New Zealand, which has - or had before feral cats arrived and possums from Australia were let loose - some of the most extraordinary fauna anywhereFeral cats aren’t really much of a problem, it was the rat and the original humans that came to New Zealand that did everything in. The original rodent invader was the Kiore, or the pacific rat (which is also found on Haiwai'i coincidentally) killed a lot of our bird life, although this will never be properly quantified and what the rats didn't get the Maori wiped further bird species out. For example, the Moa and the great eagles that hunted them both went extinct due to the actions of the Maori. When Europeans arrived it just went from bad to worse, because not only did we bring further invaders (Gorse, possums and the european ship rat, which is bigger and meaner ) we also bought guns. Birds that had been difficult to kill previously, like the Kereru (wood pidgeon) became a snap to kill with a gun and they were very nearly driven to extinction. Strong conservation laws were about the only thing that ended up saving the species from extinction.
There are numerous other biological invaders as well that we’ve bought in too. For example the beloved fishermans catch, the trout, is like a nuclear weapon in fish form for streams that it is put in. It’s an amazingly aggressive predator and in the streams it is present in has eliminated small invertebrates entirely like the freshwater crayfish Koura. Additionally, for those wanting another example of evolution in action, fish called galaxids in streams with trouts have been under intense selection in two regards: the first is to be active at night when trout aren’t around and the second is to have a much darker colouring so they aren’t seen as easily. Trout free streams have considerably more variation in galaxid behaviour and colour than streams with trout. For obvious reasons.
Thankfully, New Zealand conservation scientists are very good at figuring out ways of killing these various pests and we lead the world in biological control and trapping programs. For example, work has been ongoing in New Zealand for several years in using parasitic nematodes to sterilse possums. We've also introduced a form of spider mite that attacks gorse and chokes the ability of the plant to grow by webbing up the growing stem. The only unfortunate thing is that such measures can never bring back the numerous species, some we probably never even knew of, back from the dead.