Felines accompanying their human companions have gone on to prey on the local wildlife, and they have been blamed for the global extinction of 33 species.
Cats don't just replace the local predators. They are better at killing than most other small predators. And they are also sadistic bastards who will torture prey before they kill them. They may not bring down moose, but everything smaller than them is just a toy to be killed.
I have two cats. The male is definitely a murderer. Nothing smaller than him can come into the apartment without being killed. And as I bought him to be a mouse catcher, I'm pretty much OK with this. But I wouldn't let him outside.
"When people are least sure, they are often most dogmatic." -- John Kenneth Galbraith