What are the main organisms that are important in Yellowstone?

They're technically all important since without either one of the ones listed above, the whole ecosystem would be disrupted, and there would be new problems, like overpopulation or a decrease in the amount of resources (like food and water) available. Without the producers, there would no food source for the herbivores. As a result, the herbivores would then die off. Soon, because of their starvation, the carnivores would have no food themselves. However, the decomposers are also needed in the ecosystem to recycle the energy into the system again. As a result, they're technically all important in order to keep the ecosystem at equilibrium, and stay balanced (with no overpopulation or lack of food).