The answer is because they compete with native species for resources within a specific niche.
The new species introduced in an area where it has never been before is known as introduced species. The introduced species usually has no natural enemies (predators) so it can spread in the new area. However, as it spreads more and more, it will use resources of the native species. So, the native species must compete in order to survive. This way, they not only treat the native species but the local biodiversity. This could cause the extinction of many native species and not only those the introduced species compete directly.
