Earlier quoted context omitted.
So far, intelligence on Earth has been zero sum. We've wiped out multiple other intelligent species until we were the only ones.
I think that was more a result of shared environmental niche more than anything since it happens with non-intelligent species as well. Predators fight over same prey niche even in the same species. Stable multipredator shared prey arrangements have a clear power differential - the stronger one can chase the weaker ones away from kills and if they are slow enough that may be their main strategy. If there were say merp…
Edit: and going back to you merpeople example, the onset of humans represents a mass extinction level event for the closest thing to merpeople: ceteceans. http://www.iucn-csg.org/index.php/status-of-the-worlds-cetac...