One of the main issues is whether Mars has any indigenous life or not — how does this affect the question of colonization or terraforming?
If Mars does have any kind of biosphere, it should be preserved as much as possible. We still don't know yet if any such biosphere exists, but the possibility, which has only increased based on recent discoveries, must be taken into account. Such a precious discovery, which could teach us immensely about how life arose on both worlds, should be completely off-limits. Small colonies might be fine, but living on Mars should not be at the expense of any native habitats, if they exist. The most likely place to find life on Mars is underground. If the surface is truly as sterile and barren as it seems to be, then colonies there shouldn't be too much of a problem. It has also been suggested that Martian caves would make ideal human habitats, serving as natural protection from the harsh conditions on the surface. True, but if it turned out that something else was already taking up residence in them, then we should leave them alone.

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