Fiction perhaps, but not science. Time travel stories—especially those invoking paradoxes—are fantasy. "Tribbles" on the other hand is highly "realistic." There have been uncounted instances where Mankind has had to deal with the explosive and unexpected results of tampering with the balance of an ecosystem.
I said science fiction
not science. No where does it say science fiction has to be realistically scientific. If that was the case than Star Trek is most decidedly not a science fiction show.
Science Fiction is fiction based on science. True Science Fiction should be based on true science (or as it is understood at the time of writing). Laser beams, robots and spaceships do not make a story science fiction.
And no, Trek is not science fiction, it is fantasy.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy