Why do predators never completely exterminate the prey population? How do plants adapt to life in a moisture deficit?

First question:

1. Predators are much smaller than their victims, this is due to the rule of the ecological pyramid.

2. If the predators destroy all the victims, they will have nothing to eat.

3. Predators will die from lack of food.

Second question:

1. Succulents (plants that grow in places with a lack of moisture, deserts) often have deep roots, because there are water sources underground.

2. Their leaves are often spiny, which reduces water evaporation.

3. In addition, such plants can be fleshy (eg cacti), thus storing water.