A nonnative species may not have any natural predators when being introduced subsequently it won’t have any competition with the other organisms living in the same ecosystem. This will make the other organisms struggle for food and other resources and could potentially make them go extinct. They can also have drastic effects to the local biodiversity (for the previous reason). Also, due to the competition or newly introduced predator (assuming they’re a predator) the preexisting organisms of the ecosystem may start to relocate and start a new ecosystem or if there are neighbouring towns/cities may have to forage for food there - which would obviously be dangerous for both them and us. Hope these few examples help.
