Cats should not be allowed outside on their own. They can decimate local wildlife populations. Even if not bringing anything in, they are probably still killing and leaving it outside.
I understand your concern.
The biggest change in negative terms in the amount of wildlife in my suburban garden was when the 1930s school close by was replaced by a super-duper modern academy.
The security fence seals us off from the foxes; there are manicured lawns instead of rich feeding grounds full of wild stuff for the birds where the sports field used to be. The developer said there were no bats in the building or grounds, but we see a twentieth of the numbers we used to see on a summer evening. They decimated the trees so we no longer hear woodpeckers, or have the variety of birds we used to get. The wet area at the bottom of the sports field where I think our frogs came from (which we are slowly trying to get back into our pond) has been drained - were there any newts there? Dunno, the report said not, but I doubted it then, and now I know who paid for it.......
So cats = killers, but humans WTF.
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