Ecology, the study of the relationships of organisms with one another and with their environment, is a complex but fascinating area of biology that has many important implications for each of us. The human population is climbing rapidly, and has now attained the unprecedented level of more than 5 billion people. This severely strains the earth's capacity to sustain us all. In the face of this situation the principles of ecology assume a crucial importance and may enable us to chart a sound future. Ecology, however, is not intrinsically an action-oriented field; it is an area of scientific knowledge that is concerned with the most complex level of biological integration. Ecology attempts to tell us why particular kinds of organisms can be found living in one place and not another-the physical and biological variables that govern their distribution; the factors that control the numbers of particular kinds of organisms and maintain them at certain levels; and the principles that may allow us to predict the future behavior of assemblages of organisms.
       Ecologists consider groups of different organisms at three levels of organization that become progressively more inclusive. Thus populations of different organisms that live together are called communities. A community, together with the nonliving factors with which it interacts, is called an ecosystem. An ecosystem regulates the Bow of energy, ultimately derived from the sun, and the cycling of the essential elements on which the lives of its constituent plants, animals, and other organisms depend. Biomes are major terrestrial assemblages of plants, animals, and microorganisms that occur over wide geographical areas and have definite characteristics that identify them as distinct from other such assemblages. They include deserts, tropical forests, and grasslands. Similar major groupings can be distinguished in marine and freshwater habitats.