How does an invasive species cause negative effects on the biodiversity of an ecosystem?

Invasive species can cause negative effects on biodiversity through out-competing native species, becoming a voracious top predator and killing most of the native animals/fauna (especially since if there are no natural predators), and this all leads to knock-on effects within the ecosystem.

For example, American grey squirrels out-compete British red squirrels because they are more aggressive and reproduce faster. However, this does not only affect the reds, but greys are also more aggressive towards neighbouring bird species which reduces the overall biodivesity of that area. An example of a top predator could be the brown tree snake introduced to guam, which was a popular bird nesting island. This snake has since eaten all of the birds on guam and is close to making the fruti bats their extinct as well. In this example, it's not only the animals who are affected though, since there are no longer birds, and very few bats, the seeds of the trees are not being dispersed and therfore the forests are also dying.