Imagine that, in the near future, pollution, overpopulation and war leads to a massive extinction where almost all chordates, some invertebrates and many plants are wiped out. Small, resilient plants, insects and the like remain, but all megafauna is dead.
Then, mere decades later, an interstellar ark arrives at Earth from another world. On board are a multitude of sapient species, which fled another warlike race that has since destroyed their homeworlds. To them, Earth was like Kepler 186f is to us. They had discovered it beforehand, confirmed it to be in the habitable zone, but did not know whether or not it had any life. So, they brought with them hundreds of species from their world (Mainly from one planet, since they would have all been completely different in physiology if they were all from different worlds.). They arrive at the barren Earth and introduce the alien creatures across the globe, in whatever environment they're best adapted to.
So, the question is: If sapient species came to Earth after a mass exinction, and introduced hundreds of species from their own world, how long would it take for them to get a foothold? I know that natural selection is cruel, and not all the species would survive very long. However, with supervision, help and managing from sapient species, could they have fully functional ecosystems and food-webs all over the world? One with active predators hunting saltatorial grazers and heavy browsers, their carcasses fed on by scavengers?
If you require any more clarification, just say so (Nicely, preferrably.) in the comments and I'll edit it as soon as I can.