There was an orbital station; a huge torus, as known from sci-fi. It was a luxurious place for people from dusty planets to relax at, and spend hard-earned money in environments resembling Earth from the legends. Forests, meadows, hills, rivers and lakes. Everything powered by solar energy, protected from radiation by artificial magnetic field, and maintained by a horde of autonomous robots that would repair anything broken, repair each other if any of them breaks, and go into nearby space to hunt comets and asteroids to provide raw material for all the repairs (and fuel for more such voyages) in case they'd be running low.
Something happened, and the human population abandoned the station, leaving almost all the autonomous systems running. Someone, maybe in a fit of wry humor, switched "ecosystem regulation" off (while leaving maintenance; water, humidity, temperature etc), so it would no longer be artificially maintained at status quo, but could evolve, and also disabled the spin motors of the station.
The station took a couple thousand years to de-spin due to tidal forces, the artificial gravity gradually dropping to zero. Meanwhile, the ecosystem thrived in the changing conditions, allowed to "run wild"; robots faithfully maintaining the infrastructure, bringing resupplies of air in place of leaks, fixing breaches from meteorites, etc. And the ecosystem (oh, pick anything you like that is reasonably varied, say, moderate climate forests, or African savanna, just please, not "bottom of Pacific"), kept evolving, adapting to the new conditions.
The station is "rediscovered", say... 100,000 years later. How will the different classes of species (predators like wolf or leopard, larger herbivores like deer or antelope, small herbivores (rodents etc), birds and insects look like? Or will one or more class of these go extinct?)