**4. Ecological importance of radionuclides**

There are different kinds of atoms of each element; these are referred to as isotopes. Some isotopes are radioactive, some not. Radioactive isotopes are unstable. These decay into other isotopes releasing radiation. Each radioactive isotope, radionuclide, have a specific rate of disintegration, its half life.

Radionuclides fall into well defined groups (Tables 1 and 2). Naturally occurring nuclides are included in Table 1 while those from fallout produced by fission or uranium and other elements are found in Table 2. Fission isotopes are produced from nuclear explosions which have for the most part been eliminated and from "controlled" reactions that produce nuclear power. While most of the aforementioned nuclides are not essential for the growth of organisms, they may be incorporated in biogeochemical cycles and become concentrated in food chains, especially strontium and cesium. Thus Woodwell (1962) used cesium as a gamma radiation emitter in his well published study of an irradiated pine oak forest at Brookhaven National Laboratory, Long Island, New York. More will be said about this research later in this paper.
