8 Ecological Succession – Part 2

Renuka Gupta

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Module 20: Ecological Succession – Part 2

Contents

  1. Concept of the Climax
  2. Theories of climax
  3. Examples of Succession
  4. Hydrosere
  5. Xerosere

 

  1. 1 Concept of the Climax

 

The climax community is final, terminal and self-perpetuating community in succession, as it is in more or less equilibrium with itself and its environment. It represents a steady state of species composition, community structure and energy flow. At this stage, there is equilibrium between gross primary production and total respiration, between amount of energy and nutrients received by the plants from the environment and return back to the environment in more or less same amount by decomposition process. The individuals of the climax stage are replaced by others of the same kind. Thus the species composition maintains equilibrium.

 

The climax community has the following characteristics:

  1. It is able to tolerate its own reactions.
  2. It has a wide diversity of species, a well-defined spatial structure, and complex food chains and webs.
  3. The species of climax community are more or less stable with the climate. This climax community cannot be replaced through competition by any other group of species. In other words, only few dominant species can form climax community in a particular climatic region.
  4. The developmental stages of climax community have their own characteristics, which reflect the  type  of  climate.  Similar  to  the  development  of  organism  with  changing  age,  climax communities also undergo changes with changing climate. Clements considered the climax as a super-organism (as given in previous module).There are following three theoretical approaches to the climax:20.1.1. Monoclimax theory or Climatic Climax Theory

     

    According to Monoclimax theory, given by Clements (1916), there is only one type of climax community in a climatic region. Further, each climax is direct expression of its climate – the climate is the cause, climax is the effect, which in turn reacts upon the climate. In other words, it is the climate that controls the occurrence of the life forms of the dominant species and which in turn become the characteristic appearance of that climax community. That’s why this theory is also known as climatic climax theory. As per the theory, all successions starting from diverse habitats, be it aquatic ecosystem or terrestrial ecosystem (ponds, bogs, river valley silts, etc.) in an area, will eventually converge towards a single climax community which is decided by the climate of the area and is termed as climatic climax.

     

    All other communities existing besides the climatic complex are called as subordinate communities. Clements, although agreed with the possible control of factors other than climate on climax, but he thought that these communities would sooner or later develop into climatic climax types. Therefore, he proposed four terms for these communities: subclimax, disclimax, preclimax and postclimax.

     

    1. a) A subclimax is a stage in succession just preceding the climatic climax community. Here succession gets arrested at this stage for a long time in response to physiographic or edaphic factors.

     

    1. b) The disclimax (disturbance climax) or anthropogenic subclimax (human generated) is the community in an area replacing true climax community as a result of recurrent biotic disturbances. For example, overgrazing by livestock may produce a desert community of bush, mesquite and cactus where the local climate would actually allow grassland to maintain itself. This means that at the same place, under similar climatic conditions, at smaller distances,  grassland community may be the climax community. The desert community would be the disclimax, and the grassland would be the climatic climax.
      1. c) The preclimax is a community of lower life forms than the true climax and results from different edaphic conditions in the same area. For example, slightly drier localities in a given climatic climax area may have a self-perpetuating community different from the climatic climax, e.g., occurrence of pine forests in Himalayas.

       

      1. d) The postclimax is the community of higher life forms occurring within a climatic climax due to slightly better moisture or colder conditions in that zone, for example, a forest along a stream in a grassland community constitutes postclimax community.

       

      The monoclimax theory faced a lot of criticisms. Cowles, disagreeing the concept of stability, stated that equilibrium state is never reached and succession is in fact a variable approaching a variable rather than a constant approach. However, Cooper considers climax state as the state of minimum changes rather than finally changed state of succession. In the same climate, climax community may differ as it depends upon primary stage and also habitat characteristics. It is also possible that in same climate conditions, a lithosere and hydrosere have different pioneer community and proceeding through different successive developmental stage finally reach to similar climax communities. It may also possible that similar pioneer communities and seral communities would lead to formation of different climax communities. Under uniform climate, different climax communities can be observed due to variations in soil, topography and other factors. So, it will be inappropriate to consider climate as a determining factor for climax formation. Tansley (1935), also disagreeing with the monoclimax theory, stated that climax communities are controlled by many factors, rather than a single factor climate.

       

      20.1.2 Polyclimax theory

       

      This theory was proposed by Tansley (1935). According to this theory, the structure and stability of climax communities in a region forming a mosaic of vegetation climaxes are controlled by a number of factors besides the climate such as soil moisture, soil nutrients, topography, slope, temperature, fire, etc. and various biotic factors. For example, a south facing slope of a mountain in Northern  Hemisphere may have one type of climax community because its micro-climate is warmer, but its north facing slope may have other type of climax due to colder micro-climate. Different kinds of climaxes may develop even on the same parent material in the same locality. In an example given by Jenny et al (1969), in certain areas of the northern California, giant redwood forests occur side by side with pigmy forests of tiny, stunted trees (Fig. 20.1). The same parent material (beach deposit and sandstone) underlies both forests, but stunted pigmy forest is due to an iron-cemented hardpan located about 50 cm below the surface. The soil above the hardpan is excessively acidic (pH 2.8-3.9) and low in Ca, Mg, K and P. The impervious hardpan restricts the deeper root penetration. On the other hand, soil below redwood forest is sandy. Thus, due to differences in edaphic factors, we find two climax communities differing not only in species composition but also in stature of vegetation in the same locality. The influence of physical factor is so strong that the two different climax communities on the above sites cannot converge to single climax community, even in indefinite period.

