Water plays an array of vital roles in ecosystems across the planet. Although many other substances are necessary for life and for ecosystems to exist, without water nothing else would function to produce life as we know it.
Water is essential to life since every living cell of every living thing needs water for sustenance. Without water an ecosystem would not exist.
Water is an agent of erosion. Rainwater, river water and freezing and thawing water can break up enormous amounts of rock into small fragments, forming some of the building materials of soil.
Water in the atmosphere affects the climate, as in cloud formations that can produce snow and rain. Clouds can also reflect sunlight away from Earth, producing a cooling effect.
The cooling effect of water can also result from evaporation or evapotranspiration (the combination of evaporation in general and the evaporation of water through plants' leaves called transpiration).
Human industry has produced hydropower, power from water through dams where turbines generate electricity. This usage has an impact on ecosystems, especially migratory fish.