"But this replacement is not total. There is whole ecosystem that supports the existence of human beings, and most of it is invisible. When we go to Mars we believe we can use technology alone to survive there. But could we be wrong? The stars may only be accessible to those species that cultivate life in general. This would set them apart, and perhaps provide a rich diversity of biological capabilities that can make the impossible happen. Could there be biota that could make Mars' atmosphere breathable or warmer? Could there be a methodology to rapidly evolve life on Mars, rather than just plop it in place and hope it can be maintained with mechanical devices?

Discuss."

The problems associated with establishing a Mars colony will be immense. If you using socialistic model it will simply fail.

It is true that we are connected to a web of life- in terms long term survivial, it or something that functions similar to it is needed. But this web of life can also cause us problems- a virus could kill tens of millions if not billions of people. So, it's a challange but likely to have great benefits associated with overcoming the challenge.
As far as our connection to biosphere, if one is only looking at time periods of less than a decade, there should not be much of a problem- some people have lived in semi-isolated environments for periods of years- and perhaps a extreme and freakish example is what is commonly called "boy in the plastic bubble" whereas extreme measures are taken to live in a near sterile environment. Of course you can't separate a human from microbes:
"Of all the cells that make up the normal, healthy human body, more than 99 per cent are the cells of microorganisms living on the skin or in the gut, etc. This normal resident microbial population includes potential pathogens as well as organisms that help to keep the potential pathogens in check" http://helios.bto.ed.ac.uk/bto/microbes/
So people going to Mars are going to bring their microbes with them. And if not severely restricted humans will bring other life- their cat, goldfish, snakes, plants, whatever. Humans as a general rule love other life.

As far as globally terraforming Mars, generally speaking I don't favor the idea. It's generally part of what call the socialistic model and in general terms it won't work. But there is no doubt that human will change Mars- and I don't have any problem with this. I see the Mars world evolving with islands of transplanted life- and of course this life may be altered or become altered so as to survival better on the Mars world [even though it's enclosed in an atmosphere- and of course life may be started outside the islands at some point- Maritans *will* try to do this [again, if not somehow severely restricted from doing it].

Even if we get off our butt, it will probably take almost a century before Mars begins as a civilization instead of a waystation or some bases/outposts. So in the period of next 100 hundred year you might have a few screwballs who will stake their life and desendents on Mars, but for most part most people will visit Mars for short periods and longer periods [up to a decade or more]. During this period science will advance and we will get solutions and run across new problems.