Long after humankind has faded from this planet, cats will surely be scratching around whatever’s left of it. Cats are efficient and instinctive predators. You can teach them to be nice kitties…
But if they are outside, they will kill things. They kill birds; they kill nesting waterfowl. They will prey on eggs in nests. They catch small mammals which has impacts on the small animal population, but it also has a great impact on the predators on which those small mammals and rodents are a food source. The owl population will be impacted if there are cats killing mice. Typically animals that are hunted by cats do not recover. Cat wounds are not easy to treat as they get infected very easily.
4 billion birds are killed by outdoors cats annually; thats 10% of the population. 40 Species have gone extinct solely from outdoor cats. Chile almost had all their hummingbirds go extinct. Australia has committed in euthanizing 2 million feral cats because they’ve already wiped out many native species and continue doing so. Birds aside, feral cats pose all kinds of intractable health, safety, and philosophical dilemmas that have overwhelmed our current systems. This is not the cats fault, this is out fault for being naive. (Video of a cat catching a bird in a building in Israel has gone viral)
We’d have fewer cats at large if all cats were chipped: Lost animals could easily be returned, and abandoned animals could be traced to those who dumped them.