ECOLOGY 759 



phates are being carried into the sediments at the bottom of the sea 

 taster than they are benig returned by the actions of fish and marine 

 birds. Sea birds play an important role in returning phosphorus to the 

 cycle by depositing phosphate-rich guano on land. Man and other ani- 

 mals, by catching hsh, also recover some phosphorus from the sea. Min- 

 erals are recovered from the sea botton and made available for use once 

 more when geologic upheavals bring some of the sea bottom back to the 

 surface and raise new mountains. 



333. The Energy Cycle 



The cycles of matter are closed: the atoms are used over and over 

 again. Keeping the cycles going does not require new matter but it does 

 require energy, for the energy cycle is not a closed one. Although energy 

 IS neither created nor destroyed, but converted from one form to an- 

 other (First Law of 1 hermodynamics), there is a decrease in the amount 

 of useful energy whenever one of these transformations occurs; some 

 energy is degraded into heat and dissipated (Second Law of Thermo- 

 dynamics). 



Only a small fraction of the light energy reaching the earth is 

 trapped; considerable areas of the earth have no plants, and plants can 

 utilize in photosynthesis only about 3 per cent of the incident energy. 

 This is converted into the chemical energy of the bonds of the organic 

 substances made by the plant. When an animal eats the plant, or when 

 bacteria decompose the plant material, and these organic substances are 

 oxidized, the energy liberated is just equal to the amount used in 

 synthesizing the substances (First Law of 1 hermodynamics) but some of 

 the energy is heat and is not useful energy (Second Law of Thermo- 

 dynamics). If the animal's llesh is eaten by another animal, a further 

 decrease in useful energy occurs as the second animal oxidizes the or- 

 ganic substances of the first to liberate energy to synthesize its own 

 protoplasm. 



Eventually, all the energy originally trapped by plants in photo- 

 synthesis is converted to heat and dissipated to outer space and all the 

 carbon of the organic compounds ends up as carbon dioxide. The only 

 important source of energy on earth is sunlight-energy derived from 

 atomic disintegrations occurring at extremely high temperatures in the 

 interior of the sun. W'hen this energy is exhausted and the radiant 

 energy of the sun can no longer support photosynthesis, the carbon cycle 

 will stop, all plants and animals will die and organic carbon will be 

 converted to carbon dioxide. 



334. Physical Factors in the Environment 



No species of animal or plant is found everywhere in the world; 

 some parts of the earth are too hot, too cold, too wet, too dry, or too 

 something else for the organism to survive there. The environment may 

 kill the animal or plant directly, or it may keep the species from be- 

 coming established by preventing its reproduction or by killing off the 



