If an ecosystem was experiencing a prolonged drought, there would be a big decrease in the population. All living creatures require water, even the ones well adapted to little water. There probably would be as many planets, for the ones surviving would be ones managing to get water, or ones that can survive off little water. Now on the other hand the animals then wouldn't be getting enough food or water, so then again ones who would manage to survive would be ones adapted to little water and food or are excellent hunters.
You can also expect some animals to leave the ecosystem look for a better source of water and food.
