The Classical model of ecological succession shows that the "structural complexity and organization of an ecosystem increase and mature over time as succession progresses." The natural cycle of progression according to this model is shown below.
The model above shows the natural cycle of progression of an ecosystem. The early successional stage is characterized by a few "pioneer species" and net community production is greater than respiration. The next mature stage shows an increase in species diversity, biomass, nutrients, food chains, and net production is equal to respiration. The succession slows down when it reaches equilibrium at the level of "climax" community.
"Autogenic Succession is self-driven, resulting from the interaction between organisms and their environment." There are two types of autogenic succession. Primary Succession occurs on virgin or newly formed substrates, such as lava flows, alluvial deposits, newly exposed rock faces and glacial moraines. (Mackenzie, Ball and Virdee 2001) the Glacier Bay and Krakatau sequences are examples of Primary Succession. "Secondary succession occurs on disturbed ground where vegetation cover has been disturbed by external environmental factors like humans, animals or by fire, wind, floods.
Succession from bare moraine to mature coniferous forest has been shown in areas left bare by the retreating ice in Glacier Bay, South East Alaska. This is a prime example of primary succession. Current estimates have shown that it takes 250 years for mature forest to develop from bare moraine. (Packam, Harding, Hilton, Stuttard 1992)
The process involves colonization of the nutrient poor clay by mosses and shallow-rooted herbaceous species like mountain avens (Dryas sp.). These early species alter the soil conditions facilitating the colonization by new species. Nitrogen fixation is a free process in succession. Litter accumulation helps in soil development, which aids colonization by shrub and eventually tree species like Cottonwood and Hemlock. Marble is present in the area and the soil parent material shows a pH of 8.0 - 8.4. The development of Sitka Alder has a strong acidifying effect leading to a fall in pH from 8.0 to 5.0 approximately within 30-50 years. Alder nodules also fix atmospheric nitrogen, and the increase in nitrogen values of the soil is crucial to the initial establishment of Sitka Spruce. The Spruce eventually displaces the Alder, and it leads to a mature Sitka Spruce-Hemlock forest. In another 1250 years, the mature forest changes to Sphagnum-dominated muskeg bog in suitable sites.
Another example of Primary Succession is the Krakatau group, which lies over an orogenic hot spot in between Java and Sumatra. After the explosion in 1883, the present successional sequence was started on Rakata, and the smaller islands of Rakata Kecil and Sertung. Here volcanic ash sterilized the whole area. But the successional sequence, which started with a few blades of grass on Rakata in 1884, has led to a lush tropical forest over the century. The pioneer species included