In ecological terminology the developmental stages are known as seral  stages, and the final steady state as the climax. The entire gradient of communities that is characteristic of a given site is called a sere. Succession that begins on a sterile area where conditions of existence are not at first favorable as, for example, a newly exposed sand dune or a recent lava flow is termed primary succession. The term secondary succession refers to community development on sites previously occupied by well developed communities, or succession on sites where nutrients and conditions of existence are already favorable, such as abandoned croplands, plowed grasslands, cut over forest, or new ponds. As would be expected, the rate of change is much more rapid, and the time required for the completion of the sere is much shorter, in secondary succession. Finally, it it important to distinguish between what may be called (for lack of better terms) autotrophic succession and heterotrophic succession. The former is the widespread type in nature that begins in a predominantly inorganic environment and is characterized by early and continued dominance by autotrophic organisms. Heterotrophic succession characterized by early dominance by heterotrophs occurs in the special case where the environment is predominantly organic as, for example, in a stream heavily polluted with sewage or, on asmaller scale, in a fallen log. Energy is maximum at the beginning and leclines as succesion occurs unless additional organic matter is importedor until an autotrophic regime takes over. In contrast, energy low does not necessarily decline in the autotrophic type but is usually maintained or increased during succession.