Law of evolutionary succession in the rock record,positive affirmations for healthy mind,how to learn how to write programs 90s - Videos Download

Author: admin, 10.10.2015. Category: Positive Phrases About Life

Succession after disturbance: a boreal forest one year (left) and two years (right) after a wildfire.
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 animals and 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 a disturbance or the initial colonization of a new habitat. Precursors of the idea of ecological succession go back to the beginning of the 19th century. Henry Chandler Cowles, at the University of Chicago, developed a more formal concept of succession.
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. The developmental study of vegetation necessarily rests upon the assumption that the unit or climax formation is an organic entity. An association is not an organism, scarcely even a vegetational unit, but merely a coincidence.
Gleason's ideas were, in fact, more consistent with Cowles' original thinking about succession. 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. A more rigorous, data-driven testing of successional models and community theory generally began with the work of Robert Whittaker and John Curtis in the 1950s and 1960s. The trajectory of successional change can be influenced by site conditions, by the character of the events initiating succession (perturbations), 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.
In general, communities in early succession will be dominated by fast-growing, well-dispersed species (opportunist, fugitive, or r-selected life-histories). Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. Ecological succession was formerly seen as having a stable end-stage called the climax, sometimes referred to as the 'potential vegetation' of a site, and shaped primarily by the local climate. 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. 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. 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. Allogenic succession is caused by external environmental influences and not by the vegetation. In 1916, Frederic Clements published a descriptive theory of succession and advanced it as a general ecological concept.[10] His theory of succession had a powerful influence on ecological thought. Nudation: Succession begins with the development of a bare site, called Nudation (disturbance). Competition: As vegetation becomes well established, grow, and spread, various species begin to compete for space, light and nutrients.
Reaction: During this phase autogenic changes such as the buildup of humus affect the habitat, and one plant community replaces another. A seral community is an intermediate stage found in an ecosystem advancing towards its climax community. Succession of micro-organisms including fungi and bacteria occurring within a microhabitat is known as microsuccession or serule. According to classical ecological theory, succession stops when the sere has arrived at an equilibrium or steady state with the physical and biotic environment. It has a wide diversity of species, a well-drained spatial structure, and complex food chains.
If there is only a single climax and the development of climax community is controlled by the climate of the region, it is termed as climatic climax. When there are more than one climax communities in the region, modified by local conditions of the substrate such as soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity, it is called edaphic climax.
When a stable community, which is not the climatic or edaphic climax for the given site, is maintained by man or his domestic animals, it is designated as Disclimax (disturbance climax) or anthropogenic subclimax (man-generated). Monoclimax or Climatic Climax Theory was advanced by Clements (1916) and recognizes only one climax whose characteristics are determined solely by climate (climatic climax).
More recently another possible idea has been put forward called the theory of alternative stable states which suggests that there is not one end point but many which transition between each other over ecological time. The forests, being an ecological system, are subject to the species succession process.[19] There are "opportunistic" or "pioneer" species that produce great quantities of seed that are disseminated by the wind, and therefore can colonize big empty extensions. An example of pioneer species, in forests of northeastern North America are Betula papyrifera (White birch) and Prunus serotina (Black cherry), that are particularly well-adapted to exploit large gaps in forest canopies, but are intolerant of shade and are eventually replaced by other shade-tolerant species in the absence of disturbances that create such gaps.


