At least in the US the outdoors are no place for cats. I see dead cats on the side of the road far too often. People leave out antifreeze to poison cats so they won't poop in their yard or attack birds. Sometimes they shoot them for fun. And sometimes they saw off their back legs for kicks [1].
I've also seen way too many FIV+ cats come through the shelter system, transferred when they're attacked by FIV+ strays. If you ever need to give up your pet, FIV+ status is an immediate death sentence in many shelters. In no-kill shelters, it means a longer stay because some people are weirded out about "cat AIDS".
Ahh, so much quiter without all those birds and other critters :P
On a serious note: cats usually integrate well with their ecosystem and if they would really pose such a big threat, smaller mammals and brids would just migrate away.
edit: I'm speaking of balanced ecosystems where cats have natural enemies, foxes and whatnot.
There's no scientific evidence that letting cats outdoors cause a decline in bird populations: and that's according to the UK charity that campaigns for the protection of birds[1].
The majority of birds that cats kill are frail, in poor health, or otherwise likely to have not made it through the season anyway.
Having lived in both the U.S. and the U.K. it's interesting to see the two different attitudes to cats. In the UK, letting your cat outside is the norm. The shelter I adopted from wouldn't give you a cat unless you were able to let it out (or the cat had feline AIDS and needed to be kept indoors for protection).
But it's the complete opposite in the U.S, and although that makes sense in some places (a lot more predators in the countryside), in a city like San Francisco the risks seem identical to those of London. It's very strange. Of course, in the UK cat owners aren't legally liable for their cat's actions (cats, unlike dogs, are deemed by the law to be free agents), maybe that's not the case in the states.
Where I live, if you let your cat(s) outside, sooner or later they will be eaten by coyote or mountain lion. I am not exaggerating. There's also the issue of cats decimating the local bird population: http://theoatmeal.com/comics/cats_actually_kill
Well, I did say in my original post I wasn't talking about the countryside. One would hope there aren't mountain lions wandering the streets of, say, Brooklyn.
Cats do kill wildlife, but their impact is often overstated. The RSPB (Royal Society for the Protection of Birds) in the UK has no issues with cats being let out, but recommend they have bells on their collars. The majority of birds that cats kill are sick, frail, or otherwise in poor health.
To quote the RSPB: "there is no scientific evidence that predation by cats in gardens is having any impact on bird populations UK-wide"[1].
(We've had a lot of "missing cat" signs on the group mailboxes lately. Woods nearby. Expanding populations of fishers, coyotes, wolves, etc. Did I mention I'm 15 minutes from Parliament Hill? Bears downtown, -35 C winters (with serious wind and a whole lotta snow), +35 C summers (with 90+% humidity), wild beasties in the woods.... I really love Ottawa. I mean, I must. No rationale person would live here otherwise.)
I do understand your concern but we had a cat door for years and the cats were fine, if not happier, going outside at will. The worst that happened was they would occasionally bring in dead/mostly dead animals to show off as trophies from their hunts.
