We're flawed creatures so it's not ideal, but it sure beats being at the mercy of the forces nature uses to correct things on its own, like diseases and famine.
The only sensible definition I know of of "natural" is "according to the nature of a thing". Thus, human beings have a nature, and that nature is what determines what is good or bad for us. Arsenic isn't poisonous as such, but it is poisonous to us by virtue of our nature. We are rational animals by nature. And so, unnatural are things which depart from that nature, like the desire to eat glass or having a sexual interest in oak trees and so on. It is the nature of a thing that is the reference point that allows pathologies to be defined. By nature, we should have two arms, hence to lose or lack an arm is a defect. Similarly, psychological disorders only make sense with reference to the normative, which is defined by human nature. To say "everything is natural" renders the word meaningless, annihilating all justifiable and objectively normative statements, which is absurd. If everything is "natural", then nothing is unnatural, because natural is simply identical with everything.
Name three.
I can think of one: we landed stuff on the Moon and beyond. I think that otherwise, nature has a hard time reaching out beyond low Earth orbit.
Other than that, I can't think of anything we'd consider massive fuckups that nature didn't do better. We're definitely tamer than anything else, considering that life itself is a mass murder fest at every scale, from molecular to planetary.
But "Plastics" could be a candidate.
The stupid part is that we're not using nuclear energy, and have irrational fear of nuclear waste relative to much more dangerous, potent, and undiscussed "conventional" industrial waste.
Also naturally ocurring nuclear reactors are a thing. On the surface, too, I'm not talking about the Earth's core here.
> gross water mismanagement
Animals do intentional and unintentional water management too.
> fossil fuel mining and usage, all things plastic
Fair enough, this is unique-ish and we are mishandling it, though the use of either isn't bad per se - rather the unsustainable use.
> hunting and killing for sport and other non-nutrient-related desires
Have you ever seen a cat?
> extreme resource hoarding
That's a fundamental thing all life does.
Keep in mind that nothing in nature is thinking forwards - all life is self-destructively greedy, and nature doesn't care if e.g. some beavers dam a river and accidentally flood a whole valley, killing themselves, their offspring, and extincting a bunch of unique flora and fauna. The stability you see, that people so love and associate with nature - it's not a fixed thing, it's a temporary equilibrium in life killing other life.
Nothing about this tree growing in North America is "nature taking its course".
Humans decided to cultivate it here, and we can choose to stop. Cycles of correction, sure, but attempting to fix problems due to introduced species seems like a worthwhile effort.
But also, left alone, nature tends to rebalance on its own. Any species with a dominant advantage will eventually lose that advantage, given a few generations. Well, except for humans, who continually fight the natural rebalancing, and are only succeeding in increasingly destroying that which sustains life on this planet.
Lawns and backyards.