Why must all conditions except the independent variable be held constant for all experimental treatments?

Uhm... I believe you mean a situation when we examine a relation between two variables. In this case all other conditions must remain as constant as possible because they could affect a result and then the conclusions could be false. ie. you examine a relation between temperature and frequency of cell division/organism's activity, then you should stabilize air pressure, food composition, bacterial flora in experimental room... etc. of course when you conduct research in a field, ie. ecological research you can't do any of that but then you are obliged to explain possible aberrations of your results in an article's discussion.

Other organisms can change their behavior to take the place of the plants in the ecosystem.