But I guess what I'm trying to say is that one niche always depends on the other. If one falls out because of the other, then what is left is just a chain reaction till everything is dead. Nature made and ecosystem the way it is because of different adaptations in organisms meaning that organisms from different areas don't belong in that particular ecosystem because they don't fall into a niche.
They can occupy a certain niche in their original ecosystem and not harm a single thin, but somewhere else cause death of entire river systems.
And yes eventually the said river would eventually repopulate but in how much time? In that amount of time a country could rise and fall, it all depends on the circumstance of the fish killings and how fast it all happens. Something fast like a volcano or something progressive like an invasive species. It all just depends.
And how is it not an issue for nature when species are being killed off?
Well what filling a niche is all about is that a species can fit the same role, and if they're better suited, then they are. Of course, the specifics are a lot more complicated, for example the new apex predator could feed more on a certain species more than others whilst it could be the reverse for the old apex predator, although both eats everything, for example. That would trigger chain reactions in which other numbers increase/decrease and the likes, but it's not as if every single time this happens the entire system collapses, which is why we still have life on Earth. After all, if every change to a system - addition of diseases, or plants, or animals, or change in temperature or rainfall or whatever - disrupts it to the point of irreversible damage and so quickly then well yeah, barren lands all over would be what's around. Of course there are some systems that over time become so fine tuned that any disturbance, at all, would be bad, regardless.
But really again, nature is constantly in flux, and that's the normal state all over, not the 'stable' state. After all even without intervention of other organisms, natural occurences would disrupt a community anyways, and then it'd be in flux anyways. So yes species may not 'belong' in certain other communities, but that is not to say that it's unnatural.
Which is why it's not an issue for nature when species are killed off. Because it's more natural for species to die off actually. As of right now, the number of extinct species far outnumber the number of extant species. 'Nature' really does not 'care' what dies off and what don't, it cares more about things just doing whatever it want (quite literally) and if they're successful, they're successful. If they're not, they're not.
It's us who starts to concern ourselves about all this 'oh my god we must preserve this ecosystem exactly as it is' thing, which is funny as well because conservationists do such a great job of conserving the 'right' species anyways. All the talk about 'we must preserve this and that' and in the end they spend the millions of dollars they get on preserving stuff like koalas and the likes. Not saying that the conservationists can't preserve animals like that, but if their intent is to preserve the environment then it's more important to preserve stuff like insects and plants and the likes, but of course since they're not cute or cuddly society as a whole doesn't care anyways. But I digress, not the point of this.
Though I guess it does bring up something I want to emphasize - why we want to conserve anything is entirely based on our own personal viewpoints and needs. We want certain species of fish to be conserved for food reasons. We want others to be conserved because they look cute and cuddly. In my opinion there's nothing wrong with that, but we need to realize it's for our own (sometimes selfish) reasons, not for some 'we need to protect nature' bull crap, because nature doesn't need any help from us. Even if we bomb this planet and wipe out everything living on land and on the coasts, as long as we don't actually destroy the planet itself (doubt we can do that), then nature's cool. Organisms living in places not affected - deep caverns, underground, deep under water, etc. etc. will eventually repopulate the planet, evolving as they adapt, creating new species and the likes. Even if somehow everything is destroyed except for bacteria, they'll eventually bounce back.
So yes, again, I agree, actually, there's reasons why we want to stop invasive species and the likes - but we must face the fact that it's not for nature, it's just for us. Which is fine imo.
So many wrong things with this statement. First of all do you really those animals have a chance of surviving in the environments you speak of them being introduced?