if you were a widget maker and you heard that the price of the loompies that you needed to make your widgets was going to go up, you'd buy all the loompies that you could get your hands on. by doing that, you'd cut the supply and the price would go up temporarily. if the loompie fields turned out to have a bumper crop and there was no real shortage of loompies, the price would drop again and you would have paid a higher price because you guessed wrong.
so yes, speculators can have a temporary effect on price of something, but they are the ones taking the major risk. in the end, unless the market is manipulated, the market sets the loompie price, not the speculator.