Why are nonnative species a threat to biodiversity?

They do not have natural predators to regulate their population growth.

They compete with native organisms for resources.

A nonnative species is the one which is either intentionally or accidentally introduced into a new habitat. This species is capable of competing with the native species for resources like water, food and mates. This may result in the decline of the native species due to mortality of the members of the native species, migration and extinction.