Under succession ( Latin succedere "move up", "follow") is defined as the natural return of a location typical plant, animal and fungal companies ( biocenosis ) that after a deviation due to the prevailing environmental factors (especially climate and soil type ) hires there again. This successive development leads from a disturbed or changed initial stage - for example due to storm damage, avalanches, etc. or various man-made changes in nature such as clearing or overuse , in the extreme case from the vegetation-free soil ("initial stage") - through various intermediate stages to a stable one End stage ( climax society ), provided that no further disturbances occur.
In colloquial language, areas that were previously used and have since been left to their own devices for a longer period of time are referred to as succession areas. Succession is then often equated with bush cover .
The natural succession can be with the healing process compare a living being.
Succession can take place in all ecosystems. The starting point are newly created (e.g. exposed rock surfaces, fresh sand dunes), disturbed (e.g. forests after forest fire or storm damage) or used (e.g. meadows, heaths) habitats. But succession can also take place in natural habitats if the living conditions change (e.g. climate change). If the community follows passively gradually changing abiotic environmental conditions, as in this case , one also speaks of allogeneic (exogenous) succession.
The community itself is often a major cause or drive for the further course of the succession. Established communities can change the location factors themselves, e.g. B. by soil formation ( pedogenesis ) or when a lake silted up by dead plant litter of the reed species. Newly added species can make colonization easier for other species, or they can displace them (through competition ). Such a succession, which is driven by the action of the organisms of a community itself, is called autogenous (endogenous) succession.
There are three possible key processes involved in succession
- Funding: Predecessors promote successors by changing the living space and location in their favor, e.g. B. in that plants with high nutrient requirements can only thrive in the soil through the accumulation of nutrients through plant litter of other species. Species of later succession stages are then dependent on pioneer species as first colonists.
- Tolerance: Species can settle without benefiting from species previously growing there. This comes e.g. For example, if later emerging species can tolerate a lower level of essential resources than first colonists. They can then penetrate into a habitat, even though it is already occupied by individuals of the first settlers. Ultimately, this leads to the displacement of the first settlers.
- Inhibition: later species can only establish themselves despite the early colonists. As long as these are vital, they prevent the subsequent species from establishing.
The understanding of the succession can be z. This can be used, for example, in landscape planning and nature conservation, when it comes to restoring and securing the disturbed natural balance - as a basis for human existence - in the long term.
The assumption of a (single) ecological equilibrium towards which ecosystems would develop after disturbances by succession has been questioned in ecology by empirical studies and has given way to newer models of dynamic and multiple equilibria.
In addition to the “normal” (or “progressive”) succession to a more highly organized “climax society” described above, in exceptional cases a backward “regressive” succession can occur, in which highly developed communities are replaced by more simply structured ones. This is e.g. This is the case, for example, when the soil has been so severely impaired or eroded after a forest has been destroyed (e.g. after a forest fire) that development then proceeds in a completely different direction and not the initial state, but a more simply structured community (e.g. B. a bush or a rock heather) follows.
A distinction is made in botany:
- "Primary" succession: This starts on previously unpopulated locations.
- "Secondary" succession: The starting point here are already settled locations that have been changed by the species that occur. This can be existing vegetation, z. B. Succession from meadow to forest in case of abandonment. But even in locations without vegetation, soil developments have already taken place here (e.g. humus content, nutrient enrichment) and there is usually a seed bank ( diasporic bank) in the soil . Secondary succession also occurs when a community is disrupted that left the soil intact. Often the disturbed area then develops back into its original state.
In the case of succession on bare ground, the following sequence is typical:
Initial stage, Subsequent stages, Climax stage
In the initial stage, pioneer species open up the unpopulated area. If it has arisen due to a disturbance by humans or due to human use, one also speaks of substitute societies . Types of such pioneering societies must have effective mechanisms for remote dispersal in place (e.g. wind-dispersed seeds). In contrast to types of climax societies, they often have a greater tolerance towards extreme location factors . In the initial and early stages of succession, species with high reproductive potential, the r-strategists , predominate (named after the reproduction factor r in the logistic equation ). r-strategists reproduce quickly and in large numbers. In the course of time, other species immigrate that spread more slowly and displace the mostly weak competitive pioneer species. The pioneer species also change the location factors, e.g. B. through accumulation (accumulation) of nitrogen , other nutrients and humus, change the water balance and climate, affect the soil (see pedogenesis ) and the fauna (totality of animal species).
Due to the changed location factors, other species are now able to colonize the changed biotope. These species are more demanding (climate, water, nutrients, etc.) and are more productive. In the following stages , therefore, the K strategists increasingly prevail (named after the habitat capacity K in the logistic equation ). K strategists reproduce less rapidly, so they have fewer offspring. However, these have a higher ability to assert themselves in the struggle for survival and displace the pioneer species. The new species, for their part, also change the location factors, and the process continues; a next, more demanding and more productive society takes over. The biodiversity increases over time, while the rate of change continues to decrease.
The climax stage is reached when the species composition no longer changes or changes only very slightly. It is traditionally assumed that the climax stage also has the highest production of biomass that is possible at a location. The climax stage uses its resources most effectively (if usable resources were still free, they could be used by additional species. The succession would thus continue, the end point would not yet be reached).
Klimax societies / climax stages
The term climax in botany goes back to Frederic Edward Clements . In its original version, it assumed a uniform plant community for each climatic zone, which, if the development time was long enough, would ultimately prevail everywhere and in all locations ("monoclimax"). In the modern ecological discussion, the term is mostly relativized and used with a modified meaning. As a rule, it is assumed that very different locations would not level each other completely (“Polyklimax”). In addition, (e.g. cyclical) changes are also accepted as a possibility in the climax stage.
The climax vegetation (largely) corresponds to the term “ potential natural vegetation ” introduced by the vegetation expert Reinhold Tüxen . (Difference: permanent location changes in the course of succession are not taken into account).
According to the prevailing opinion, the climax vegetation in Central Europe is a largely closed forest except in extreme and exceptional locations. Most of the sites are in a beech forest with few species of plants . Other climax societies can only be found at special locations. Bogs usually form raised bogs as a climax stage; High mountain areas and mud flats in turn form other climax stages. Other exceptions include a. azonal forest communities , such as B. Alluvial forests , swamp forests .
It should be noted, however, that some societies are subject to natural dynamics (e.g. floodplains with regular floods). Due to these (natural) disturbances, the climax vegetation may never be reached here. Continuous clearings in the climax forest due to (natural) disturbances by large herbivores are also discussed (cf. megaherbivore hypothesis ).
Mosaic cycle theory
The mosaic cycle theory (also known as the mosaic cycle concept ) goes back to Kurt Michael Zukrigl and has been propagated in the last few decades mainly by Hermann Remmert . Instead of a uniform climax stage, it assumes a mosaic-like structure in which the most diverse succession stages exist side by side.
- Hermann Remmert : The mosaic cycle concept and its significance for nature conservation. 1991.
- Joseph H. Connell; Ralph O. Slatyer (1977): Mechanisms of Succession in Natural Communities and Their Role in Community Stability and Organization. American Naturalist 111 (982): 1119-1144.
- FE Clements: Nature and Structure of the Climax. (PDF; 1.8 MB) In: The Journal of Ecology. 1936, 24 (1): 252-284.
- Hermann Remmert: The mosaic cycle concept and its significance for nature conservation. 1991.