Robbert Murphy sees a significantly ideological, rather than scientific, basis for the disfavour shown towards succession by the current ecological orthodoxy and seeks to reinstate succession by holistic and teleological argument. The E?engineE? of succession, the cause of ecosystem change, is the impact of established species upon their own environments.
Succession may be initiated either by formation of new, unoccupied habitat, such as from a lava flow or a severe landslide, or by some form of disturbance of a community, such as from a fire, severe windthrow, or logging. 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-cutting. 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).
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.
Some of these factors contribute to predictability of succession dynamics; others add more probabilistic elements. As succession proceeds, these species will tend to be replaced by more competitive (k-selected) species. 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. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of ecosystems dynamics. The short-lived and shade intolerant evergreen trees die as the larger deciduous trees overtop them. The stages of primary succession include pioneer plants (lichens and mosses), grassy stage, smaller shrubs, and trees. Dynamics in secondary succession are strongly influenced by pre-disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. 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. 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. 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.
In many cases more than one seral stage evolves until climax conditions are attained.[16] A prisere is a collection of seres making up the development of an area from non-vegetated surfaces to a climax community. This type of succession occurs in recently disturbed communities or newly available habitat, for example in recently dead trees, animal droppings, exposed glacial till, etc.
There is equilibrium between gross primary production and total respiration, between energy used from sunlight and energy released by decomposition, between uptake of nutrients from the soil and the return of nutrient by litter fall to the soil. Succession ends in an edaphic climax where topography, soil, water, fire, or other disturbances are such that a climatic climax cannot develop. For example, overgrazing by stock may produce a desert community of bushes and cacti where the local climate actually would allow grassland to maintain itself. If the community has life forms lower than those in the expected climatic climax, it is called preclimax; a community that has life forms higher than those in the expected climatic climax is postclimax. The processes of succession and modification of environment overcome the effects of differences in topography, parent material of the soil, and other factors. It proposes that the climax vegetation of a region consists of more than one vegetation climaxes controlled by soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity. The climax pattern theory recognizes a variety of climaxes governed by responses of species populations to biotic and abiotic conditions.
It is therefore normal that between the two extremes of light and shade there is a gradient, and there are species that may act as pioneer or tolerant, depending on the circumstances. 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. In 1859 Henry David Thoreau wrote an address called "The Succession of Forest Trees"[6] in which he described succession in an oak-pine forest. 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). Clements and his followers developed a complex taxonomy of communities and successional pathways. 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. Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development.
Net Primary Productivity, biomass, and trophic properties all show variable patterns over succession, depending on the particular system and site. Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable.
Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid. As an example, secondary succession has been occurring in Shenandoah National Park following the 1995 flood of the Mormon River, which destroyed plant and animal life.


Animals also play an important role in allogenic changes as they are pollinators, seed dispersers and herbivores. Climatic climax is theoretical and develops where physical conditions of the substrate are not so extreme as to modify the effects of the prevailing regional climate. Preclimax strips develop in less moist and hotter areas, whereas Postclimax strands develop in more moist and cooler areas than that of surrounding climate. According to this theory the total environment of the ecosystem determines the composition, species structure, and balance of a climax community.
Once they have produced a closed canopy, the lack of direct sun radiation at soil makes it difficult for their own seedlings to develop.
It is of paramount importance to know the tolerance of species in order to practice an effective silviculture. 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"). Gleason argued that species distributions responded individualistically to environmental factors, and communities were best regarded as artifacts of the juxtaposition of species distributions. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. For example, when larger species like trees mature, they produce shade on to the developing forest floor that tends to exclude light-requiring species. 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.
The tundra vegetation and bare glacial till deposits underwent succession to mixed deciduous forest. Changes of pH in a habitat could provide ideal conditions for a new species to inhabit the area. A rapid development of herbaceous vegetation follows until the shrub dominance is re-established.
Communities other than the climax are related to it, and are recognized as subclimax, postclimax and disclimax. The environment includes the species responses to moisture, temperature, and nutrients, their biotic relationships, availability of flora and fauna to colonize the area, chance dispersal of seeds and animals, soils, climate, and disturbance such as fire and wind. It is then the opportunity for shade-tolerant species to become established under the protection of the pioneers. Additions to available species pools through range expansions and introductions can also continually reshape communities.
The greenhouse effect resulting in increase in temperature is likely to bring profound Allogenic changes in the next century. The animals found during this stage include nematodes, insects larvae, ants, spiders, mites, etc. In some cases the new species may outcompete the present ones for nutrients leading to the primary species demise. 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.
Geological and climatic catastrophes such as volcanic eruptions, earthquakes, avalanches, meteors, floods, fires, and high wind also bring allogenic changes. The animal population increases and diversifies with the development of forest climax community.
Changes can also occur by microbial succession with variations in water availability and temperature.
The climax community represents a pattern of populations that corresponds to and changes with the pattern of environment.
These species are capable of growing beneath the canopy, and therefore, in the absence of catastrophes, will stay.
Debates continue as to the general predictability of successional dynamics and the relative importance of equilibrial vs.
The fauna consists of invertebrates like slugs, snails, worms, millipedes, centipedes, ants, bugs; and vertebrates such as squirrels, foxes, mice, moles, snakes, various birds, salamanders and frogs. Theories of macroecology have only recently been applied to microbiology and so much remains to be understood about this growing field. A recent study of microbial succession evaluated the balances between stochastic and deterministic processes in the bacterial colonization of a salt marsh chronosequence. When a catastrophe occurs, the opportunity for the pioneers opens up again, provided they are present or within a reasonable range.



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