Light

The energy for almost all living things on earth comes directly or indirectly from sunlight. The amount of sunlight striking a given area of the earth's surface varies with latitude. Areas around the equator receive sunlight of the strongest intensity, while areas around the North and South Poles receive light of weakest intensity. At the North and South Poles, the sun never rises sbove the horizon during the six winter months of each year. During the summers, the sun never sets. In regions between the equator and the poles, the relative lengths of day and night vary with the season, with more hours of daylight in the summer and fewer in the winter. These variations in the amount of sunlight striking the earth are caused by the daily rotation of the earth, the movement of the earth around the sun, and the tilt of the earth's axis.

Soil and Minerals	Biotic Organization	Authotrophic and Heterotrphic Nutrition	The Nitrogen Cycle
Temperature	Competition In Ecosystems	Symbiotic Relationships	The Carbon and Oxygen Cycles
Water		Producers, Composers and Decomposers	The Water Cycle
		Food Chains and Food Webs

The intensity the duration of sunlight are basic factors affecting the growth and flowering of plants. Some plants require high light intensity and long days, while others grow where the light intensity is low and the days short. In many animals, migration, hibernation, and reproductive behavior are influenced by the relative lengths of day and night.

Light conditions also vary in aquatic environments. Light is absorbed as it passes through water. Thus the amount of light present decreases with increasing depth. The layer of water through which light penetrates is called the photic zone. Approximately 80% of the earth's photosynthesis takes place in the photic zone. Below the photic zone is the aphotic zone, where there is no light. Except for a few chemotrops, organisms that live in the aphotic zone are all heterotrops deriving their energy from organisms that drift or migrate down from the photic zone.