Process for determining how a new species affects other species

Question

We are using Earth as the basis for our planet, so we already have very detailed information on the geography, species, etc. to work with. There are additional species we are adding, and I thought the best approach would be to add one at a time and see what would change. After all species are added, then we would have our new modified Earth.

Since I've been arguing with a friend for a while now, I would prefer a concrete process which works for almost all if not all cases, and produces verifiable logical results. Generalize if needed but not if it makes the process less useful.

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This step would be repeated for each new species to add, in the chronological order that they diverged from their parent species.

What we know:

• What the new species is (details)
• What species it diverged from
• When it diverged (approximate time period)
• Where it diverged (approximate geographical area)

What we want to know:

• What new species would appear because of it
• What species would go extinct because of it
• What adaptations it would force in other species
• Which of the above that would have occurred are now denied by it

Other considerations:

• Some added species share a common ancestor with humans, so we can expect them to have similar technological progress, or be a little behind. Factors specific to these advanced species (culture, war, etc.) should only be included if there is a way to simplify and predict them, otherwise it ruins the result integrity. As for the species that aren't related to humans, we can assume they're primitive.
• We only care about our new species affecting other species, not the other way around, except for symbiotic relationships, since that can in turn have effects on other species.

Answer 1

Short answer we can't.

It sounds like you are talking about how new species will affect the local and global ecosystem. in which case the answer is we basically have no clue. there are just too many variables and interactions for us to have any predictive power with this. we can make some broad vague generalizations, like new predators will cause some changes in prey of some kind but that's about the best we can do.

worse even when we can give even some form of answer it is highly contingent on no other changes, and specific to that one environment, so what you want is basically impossible with current technology.

Answer 2

As mentioned in the comments the missing question is "why did the new species emerge" and the factors behind that could be quite random. If there is competition in a specific ecosystem a new species may evolve to either take advantage of an under utilized niche (up in the trees, out in the water, under the surface) or deal with an existing environment in a more efficient way. These two scenarios would have a dramatically different impact on the parent species. Though there's nothing to say that an initial lack of competition wouldn't change over time (species Y evolved to take advantage of the treetops, thrived, exhausted their food supply and descended back on species X). The intriguing bit is this unknown "why" factor that can't be distilled into a predictable algorithm.