Ecological succession Ecological succession is the process of how species compositions change in an Community (ecology), ecological community over time. The two main categories of ecological succession are primary succession and secondary succession. Primary successi ...

can be understood as a process of changing

species composition Relative species abundance is a component of biodiversity and is a measure of how common or rare a species is relative to other species in a defined location or community.Hubbell, S. P. 2001. ''The unified neutral theory of biodiversity and biogeog ...

within a community due to an ecological disturbance, and varies largely according to the initial disturbance prompting the succession. Joseph Connell and Ralph Slatyer further developed the understanding of successional mechanisms in their 1977 paper and proposed that there were 3 main modes of successional development. These sequences could be understood in the context of the specific life-history theories of the individual species within an ecological community.

The 1977 study

Connell and Slatyer chose to focus on

autogenic succession "Auto-" meaning self or same, and "-genic" meaning producing or causing. Autogenic succession refers to ecological succession driven by biotic factors within an ecosystem and although the mechanisms of autogenic succession have long been debated, ...

, which occurs on newly exposed landforms and is initiated by changes from within the community rather than a geophysical transformation.Connell & Slatyer They targeted plant and immobile aquatic organisms that demanded the greatest surface area within an environment and could modify the physical landscape. They defined community as "the set of organisms that occur together and that significantly affect each other's distribution and abundance".

The models

''The key factor distinguishing the three models is how the process of succession affects the original, pioneer species (i.e. their relative success in later-successional stages).''Connell & Slatyer, 1121-1124.

Facilitation model

Based on the assumption that only particular species with qualities ideal for "early succession" can colonize the newly exposed landforms after an

ecological disturbance In ecology, a disturbance is a change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, to alter the physical structure or arrangement of biotic and abiotic element ...

. ::*These "colonizing" qualities include: highly effective methods of dispersal, the ability to remain dormant for long periods of time, and a rapid growth rate. However, the

pioneer species Pioneer species are resilient species that are the first to colonize barren environments, or to repopulate disrupted biodiverse steady-state ecosystems as part of ecological succession. Various kinds of events can create good conditions for pi ...

are often subsequently less successful once an area has been heavily populated by surrounding species due to increased shade, litter or concentrated roots in the soil, etc. ::*Thus, the presence of early successional species often changes the environment so that the habitat is less hospitable for the original species’ own ecological demands and facilitates the invasion of later-successional species. (Note: See

Ecological Facilitation Ecological facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither.Stachowicz, J. J. 2001. Mutualism, facilitation, and the structure of ecological communities. BioScience 51 ...

Tolerance model

In this case, new pioneer species neither inhibit nor facilitate the growth and success of other species. The sequences of succession are thus entirely dependent on life-history characteristics such as the specific amount of energy a species allocates to growth. ::*The

climax community In scientific ecology, climax community or climatic climax community is a historic term for a community of plants, animals, and fungi which, through the process of ecological succession in the development of vegetation in an area over time, hav ...

is composed of the most "tolerant" species that can co-exist with other species in a more densely populated area. Eventually, dominant species replace or reduce pioneer

species abundance In ecology, local abundance is the relative representation of a species in a particular ecosystem. It is usually measured as the number of individuals found per sample. The ratio of abundance of one species to one or multiple other species livin ...

through

competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indi ...

Inhibition model

Earlier successional species actually inhibit growth of later successional species and reduce growth of colonizing species already present. ::*Example: Pioneer species might modify the environment through rapid growth and make the area increasingly shady (essentially increasing competition for light). ::*The environment is thus less hospitable to other potential colonizing species. ::*The only possibility for new growth/colonization in this successional sequence arises when a disturbance leads to dominating species being destroyed, damaged, or removed. This frees up resources and allows for the invasion of other species that were not previously present.

Examples of each model

:1. Facilitation Model ::Essentially, the facilitation model suggests that the presence of an initial species aids and increases the probability of the growth of a second species.Ricklefs, 400. For example, in "Physiological Controls Over Seedling Growth in Primary Succession on an Alaskan Floodplain" by Lawrence R. Walker and F. Stuart Chapin, III, the presence of

alder Alders are trees of the genus ''Alnus'' in the birch family Betulaceae. The genus includes about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species ex ...

plants aids the growth of

willow Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...

and poplar seedlings in an

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

n floodplain. ::Alder roots contain

nitrogen-fixing bacteria Diazotrophs are organisms capable of nitrogen fixation, i.e. converting the relatively inert diatomic nitrogen (N2) in Earth's atmosphere into bioavailable compound forms such as ammonia. Diazotrophs are typically microorganisms such as bacteria ...

, which greatly increase the amount of inorganic nitrogen present in soils. This increased availability of nitrogen aids the growth of both willow and poplar seedlings in areas without other competition. Eventually, however, willow and poplar grow more rapidly than alder, leading to a reduction in the abundance of the pioneer species, and eventually, spruce becomes a later-succession species, due to its increased ability (over alder) to grow in shaded areas. ::Another case of facilitation comes from the colonization of lakeshore

sand dunes A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...

