what is meant by term ecological succession?? provide details regarding ecological succession on land and water.
Ecological succession on land
The succession initiated on bare rock or wind blown sand where water is low is termed as xerarch. The first plants to colonize here are crustose lichens like Rhizocarpon, Rhinodina and Lecidea. Foliose lichens (Dermatocarpon and Parmelia) replaces the crustose lichens which in turn are replaced by mosses like Polytrichum, Tortula etc. After mosses herbs occupy the land which is replaced by shrubs like Rhus and Phytocarpus. Finally the climax species i.e. forests becomes dominant in the land.
Ecological succession on water
Succession occurring on water starts with phytoplanktons which colonize the nutrient poor water. Submerged hydrophytes like Hydrilla, Potamogeton and Vallisneria replaces the phtoplanktons. After submerged hydrophytes comes the rooted plants which have their leaves floating on water surface like Nymphaea, Trapa and Monochoria. Next stage in succession on water is amphibian stage dominated by plants whose shoots are exposed to air like Typha, Sagittaria etc. Then comes sedge-meadow stage defined by Juncus, Cyperus and Eleocharis. After this certain species of shrubs and trees appear followed by the forests.
A gradual process incurred by the change in the number of individuals of each species of a community and by establishment of new species populations that may gradually replace the original inhabitants.
Ecological succession, is the 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 was among the first theories advanced in ecology and the study of succession remains at the core of ecological science. 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.
History of the theory
Precursors of the idea of ecological succession go back to the beginning of the 19th century. The French naturalist Adolphe Dureau de la Malle was the first to make use of the word succession concerning the vegetation development after forest clear-felling. In 1859 Henry David Thoreau wrote an address called "The Succession of Forest Trees" in which he described succession in an Oak-Pine forest.
H. C. Cowles
Henry Chandler Cowles, at the University of Chicago, developed a more formal concept of succession. Inspired by studies of Danish dunes by Eugen Warming, Cowles studied vegetation development on sand dunes on the shores of Lake Michigan (the Indiana Dunes). He recognized that vegetation on dunes of different ages might be interpreted as different stages of a general trend of vegetation development on dunes (an approach to the study of vegetation change later termed space-for-time substitution, or chronosequence studies). He first published this work as a paper in the Botanical Gazette in 1899 ("The ecological relations of the vegetation of the sand dunes of Lake Michigan"). In this classic publication and subsequent papers, he formulated the idea of primary succession and the notion of a sere -- a repeatable sequence of community changes specific to particular environmental circumstances.
Clements vs. Gleason
From about 1900 to 1960, however, understanding of succession was dominated by the theories of Frederic Clements, a contemporary of Cowles, who held that seres were highly predictable and deterministic and converged on a climatically determined stable climax community regardless of starting conditions. Clements explicitly analogized the successional development of ecological communities with ontogenetic development of individual organisms, and his model is often referred to as the pseudo-organismic theory of community ecology. Clements and his followers developed a complex taxonomy of communities and successional pathways (see article on Frederic Clements).
Henry Gleason offered a contrasting framework as early as the 1920s. The Gleasonian model was more complex and much less deterministic than the Clementsian. It differs most fundamentally from the Clementsian view in suggesting a much greater role of chance factors and in denying the existence of coherent, sharply bounded community types. Gleason argued that species distributions responded individualistically to environmental factors, and communities were best regarded as artifacts of the juxtaposition of species distributions. Gleason's ideas, first published in 1926, were largely ignored from their initial publication until the late 1950s.
Two quotes illustrate the contrasting views of Clements and Gleason. Clements wrote in 1916:
" The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. As an organism the formation arises, grows, matures, and dies... Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity its development."
while Gleason, in his 1926 paper, said:
“An association is not an organism, scarcely even a vegetational unit, but merely a coincidence.”
This classification seems not to be of fundamental value, since it separates such closely related phenomena as those of erosion and deposition, and it places together such unlike things as human agencies and the subsidence of land.
Beginning in the 1950s and 1960s, beginning with the work of Robert Whittaker and John Curtis, more rigorous, data-driven testing of successional models and community theory generally began. Succession theory has since become less monolithic and more complex. J. Connell and R. Slatyer attempted a codification of successional processes by mechanism. Among British and North American ecologists, the notion of a stable climax vegetation has been largely abandoned, and successional processes have come to be seen as much less deterministic, with important roles for historical contingency and for alternate pathways in the actual development of communities. Debates continue as to the general predictability of successional dynamics and the relative importance of equilibrial vs. non-equilibrial processes.
Factors influencing succession
The trajectory of successional change can be influenced by site conditions, by the character of the events initiating succession, by the interactions of the species present, and by more stochastic factors such as availability of colonists or seeds or weather conditions at the time of disturbance. Some of these factors contribute to predictability of succession dynamics; others add more probabilistic elements.
In general, communities in early succession will be dominated by fast-growing, well-dispersed species (opportunist, fugitive, or r-selected life-histories). As succession proceeds, these species will tend to be replaced by more competitive (k-selected) species.
Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors. Net Primary Productivity, biomass, and trophic properties all show variable patterns over succession, depending on the particular system and site.
Ecological succession was formerly seen as having a stable end-stage called the climax (see Frederic Clements), sometimes referred to as the 'potential vegetation' of a site, and shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of ecosystems dynamics. Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. Additions to available species pools through range expansions and introductions can also continually reshape communities.
The development of some ecosystem attributes, such as soil properties and nutrient cycles, are both influenced by community properties, and, in turn, influence further successional development. This feed-back process may occur only over centuries or millennia. Coupled with the stochastic nature of disturbance events and other long-term (e.g., climatic) changes, such dynamics make it doubtful whether the 'climax' concept ever applies or is particularly useful in considering actual vegetation.
Types of succession
Primary and secondary succession
Successional dynamics beginning with colonization of an area that has not been previously occupied by an ecological community, such as newly exposed rock or sand surfaces, lava flows, newly exposed glacial tills, etc., are referred to as primary succession.
Successional dynamics following severe disturbance or removal of a pre-existing community are called secondary succession. Dynamics in secondary succession are strongly influenced by pre-disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid. Secondary succession is much more commonly observed and studied than primary succession. Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture.
Seasonal and cyclic dynamics
Unlike secondary succession, these types of vegetation change are not dependent on disturbance but are periodic changes arising from fluctuating species interactions or recurring events. These models propose a modification to the climax concept towards one of dynamic states.
Causes of plant succession
Autogenic succession can be brought by changes in the soil caused by the organisms there. These changes include accumulation of organic matter in litter or humic layer, alteration of soil nutrients, change in pH of soil by plants growing there. The structure of the plants themselves can also alter the community. For example, when larger species like trees mature, they produce shade on to the developing forest floor that tends to exclude light-requiring species. Shade-tolerant species will invade the area.
Allogenic succession is caused by external environmental influences and not by the vegetation. For example soil changes due to erosion, leaching or the deposition of silt and clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes as they are pollinators, seed dispersers and herbivores. They can also increase nutrient content of the soil in certain areas, or shift soil about (as termites, ants, and moles do) creating patches in the habitat. This may create regeneration sites that favor certain species.
Climatic factors may be very important, but on a much longer time-scale than any other. Changes in temperature and rainfall patterns will promote changes in communities. As the climate warmed at the end of each ice age, great successional changes took place. The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. The greenhouse effect resulting in increase in temperature is likely to bring profound Allogenic changes in the next century. Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high wind also bring allogenic changes.
Clement's theory of succession/Mechanisms of succession
F.E. Clement (1916) developed a descriptive theory of succession and advanced it as a general ecological concept. His theory of succession had a powerful influence on ecological thought. Clement's concept is usually termed classical ecological theory. According to Clement, succession is a process involving several phases:
- Nudation: Succession begins with the development of a bare site, called Nudation (disturbance).
- Migration: It refers to arrival of propagules.
- Ecesis: It involves establishment and initial growth of vegetation.
- Competition: As vegetation became well established, grew, and spread, various species began to compete for space, light and nutrients. This phase is called competition.
- Reaction: During this phase autogenic changes affect the habitat resulting in replacement of one plant community by another.
- Stabilization: Reaction phase leads to development of a climax community.
A seral community is an intermediate stage found in an ecosystem advancing towards its climax community. In many cases more than one seral stage evolves until climax conditions are attained. A prisere is a collection of seres making up the development of an area from non-vegetated surfaces to a climax community. Depending on the substratum and climate, a seral community can be one of the following:
- Community in freshwater
- Community on rock
- Community on sand
- Community in dry area
- Community in saline body (e.g. a marsh)
Changes in animal life
Animal life also exhibit changes with changing communities. In lichen stage the fauna is sparse. It comprises few mites, ants and spiders living in the cracks and crevices. The fauna undergoes a qualitative increase during herb grass stage. The animals found during this stage include nematodes, insects larvae, ants, spiders, mites, etc. The animal population increases and diversifies with the development of forest climax community. The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mouse, moles, snakes, various birds, salamanders and frogs.
Succession of micro-organisms like fungi, bacteria, etc occurring within a microhabitat is known as microsuccession or serule. This type of succession occurs within communities, for example in dead trees, animal droppings, etc. Microbial communities may also change due to products secreted by the bacteria present. Changes of pH in a habitat could provide ideal conditions for a new species to inhabit the area. In some cases the new species may outcompete the present ones for nutrients leading to the primary species demise. Changes can also occur by microbial succession with variations in water availability and temperature.
The climax concept
According to classical ecological theory, succession stops when the sere has arrived at an equilibrium or steady state with the physical and biotic environment. Barring major disturbances, it will persist indefinitely. This end point of succession is called climax.
The final or stable community in a sere is the climax community or climatic vegetation. It is self-perpetuating and in equilibrium with the physical habitat. There is no net annual accumulation of organic matter in a climax community mostly. The annual production and use of energy is balanced in such a community.