> human induced changes in ecosystems can cause problems for humans
I mean, there's totally some degree of selfishness in us wanting to keep nature at some idealized level of "pristine": It has value to us as humans, both in terms of resources and ecosystem services, but also just at an emotional level -- some of us really value nature (as we roughly know it).
Even if you don't use loaded terms like "damaging" or "unnatural", you can still objectively measure a decrease in biodiversity (as in the # of different kinds and numbers of inter-species populations and cross-dependencies and an ecosystem's overall ability to respond dynamically to challenging events like harsh weather or wildfire). When big changes are introduced to a small system too quickly for it to handle, it doesn't so much matter what you call it... the same thing typically still happens: local flora and fauna, which are often endemic to an area (only found there), are displaced by "generic" versatile scavenger species like rats, pigeons, and crows. (Or in this case, predatory cats).
Is a marbled murrelet, a kind of bird, more worthy of preservation than the common crow? I guess that depends on who you ask. Yes, there's some amount of value judgment there -- most of us don't want to see a world with only like twenty remaining species because we killed the rest. We don't have de-extinction technology yet either, and we don't have the capability to regrow old-growth forests in human bureaucratic lifespans. It's true that we don't have all the answers, and all the proper "values" so to speak, to be able to measure the worth of any one species over another, necessarily.
> I don't really agree its damaging to or causing problems for nature/ecosystems, or that its unnatural
But even then, there are some we know to be "keystone" species whose disappearance will cause cascading effects across its local ecosystem, while others just have roles we don't fully understand yet (or maybe really are just "worthless"). But once we kill them off, we don't have any way to bring them back, and that's a few million years of evolution potentially lost forever. A simple precautionary principle might also apply there; we shouldn't necessarily go around destroying everything just because we don't see its immediate value or how other parts of the system depend on it.
This is the kind of thriving, diverse life that many people, environmentalists or not, value. Is it "natural"? If you don't like that word, don't use it, and maybe biodiversity is another metric we can use instead? Does THAT have value? Again, it's a judgment.
A roundish rock with magma lakes is also "natural", but it's not exactly teeming with life. A planet with only microbes is also alive, but not very diverse. Or a city with only imported palm trees and glass windows. The modern web of life took a looooooong time to get to this point, but we can easily lose a lot of it in a few short years. A lot of species value biodiversity -- not just humans, but many animals will prefer certain kinds of forests over others, and decomposers will converge on different kinds of debris, different birds have favorite foods, etc. -- life begets more life. If you don't value any of that, that's totally within your rights, but maybe I'd ask "Why not?"
Is it that you really love cats, and don't want to keep them inside all the time? (I have a cat too and I feel bad that it's an indoor cat... but for entirely different reasons, not biodiversity). Even then there are tools to limit the damage they can cause (like silly poofy collars that birds can easily see: https://www.birdsbesafe.com/)
Do you just not care for nature, feeling kinda meh about trees and birds and shit? That's fine, though it does seem like jumping to conclusions a bit quickly. How do we know these things are worthless?
Is it a more philosophical stance on nihilism and accelerationism...? Like if "nature" goes through cycles and things live and die and eventually our sun will burn out and entropy will increase and the universe will reach its heat death anyway, what does it matter what we do now?
I dunno. I shouldn't put words into your mouth. What do you think?