Let's begin our definiton of SUCCESSION based on a more common issue. Succession is generally a series of change - any change that one can think of. Queen Elizabeth II (the reigning queen of United Kingdom), will be succeeded by her son Prince Charles. Prince Charles will be succeeded by his son with Lady Diana, Prince William. This is a POLITICAL SUCCESSION. The ruling/sitting monarchs are CHANGED or REPLACED by succeeding generation - this can be brought about by death or abdication.
In a more local nature, succession can be brought by removal. Political SUCCESSION in the Philippines is very dramatic such as the succession of power from Marcos to Cory Aquino and that from Erap Estrada to Gloria Arroyo. Upon lawful election, political succession peacefully paved way to the succession of Noynoy Aquino.
Bearing in mind that succession is a change which may be brought naturally (such as death) or harsh (such as removal), one can define ECOLOGICAL SUCCESSION as change in ECOLOGICAL COMMUNITY.
Succession generally follows two stages: Primary Succession and Secondary Succession.
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Primary succession are those in the form of lava flows caused by volcanic erruptions, substrate exposed by the retreat of glaciers and freshly formed san dunes (Townsend, Begon and Harper). The key determinant for a primary succession is that the area is never occupied by a community - "sterile"area.
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On the other hand, Secondary Succession are those communities/areas previously occupied by well-developed communities. Examples of these communities which are considered secondary succession are those areas which were damaged by fire.
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Ecologists have different opinions on the nature of ecological succession. Frederic Clements believe that ecological succession is a directional and orderly change. His concept on ecological succession is commonly referred to as CLASSICAL ECOLOGICAL THEORY. He enumerated the process of ecological succession in several phases:
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1. Nudation. Nudation is the development of an area without any life form. Lava flow is one example of nudation. Another example is the exposure of a surface brought about by glacier retreat. Nudation may be caused by topography (lanslide, volcanic actvity), climate (glacier retreat) and biotic factors (anthropogenic activities).
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2. Invasion. Invasion is the successful establishment of species in the bare, nude area. Hence, invasion includes migration (seeds brought by birds or winds), ecesis (seeds grow while adjusting to the prvailing conditions in the area), and aggregation (colonization of the successful offspring and/or new migrants)
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3. Competition. Because species become established and that they begin to grow and reproduce, competition for space, light and nutrients take place. Competition could be intraspecific (between same species) and interspecific (between different species).
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4. Reaction. Reaction basically refers to the reaction of the biotic and abiotic factors with each other. Because living organisms now inhabit the area, soil, water, light conditions and temperature will change. The environment becomes modified such that the existing community not suited to such changes may be replaced by another community (seral community = intermediate stage in ecological succession advancing towards climax community)
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5. Stabilization. The stabilization point refers to the final community or the climax community.
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What is a CLIMAX community?
A climax community is a community which possesses the following characteristics:
a. the vegetation found in the area is TOLERANT of environmental conditions.
b. the area holds a wide variety of species and a complex food chain.
c. the ecosystem is balanced in terms of:
gross primary production = total respiration
used energy from sunlight = released energy by decomposition
uptake of nutrients from soil = return of nutrient to soil by litterfall
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d. indivduals are replaced by others of the SAME kind.
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Other ecologists follow the theory that ecological succession is disorderly and unpredictable. This could be referred to as Gleasonian framework. This theory is also followed by Cowles.
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With these contrasting beliefs, let us summarize the different theories explainign ecological succession.
1. Monoclimax or climatic climax theory. This theory is the one developed by Clements. This theory assumes that climax communities are determined by only one (mono) factor, that is CLIMATE.
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2. Polyclimax theory. This theory was proposed by Tans;ey. It assumes that climax vegetation is influenced not by climate alone but also by soil moisture, soil nutrients, topography, slope exposure, fire and animal activity.
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3. Climax Pattern theory. While polyclimax theory recognizes other abioitc factors, this climax pattern theory recognizes biotic and abiotic factors to be influential to the climax community.
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If you are a little bit confused about these contrasting or differing theories, just think of it as "these ecologists are trying to know/establish what will become of a community. All of them believe in the so-called climax community - a relatively STABLE community". But how do we get there? These ecologists explain factors that would define what communities will become in its stable stage.
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Why does succession happen? Or why do species replace each other?
Three hypotheses are offered in response to this query.
1. Facilitation hypothesis. Later species depends on the conditions created by earlier colonists. The earlier colonists modify the area in such a way that it becomes more favorable to the later species. This modification increases the competetive ability of the later species. The later species can now displace the earlier species. (They may not know they are doing this!)
2. Tolerance hypothesis. Only those species which can tolerate the existing environmental condition can survive. If the later species could tolerate the environemnt better than the earlier species, succession happens.
3. Inhibition hypothesis. This could be opposite of facilitation hypothesis. In inhibition, earlier species may make the are more unsuitable for occupation of later species. (Well, earlier species would always want to resist invasion).
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All these hypotheses/models of succession do not occur solely. We return again to the concepts of conditions and resources and the intricate nature of nature.
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The "story" of succession puts into picture the relationship between biotic and abiotic factors. Abiotic factors define what organisms could thrive in a particular habitat. After some time, these organisms (biotic factors), alter the abiotic factors (temperature, amount of nutrients available, humidity, water). With the altered abiotic factors, other species (later species) may find this new set of abiotic factors favorable... and then because of their existence in the area, may also alter AGAIN the abiotic factors resulting to inhabitation of other species. [Generally: environemnt affects species; species affects the environment]
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:)
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