.Connell & Slatyer, 1124. Adjacent pioneer plants colonize the otherwise moving sands and alter the environmental constraints of the sandy environment to better suit other plant species, which can then allow for soil binding to take place. The giant saguaro cactus, in this respect, can only survive in the shade of other plants (or in some cases rocks) – pioneer species facilitate their existence by providing shade. (The argument has also been made that this type of interaction is exemplary of the tolerance model; see below). :2. Tolerance Model ::The tolerance model is completely dependent upon life history characteristics. Each species has an equally likely chance to establish itself in the early stages of succession and their establishment results in no environmental changes or impacts on other species.Moorcroft. Eventually, early species, typically dominated by r-selected species, which prioritize fast rates of reproduction, are out-competed by K-selected species (species that become more dominant when there is competition for limited resources). ::For example, we can examine succession in the

Loess Plateau The Loess Plateau is a plateau in north-central China formed of loess, a clastic rock, clastic silt-like sediment formed by the accumulation of Dust#Atmospheric, wind-blown dust. It is located southeast of the Gobi Desert and is surrounded by t ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. In the graph on page 995 of the paper "Plant Traits and Soil Chemical Variables During a Secondary Vegetation Succession in Abandoned Fields on the Loess Plateau" by Wang (2002), we can see the initial dominance of the '' Artemisia scoparia'', the pioneer species. Over time, however, the ''

Bothriochloa ischaemum ''Bothriochloa ischaemum'' is a species of perennial grass in the family Poaceae, found throughout much of the world. It is commonly known as yellow bluestem. Two varieties are recognized, of which ''Bothriochloa ischaemum'' var. ''ischaemum'' i ...

'' becomes the dominant species and the abundance of ''A. scoparia'' greatly declines. This is due to the rapid rate of reproduction of the ''A. scoparia'', resulting in the species' early abundance, and the dominant competition from the K-selected ''B. ischaemum'', resulting in that species' later abundance. ::A characteristic that is often associated with the tolerance model and well documented in forest succession is survival in conditions of shade. As an uninhabited area becomes populated by different plant species, shade increases – which makes less light available for the next generation. Species that are better adapted to shady conditions will then become dominant. All of the early species inhabiting the terrain have modified the environment in a way that favors a specific k-selected characteristic. :3. Inhibition Model ::In this model, one species inhibits the presence of another, either through direct means, such as predation (by eating the other species or attacking them), or indirect means, such as competition for resources. ::Sometimes in inhibition models, the time of establishment of a species determines which species becomes dominant. This phenomenon is referred to as the priority effect and suggests that the species that became established earlier are more likely to become the dominant species. One example of the inhibition model, and the priority effect, occurs in South Australia. In areas where

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns are established first,

tunicates Tunicates are marine invertebrates belonging to the subphylum Tunicata ( ). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

and

sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...

cannot grow. ::The inhibition model has also been observed at work in forest ecosystems; in these systems the early arrivers hold a monopoly on the land, keeping other species out. Closed shrub canopies have been known to prevent tree growth and access to land for periods of up to 45 years – in an experimental study on inhibition it was found that areas occupied by large areas of

Lantana ''Lantana'' () is a genus of about 150 species of perennial plant, perennial flowering plants in the verbena family, Verbenaceae. They are native to tropics, tropical regions of the Americas and Africa but exist as an introduced species in num ...

sprawling shrubs excluded and inhibited the growth of tree species.Connell & Slatyer, 1125-1126.

References

Works cited

*Connell, Joseph H. and Ralph O. Slatyer. “Mechanisms of succession in natural communities and their role in community stability and organization.” The American Naturalist 111 (982) (Nov. - Dec. 1977): 1119-1144. *Moorcroft, Paul. “Terrestrial Succession.” Organismic and Evolutionary Biology 55. Maxwell Dworkin, Cambridge. 4 April 2011. *Ricklefs, Robert E. The Economy of Nature. (New York, NY: W.H. Freeman and Co., 2008). *“Sand Dunes.” National Biological Information Infrastructure- Home. Accessed 29 Apr. 2011. . *Walker, Lawrence R. and F. Stuart Chapin, III. “Physiological Controls Over Seedling Growth in Primary Succession on an Alaskan Floodplain.” Ecology 67 (6) (Dec.,1986): 1508-1523. *Walker, et al., “The Role of Life History Processes in Primary Succession on an Alaskan Floodplain,” 1243-1253. *Wang, Guo-Hong. “Plant Traits and Soil Chemical Variables During a Secondary Vegetation Succession in Abandoned Fields on the Loess Plateau.” Acta Botanica Sinica 44 (8) (2002): 990-998. {{DEFAULTSORT:Connell-Slatyer model of ecological succession Ecological succession