Plenty of animals and plants alter their environment to the detrinent of others.
Trees are tall and shady in order to reduce the sunlight in the environment and starve nearby plants to death at absurd individual expense: it takes a lot of nutrients to make those trunks that serve no purpose other than spite.
Beavers construct big dams to turn meadows into swampland, often clearing out the local plant life. I know from personal experience that operating a large farm in a wooded area is a constant battle of construction and destruction against the beavers.
Yeah, humans are better at altering the environment than most everything else, but altering the environment at the expense of other life is hardly unnatural.
Definite case in point: photosynthesizing organisms (green plants, algae, etc.)
We know that the Earth had a large-scale anaerobic ecosystem before the advent of photosynthesis. While anaerobic organisms still exist, they're only found in very narrow niches. >99% of them were wiped out, root and branch, apparently killed off by rising oxygen levels (oxygen is a poison to them).
Because if it's not harmonius is collapses. And we all die.
But yes, there's no real reason for it to be harmonius, just like there's no real reason humans (or any other life form on Earth) need to exist.
The answer is obvious conservation until we as a species are immune to single planitary extinctions, but the romanticised view of our worlds ecology as some living entity, rather than a number of organisms interacting independently and adapting doesn't give a clear objective of what we should be doing and more importantly why.
That's part of the harmony. Harmony ends when one of these organism takes over, it destroys the entire ecology and in the process it ultimately destroys itself.