What is the consequence of bringing a nonnative species into an ecosystem?

Disastrous, but they keep trying anyway. I'll just give some examples. 

Rats from ships come into Florida spread disease make animals and people sick. Minks introduced to eat the rats. Minks like native Florida birds and their eggs better, Florida birds go extinct. 

Wild Boars introduced to Americas for sport hunting. Escape preserves and ravage cropland costing farmers million and millions of dollars every year. Also a very dangerous animal. 

The ecosystem will over power of that species Causing over-crowding, taking up space and parts in the ecosystem other organisms need to survive. therefore the ecosystem will change dramatically :)