What’s the minimal requirement for a self-sustaining ecosystem?

Ecosystems crash all the time, closed ones included. So what does a fully closed, man-made, functional ecosystem look like?

What processes lead to living planets?

What’s required for stable ecosystems and long-term stability?

Are we capable of building an autonomous ecosystem?

Are there general signs of the impact of intelligent species on ecosystems?

What leads to stable planetary habitability?

Can a non-living planet be turned into a living one?

How can a living planet be turned into a non-living planet? - Is our own an example?