The other day, my boyfriend and I were standing in our kitchen, which overlooks a grassy spread that people often use as a shortcut. We heard a most distressed eeping noise, and when we looked down, we saw that one of our neighbor's cats had caught a bird. It disappeared with the bird by the time we made it down the stairs, thank goodness, because otherwise my boyfriend would have been stuck with the task of snapping the bird's neck (something he knows how to do, but really would rather not do it).
In the Netherlands, there are no natural predators (minus birds). The last wolf was shot in the 1890s, and you have to go all the way to Maastricht before you start to encounter lynx. So hunting plays a huge role in maintaining stable and healthy populations of just about everything. One of my colleagues tells me that, unless you trap muskrats regularly, they'll tunnel through and damage the dikes. (Ironically enough, there is no real "gun culture" here, perhaps because gun laws are so strict.)
However, there are many, many different species of songbirds (and waterfowl, and birds of prey, and gulls), many of which nest on the ground--not to mention small animals like hare and rabbits. Needless to say, where it's normal to let your cat outside, there's a dearth in these animals.
"But it's only natural for the cat to go out!" some people say, as they defend letting their cat out. Look, I completely understand letting your cat out if you're on a farm and need mousers to keep the rats out of the barn. But no cat needs to be outside. My cats are happy as clams sitting on our balcony, sunning themselves and chewing on the cucumber vine.
Consider that, on average, a cat has the intelligence of a three-year-old child (and some have less, *cough* Shadow). You wouldn't let a three-year-old run around by himself, would you? Yes, they might be smart enough to avoid cars and dogs--but will they be smart enough to not eat mouse poop (which contains parasites), stay away from rat poison, not drink antifreeze (some human children will do this, for Chrissakes, never mind cats), differentiate between a dove and a hawk that can kill them, and not get lost? There's a reason why the average life of a feral cat is 5 years, while an indoor-only housecat lives three times that.
"But I can't keep my cat inside!" If you can't outsmart a cat and keep it inside, then you've got no business owning a cat.
"But if I don't let it out, he'll scratch/pee/poop all over!" It's called training. And believe it or not, it can be done with cats. My own cats are very well-behaved. They know their names. They know the signal that I want them to come to me. They sit still when it's time to clip their claws. And they don't scratch the antique furniture. I don't really know anything about training a cat, so if I can do it, you can figure it out, too.
The point is, letting cats run around outside is basically a sentence of genocide for all the wildlife that lives in your area--even if you don't see it (and believe me, that's kind of the point of most creatures, not to be seen)--and a death sentence for your cat. So please, keep your cats inside.