Seres are sometimes classified according to the predominant force that is bringing them about. These forces are biotic, climatic, physiographic, and geologic and their resultant seres are commonly called bioseres, cliseres, eoseres and geoseres.
The succession may be of the following two types in any of the basic environments such as terrestrial, fresh-water or marine:
1. Primary succession:
It is the process of species colonization and replacement in which the environment is initially virtually free of life. That is, the process starts with base rock or sand dune or river delta or glacial debris and it ends when climax is reached. “The sere involved in primary succession is called presere.
2. Secondary succession:
It is the process of change occurs after an ecosystem is disrupted but not totally obliterated. In this situation, organic matter and some organisms from the original community will remain; thus the succession process does not start from scratch. Consequently, secondary succession is more rapid than primary. It is seen in areas burned by fire or cut by farmers for cultivation. The sere involved in secondary succession is called subsere.
The primary and secondary successions may be of the following types depending on the moisture contents:
(a) Hydrach or hydrosere:
The succession when starts in the aquatic environment such as ponds, lakes, streams, swamps, bogs, etc., it is called hydrach or hydrosere.
The succession when begins in an area, where adequate moisture is present, it is called mesarch.
(c) Xerach or xerosere:
The succession when starts in xeric or dry habitat having minimum amounts of moisture, such as dry deserts, rocks, etc. It is called xerach. A temporary community in an ecological succession on dry and sterile habitat is called xerosere. It may be of three types: lithosere-succession initiating on rocks, psammoscre-succession initiating on sand; and halosere- succession starting on saline water or soil.
Further, a succession progressing entirely as a result of interactions of the organisms and their environment (i.e., “driving force” is internal to the community) is known as autogenic community (e.g., succession on sand dunes); and the one moving under the influence of external factors, as input of nutrients, is called allogenic community (e.g., succession in a small pond or bog).
Sometimes, succession is classified as autotrophic and heterotrophic on the basis of community metabolism. Autotrophic succession is characterized by early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic and the energy flow is maintained indefinitely.
Heterotrophic succession is characterized by early dominance of heterotrophs, such as bacteria, actinomycetes, fungi and animals. It begins in a predominantly organic environment and there is a progressive decline in the energy content. The miniature succession of micro-organisms such as bacteria and different types of fungi on the fallen logs of the decaying wood, tree bark, etc., is called serule.