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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 accordingly called bioseres, cliseres, eoseres and geoseres.
Similarly, on the basis of these forces the succession is also called biotic succession, Climatic succession, Physiographic succession, and Geologic succession.
1. Biotic Succession:
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This succession is brought about by the forces inherent within the community and in the (activities of animals and plants themselves. The most important of these activities are their reaction; and coactions that produce modifications in the habitat and interrelations between species. Important reactions involve filling in of ponds with animal and plant remains, the addition of organic nutrient; to sterile soil, and the reduction in light intensity by increasing density of plants.
With progressive improvement of the soil and changing light and moisture conditions, a series of new ecological dominants come into the area. When invasion of new species occurs, intense competition develops and if the invaders are strong and successful the old species disappear because a new community replaces them.
2. Climatic Succession:
With changes in climate, environmental conditions often surpass the limits of tolerance of established animals and plants. The result is the replacement of the existent community by another a most interesting closure is the one that has occurred since the northward retreat of the continental glacier of Pleistocene time.
Stages in this closure may be detected and even the relative duration of each stage measured from an analysis of the number and persistence of different kinds of pollen grains at various depths in peat bogs. To make such studies a core of peat is obtained from the deepest part of the bog by mean of a special hand auger.
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The lowest portion of the core is oldest the most recently formed is as at the top. Samples of the core at various depths are suitable prepared, examined under a microscope, and the pollen grains identified are counted. The predominant kind of pollen at any level of the core represents the probable prevailing species of plants in the vicinity at the corresponding period of time.
3. Physiographic Succession:
Changes in the earth’s surface bring a change in communities. Mountain- building brings the replacement of low- land communities with new ones that invade at higher elevations. As mountains erode, the eosere progresses in the opposite direction, until base-level or peneplanation is attained. The development that the eosere will undergo with continued erosion is sometimes locally apparent in the difference between upland and floodplain forests. Stages in the erosion cycle may be distinguished by examination of habitat and animal life progressively from headwaters to mouth of a stream.
4. Geologic Succession:
The first organisms to appear on earth were unicellular forms confined to the sea. At the beginning of the Cenozoic Era, ancient types of animals, including the great reptiles and many types of invertebrates, became extinct and mammals rose to predominance. It is significant that the rich development of mammals, birds and insects came after the worldwide establishment of the angiosperms with their rich nutrient seeds, fruits and grasses.
Pleistocene brought a major change in the habitat of these animals, and some large mammals disappeared. The last stage of the geosere, the recent epoch, brought the dominance of man. Now only future ages will determine whether this stage is climax or new and different types of animal and plant life will someday evolve to replace man and to continue the succession into the indefinite future.
An important aspect of geosere is the dispersal of new types of animals and plants, for the importance of a new form depends on the extent of its distribution and the size of its population. Speciation in animals is initiated only when one population becomes isolated from another similar population so that interbreeding is checked. This permits variation and natural selection to proceed independently in the two groups. Isolation is usually affected by geographic barriers and generally involves occupancy of new niches and development of new coactions with other members of the community.
The history of the past ecological relations of species and of the whole community is the subject matter of palaeoecology. The dynamic forces involved in these processes of speciation are among the forces determining the geosere.
