[6]An ecosystem is a natural community, taken as a whole, including all biological and environmental factors. [7]Ecologists are scientists concerned with the interrelations of organisms and their environments. [8]A thermonuclear device is an explosive, such as a hydrogen bomb, based on a fusion reaction. In other atomic weapons the energy is derived from nuclear fission. [9]The living organisms. [10]Plankton are the floating, minute plants and animals that live in the sea (and also in fresh water), including diatoms, algae, protozoans, and crustaceans. [11]For more on this program, see _Plowshare_, a companion booklet in this series. [12]The half-life of a radioactive element is the time required for half its atoms to lose their radioactivity. [13]Atmospheric tests of nuclear weapons through 1962 produced a fission yield equivalent to 191 million tons of TNT and introduced about 10.01 megacuries of strontium-90, for example, as fallout entering the environment. [14]Floating one-celled animals. [15]A picocurie is one trillionth of a curie; a curie is the basic unit of intensity of radioactivity, approximately equal to that in 1 gram of radium. [16]For more about these studies, see _Fallout from Nuclear Weapons Tests_, a companion booklet in this series. [17]Information on this research is found in _Radioactive Wastes_, a companion booklet in this series. [18]The lake, drained in 1955, makes an interesting natural basin in which residual radionuclides are used in studies of mineral cycling. [19]A roentgen is a unit of exposure to radiation, measuring the alteration of the atoms (ionization) of the radiated tissues. The rat dosages compare with recommended limits of exposure to man-made radiation for average individuals in human populations of an amount that approximates 0.5 roentgen per year. [20]The area where highest readings were obtained in the survey. These studies are described in more detail in _Whole Body Counters_, a companion booklet in this series. Transcriber’s Notes --Retained publication information from the printed edition: this eBook is public-domain in the country of publication. --In the text versions only, text in italics is delimited by _underscores_. --In the text versions only, superscript text is preceded by caret. --In the ASCII version only, subscripted numbers are preceded by underscore and delimited by brackets. End of the Project Gutenberg EBook of Atoms, Nature, and Man, by Neal O. Hines