How long would it take for an array of introduced alien fauna to form functional ecosystems on a near-future "Dead Earth"?

Question

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.

Answer 1

Plan for about a century or two.

This is a case of invasive species. You have a shocked Earth ecosystem that you’re about to introduce true alien species into. There are two major things to consider here for each species you want to transplant:

1. How viable is this species within the existing ecosystem that matches its ideal climate?
2. How long does it take to propagate in said ecosystem?

The first question is the most critical. Invasive species tend to be so successful because the native species have no previous experience competing or surviving alongside them. As long as this foreign land is familiar enough to the invading species, it typically gains a foothold and then some. Ecological similarities might be a big sticking point for your aliens because rather than being transplanted from a separate region on the same planet, they’re from an entirely different planet. Even small atmospheric or planetary differences could prevent viability. If so, those species will die.

Propagation time depends on reproductive cycles and life spans. Invasive plants in the Americas have become immensely successful in a century or two, often in spite of active campaigns to eradicate them. You may very well be able to improve those times with active support of your flora. Fauna can take control of ecosystems even faster – Burmese Pythons in Florida have taken hold in the ecosystem in a matter of decades.

A note of caution here: you need both alien flora and fauna to have certain compatibilities with Earth life. Successful propagation of Earth plant species is often closely tied to insects and animals – if insects can’t assist in cross-pollination of your alien plants, that could be a serious problem. Likewise for animals – if they need to subsist on Earth fauna or flora, their digestive systems will need to be able to gain nutrients from the foreign materials efficiently.

Extinctions will, of course, be common. Given the general advantage of the invading species, it’s likely that native Earth species will lose the most. It’s inevitable, however, that some alien species won’t adapt well. All told this won’t be a fast process, but with active help from intelligent life, it won’t necessarily be too long either.

Answer 2

It depends a lot on:

1. how similar the ecosystem they are trying to introduce is to Earth's existing, surviving, lifeforms

and

2. how much of their "native" ecosystem in terms of both diversity and the vertical food-chain they have.

For example a species with left chirality will be starting from scratch with basic carbon capture etc... because all life on Earth uses right chiral enzymes (I'm not sure if it's even possible to have life with left chiral enzymes). Surprisingly enough if they have the necessary material, encompassing everything from primary producers in the form of their equivalent of Cyanobacteria to the higher animals, this is actually the best case scenario for fuss free colonisation; their crops etc... have no natural predators except what they bring with them because nothing on earth can interact effectively with their molecular biology. In this case they spread as fast as their native species can broadcast off-spring since their is little competition for basic resources with so much of "our" ecosystem destroyed and none of what's left can attack the new colonists.

At the other extreme if they're right chiral and DNA based they're prey to everything left living on Earth and probably have no natural resistance to any terrestrial bacteria, fungus or virus, because being DNA based doesn't mean they have anything like a terrestrial immune system. In this case it will be War of the Worlds all the aliens die within a few months of landing because something basic and intrinsic to us, like mitochondria, present a lethal pathogen.

I'd say the most likely scenario is something in-between; there are probably points of convergence in biological structure that make the aliens somewhat compatible with "life as well know it" so while they have little competition some of our native lifeforms do predate part or all of some of theirs at certain points in their life-cycle (whether that's while they're alive or one they're dead) so there will be some give and take. Colonisation will be possible but problems will crop up due to unexpected interactions.

Answer 3

This is like the Columbian Exchange

The Columbian Exchange on Earth provides the nearest real-life analogue. Look at how various creatures on Earth survived when introduced to the New from the Old.

There was a mass extinction of megafauna in the Americas. America's former horses went extinct (along with llamas and oxen and pretty much every other horse analogue except the American Bison). Horses were reintroduced in 1519 and rapidly expanded. On the other hand, it helped that Old World horse were probably the same species as the extinct horses.

Apart from horses, the earliest and most widespread case, there are also plenty of feral wild boar in North America, and water buffalo in Australia (technically not the Columbian Exchange, but the same concept). Neither of these animals had particularly close relatives that went extinct. Wild boar seem to have occupied the niche formerly occupied by black bears; while water buffalo compete are probably the analogue of extinct giant wombats. Both these introduced species expanded relatively rapidly; since widespread introduction in the 1800s they have feral populations in the millions (for boar) and up to 350,000 for buffalo in Australia before heavy culling.

Answer 4

I believe that if these species' is spacefaring to the level of interstellar travel, they should have more advanced knowledge in genetic engineering. In that case, I do not see a problem in integration with any natural environment.

Additionally, one could argue that because of their warlike origins, they should have even better technological advances on survival aids. Therefore, I do not think they or their fauna should take more than a decade to integrate with Earth's ecosystem even if the other civilization attempts to conserve the local life and not alter that.