I'm sure most pets wouldn't be able to survive on their own, so its pretty obvious that we are putting an inflated strain on the food chain by providing meals for them.

I wonder if there are a lot more carnivores than would otherwise naturally exist (with pets included), or if there is a similar number of carnivores but less diversity (e.g. an ecosystem that would have housed other carnivores is now hosting pigs and cows).

I never would have thought I would see "pets" as environmental argument but its a no brainer.