[qrendel]

Introducing a dominant civilization to a planet would more likely cause a sort of regression towards the mean in ecosystem diversity, at least over the short- to medium-term. In a highly complex and delicately-balanced ecosystem (i.e. Earth) it tends to reduce diversity as species and ecosystems are pushed aside, whereas in a non-existent ecosystem (i.e. Mars) it would cause something to exist in the first place, even if the system isn't as complex as what evolved on Earth over billions of years.

There would certainly be more life on Mars after it was colonized, not less - even if that at first only consists of a few dozen species used to support humans.

[guard-of-terra]

You should not overstate how Earth ecosystem is "delicately balanced". Large deserts where vegetation used to be. Ice ages (and inter-ice periods) really messed up northern fauna: all megafauna went extinct, wooly rhinos, mammoths, sabertooth tigers, etc. Without them the ecosystem is incomplete and disbalanced.

[Retra]

Compared to what? What parameters are you 'balancing?'

[guard-of-terra]

In my opinion, balanced ecosystem is one that a) diverse, b) produces as much as life as possible for a given landscape.

Why diverse? Diverse ecosystems are more resilient against shocks, changes and diseases.