The extreme wet-dry cycle that playas experience is the lifeblood of their ecosystem. When dry, the clay soils contract and form large cracks in the bottom of the playa basin. Plant seeds and invertebrate eggs from the last wet period lay dormant in the soil, waiting for the next large rainfall to germinate and hatch. When the rain comes, the first flush of water runs into the playa and through the cracks, beginning its journey to the underlying aquifer. As the runoff continues, the clay soils expand; the cracks seal and the playa begins to fill with water. Wetland plants and invertebrates complete their life-cycle, and birds and mammals use the playa for food, water, and shelter.