Cycles in an Ecosystem

Chapter 25.3

CyclesAll organisms

need certain substances to live, grow, and reproduce.

Many substances, such as water, pass through natural cycles in which they are used and reused.

Some cycles include nonliving things.

Many of the chemicals move in and out of the living realm.

The Water Cycle

Water moves continually throughout the earth by means of the water cycle.

evaporationEnergy from the

sun changes water from a liquid to a gas through the process of evaporation.

condensation

water vapor changes back to a liquid by the process of condensation.

precipitationThe condensed

water forms clouds, steam, or fog, and eventually falls back to the earth as precipitation.

The Nitrogen CycleNearly 80 percent

of the earth's atmosphere is made up of nitrogen.

Changing atmosphericnitrogen into usable compounds is callednitrogen fixation.

The Oxygen-Carbon Dioxide Cycle

When you inhale, you take in air that contains oxygen.

When you

exhale, you release carbon dioxide back into the atmosphere.

Another way inwhich carbon

dioxideenters the

atmosphereand becomes part

ofthe cycle is throughthe decay of deadorganisms.

Sometimes the bodies of dead organisms do not decompose.