I'm currently working on a few ideas for a high-powered GURPS fantasy setting (I'm talking of power levels appropriate for Exalted here).

It takes place on an artificial world created by powerful aliens about 8,000 years ago, and abandoned 6,000 years ago. They seeded ecosystems (and sapient beings) from six different planets on this world. What I want to come up with is some sort of justification why the different ecosystems haven't intermingled totally. Different climate zones wouldn't work - the original zones were split up evenly along the entire spectrum of available climates.

The best justification I can come up with is that the vegetation of each ecosystem is dependent radically different sets of microorganisms - so when one ecosystem attempts to spread into a new region, the microorganisms of the former inhabitants have to be purged or at least weakened sufficiently so that the new life forms can spread into this region. For this region, the sapients living in this world sometimes set fire to patches of "alien" vegetation when they want to spread into new territories.

How plausible would this be? Or can anyone come up with a better explanation which allows for some intermingling and changing territorial boundaries, but not total integration?