6.5.4. Ecological succesion

Ecological succession is the observed process of change in the species structure of an ecological community over time. The community begins with relatively few pioneering plants and animalsand develops through increasing complexity until it becomes stable or self-perpetuating as a climax community.

It is a phenomenon or process by which an ecological community undergoes more or less orderly and predictable changes following disturbance or initial colonization of new habitat. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe windthrow, logging) of an existing community. Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession, whereas succession that follows disruption of a pre-existing community is called secondary succession.

Primary succesion

Primary succession is one of two types of biological and ecological succession of plant life, occurring in an environment in which new substrate devoid of vegetation and usually lacking soil, such as a lava flow or area left from retreated glacier, is deposited. In other words, it is the gradual growth of an ecosystem over a longer period.

800px-Forest_succession_depicted_over_time

The primary succession has r-selected organisms– species that can survive at high disturbance areas and are fast growing organisms. There are several factors that can cause disturbance- (1) people, (2) diseases, (3) fire, (4) precipitation/floods, (5) wind/hurricanes/tornadoes, (6) temperature extremes (like freezing or extreme heat), and (7) volcanoes. In other words, the communities that are in early succession will be dominated by fast-growing, well-dispersed species. The other stage of succession (as succession moves forward) will be replaced by more competitive species.

In primary succession pioneer species like lichen, algae and fungi as well as other abiotic factors like wind and water start to “normalize” the habitat. This creates conditions nearer optimum for vascular plant growth; pedogenesis or the formation of soil is the most important process.

These pioneer plants are then dominated and often replaced by plants better adapted to less odd conditions, these plants include vascular plants like grasses and some shrubs that are able to live in thin soils that are often mineral based.

For example spores of lichen or fungus, being the pioneer species, are spread onto a land of rocks. Then, the rocks are broken down into smaller pieces and organic matter gradually accumulates, favouring the growth of larger plants like grasses, ferns and herbs. These plants further improve the habitat and help the adaptation of larger vascular plants like shrubs, or even medium- or large-sized trees. More animals are then attracted to the place and finally a climax community is reached.

Secondary succesion

Secondary succession is a process started by an event (e.g. forest fire, harvesting, hurricane) that reduces an already established ecosystem (e.g. a forest or a wheat field) to a smaller population of species, and as such secondary succession occurs on preexisting soil whereasprimary succession usually occurs in a place lacking soil.

95198-036-2619E3FASimply put, secondary succession is the succession that occurs after the initial succession has been disrupted and some plants and animals still exist. It is usually faster than primary succession as:

  1. Soil is already present, so there is no need for pioneer species;
  2. Seeds, roots and underground vegetative organs of plants may still survive in the soil.

The secondary succession communities have k-selected organisms-organisms that use the limiting resource the best and therefore can out-compete other individuals The secondary succession may include a seral community– an intermediate stage found in ecological succession in an ecosystem that advances towards its climax community.

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