Succession

Charmi Patel

Expected changes in Biodiversity

Over time, as environmental conditions change, organisms and communities also change. Eventually, new communities replace current communities and the ecosystem changes. Thus, biodiversity increases. Grasses and other small organisms are first to appear, forming a low layer of vegetation. This changes the burned area, allowing shrubs and small trees to become established. The shrubs and small tree species change the environment, and eventually larger tree species will become established. As plants become established, small animals, then larger ones return and re-populate the area, building up biodiversity.

Disturbances

When a volcano erupts, the area becomes lifeless where soil has not yet formed, which is also known as primary succession. Often the only life-forms initially present are autotrophic prokaryotes and heterotrophic prokaryotes and protists. Lichens and mosses, which grow from windblown spores, are commonly the first macroscopic photosynthesizers to colonize such areas. Soil develops gradually as rocks weather and organic matter accumulates from the decomposed remains of the early colonizers. Once soil is present, the lichens and mosses are usually overgrown by grasses, shrubs, and trees that sprout from seeds blown in from nearby areas or carried in by animals. Eventually, an area is colonized by plants that become the community’s prevalent form of vegetation. Producing such a community through primary succession may take hundreds or thousands of years.


When a mature forest is cut, the area does not become lifeless since soil is already present. This is known as secondary succession, which occurs when an existing community has been cleared by some disturbance that leaves the soil intact. The earliest plants to recolonize are often herbaceous species that grow from windblown or animal-borne seeds. Woody shrubs may in time replace most of the herbaceous species, and forest trees may eventually replace most of the shrubs.

Differences in Plant Diversity

After a disturbance (such as fire or agriculture), plant and animals species begin to reoccupy the habitat, grow, and get replaced or out-competed by other species. This pattern of gradual temporal shift in the species composition of a community is called succession. It results from a variety of processes including migration, dispersal, growth, competition and environmental change. Recent changes in the composition of plant and animal communities occur on ecological rather than geological time scales as the result of processes such as immigration, competition and predation. The overall diversity of a particular region will depend on its capacity to support life - its size and productivity - and on the variety of habitats that it includes. For plants, diversity increases with succession until woody species (trees and brushes) establish, whereby diversity then decreases.

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Biotic/ Abiotic Factors

The interaction and competition between species in a particular habitat is a biotic factor of ecological primary succession. When succession begins and the very first species, known as pioneer species, change the environmental structure, new species now tolerable to the new conditions move in. The diversity among the species present is high at this point. However, in time, competition and interaction causes a significant drop in species diversity where the dominant species thrive and the rest die off.

The soil, an abiotic factor, of an environment affects ecological primary succession greatly. Different species of plants require different soil conditions. Trees tend to be the largest driving organism in this portion of ecological succession. The pH levels of the soil is often affected most by the inhabiting trees and determines what type of plants can thrive there. The type of soil (loamy sand, sandy, top soil with humus, etc.) also plays a large role in what species can inhabit an area. In sandy areas, only a select few species are able to take root and survive. The moisture level of the soil determines what sort of trees inhabit an area. Swampy areas tend to house trees with higher pH level requirements where drier soils tend to house trees with lower pH level requirements.
Climate, an abiotic factor highly involved in both primary and secondary succession, plays a significant role in determining the direction of succession in an environment. If an environment receives a period of low precipitation, it becomes more susceptible to fires caused by lightning. This leads to secondary succession in which fire-resistant and tolerable species thrive and the others die out. Wind has the ability to reform the landscape over time by way of erosion. Winds can also drive wildfires to further cause disturbance. However, when an environment receives high levels of precipitation, it becomes more suitable for certain species that are tolerable of high moisture levels, which is an example of climatic effect on primary succession.