The atmosphere of Mars is not the same as the atmosphere of our own planet. Whereas Earth has a sizable amount of nitrogen and oxygen, with small amounts of carbon dioxide and other gases, Mars is composed mostly of carbon dioxide (95.3%), with nitrogen (2.7%), argon (1.6%), oxygen (.2%), and other trace gases. This obviously presents a serious problem for Martian colonization. Humans cannot survive in such an atmospheric composition even if the air pressure were at Earth sea level.
In contrast the composition of Earth's atmosphere contains 78.1 percent nitrogen, 20.9 percent oxygen, 0.9 percent argon and 0.1 percent carbon dioxide and other gases. It is obvious that carbon dioxide levels are much too high for animals and humans, but nitrogen levels are very low also, something that many people do not realize. Plants require CO2, but also need nitrogen to survive.
There are two main options that we have available. One would be to raise carbon dioxide levels farther to harbor an explosion of plants on the surface of Mars and an increase in temperature, or to slowly introduce both plants and biologically engineered animals to bring levels to a tolerant level. Either way, the process could take generations. Estimates vary from a few weeks to 100,000 years or more. At RC, our general theory is that genetically engineered animals could be supported on the surface by about the year 2350 if terraforming is started before the end of the century.
Part of the balance of Earth's ecosystem is that animals breathe in oxygen and breathe out carbon dioxide, whereas plants breathe in carbon dioxide and exude oxygen. Terrestrial plants would have to undergo less genetic alteration to survive in this carbon dioxide rich atmosphere. Alternatively, if terraforming were not the objective, then domed greenhouses would have little trouble compressing the Martian atmosphere to support Terrestrial or near-Terrestrial plant life. If enough organic material from Earth were imported, these greenhouses could survive until local sources of organic material could be found or locally made.
These greenhouses might even have better outputs than some Terrestrial counterparts, since they would most likely be kept at nearly 100% carbon dioxide. It is a distinct possibility that this would help any colonization effort to become self sufficient, and reduce costs by enormous amounts due to drastically reduced imports from Earth.