Ohka:

I also reject the notion that we know what is "bad" and "good".

I'm having a tough time seeing how snakeheads killing off everything else in a pond can be seen as good for anybody but the snakeheads. It's possible something like that would have eventually occurred naturally...but that still doesn't mean it'd be a positive change.

And I'm curious as to where you draw the line on "human activity." If you're talking about unforseen events, like organisms on an ocean liner, I can see where you're coming from. But are you suggesting that it's okay for me to dump my snakeheads into the local pond because they might somehow have ended up there, anyway?