalgic after thought try to transplant life on Mars. I think in the end we will domesticate all of life and like we carried our cows, chickens, pigs, sheep, and goats we will take life with us into space, possibly space habitats that contain huge areas of tame wilderness...