Source: Odum and Barrett (2005). Fundamentals of Ecology. Cengage Learning, New Delhi.

 

To describe the existence of climax communities under different habitats, Tansley recognized following five types of climaxes, some of these are primary climaxes whereas others are secondary climaxes:

  1. Climatic climax: Climax under the normal conditions of climate, soil and topography and no disturbance.
  2. Topographic Climax: The climax community stabilized by topographical factors of an area like hills, mountains, slopes etc. is known as topographical climax. Difference in topography may give rise to different local micro-climates which in turn support different climax communities in the same area.
  3. Edaphic Climax: When stable and self-perpetuating communities develop in parts of the same area due to peculiarities in soil i.e. variation in edaphic factor and are different from climatic climax, then they are known as edaphic climax.
  4. Fire Climax: When climax community occur in response to recurrent burning of vegetation which eliminate the fire susceptible species, then such climax community is known as Fire climax.
  5. Biotic Climax: The climax of biotic community stabilized by living factors including man, herbivores, and other animals are known as anthropogenic climax, grazing climax and zootic climax respectively.

  Climatic, topographic and edaphic climaxes are considered as primary climaxes, whereas fire and biotic climaxes are the secondary climaxes.

 

21.1.3 Climax Pattern Theory

 

Whittaker (1951) rejected classification approach of describing climax and proposed Climax Pattern theory. He believed that since species composition and the balance of the climax community is determined by the total environment (including both biotic and abiotic factors) of the ecosystem, any change in the environment will shift the balance among populations. As a result, the climax community represents a pattern of populations that corresponds to the changes of environmental gradients forming ecocline (e.g. thermocline). According to this theory, the communities that occupy the largest area in this ecocline are known as the ‘Prevailing climax’ or climatic climax. This theory recognizes a spatial pattern of climax vegetation which reflects the spatial variation in the  environmental conditions at that point. There is, thus, no discrete number of climax communities and not a single factor determines the structure and stability of a climax community.

 

Whereas the monoclimax theory allows for only one climatic climax in a region and the polyclimax theory allows several climaxes, the climax-pattern hypothesis allows a continuity of climax types varying gradually along environmental gradients and not clearly separable into discrete climax types. So in a nutshell, we can say that the end point of ecological succession, climax community, is not completely stable. The climate of an area has a significant control on the occurrence of species, but within each broad climatic zones there are other several factors like soil, nutrients, topography, biotic factors, etc. which lead to many climax situations. Climax communities do not necessarily represent a halt to successional change.

 

20.2 Examples of Succession

 

20.2.1 Hydrosere

 

Hydrosere is the ecological succession which occurs in a water body like pond, pool or lake. In a newly formed pond (e.g. due to a landslide), the process starts with the colonization of phytoplanktons, the pioneer community and through a number of intermediate seral stages, finally reaches a forest as climax community. The whole process of hydrosere from an open water body to a forest may take place at least two hundred years (probably much longer). Changes occur both in plant community as well as in animal community during the course of succession. The different stages of hydrosere (as shown in Fig 20.2) are as follows:

  1. Plankton Stage

The pioneer colonizers of the hydrosere include phytoplanktons like bluegreen algae (Cyanobacteria), green algae (e.g., Spirogyra, Oedogonium) and diatoms. This community begins when spores of phytoplanktons reach the site through air or animals. They are then consumed by zooplanktons like protozoans (e.g., Amoeba, Paramecium, Euglena, etc.) and fishes such as blue gill fish, sun fish etc.

When these organisms die, after their death, decompose and result in the release of minerals which in turn will mix up with the silt, brought from the surrounding land by rain water and by wave action of pond water. These autogenic influences result in development of soft mud at the bottom of pond and enrichment of aquatic habitats which now can support different types of organisms.

 

  1. Rooted submerged stage

 

The soft nutrient-rich mud at the bottom of pond now becomes suitable habitat for the growth of rooted hydrophytes like Myriophyllum, Elodea, Hydrilla, Potomogetan, Vallisneria, Utricularia etc. These submerged plants have roots at bottom and remaining portion in water (Fig. 20.3). The hydrophytes die and are decomposed by microorganisms, thus, releasing nutrients and further build up the substratum. The depth of water level decreases and pond become less deep or more shallow, turbid and nutrient rich. This new habitat now replaces these plants giving way to another type of plants of floating types.

 

  1. Floating stage

 

Now rooted hydrophytes like Nelumbo, Trapa, Nymphaea, Limnanthemum, Monochoria, etc. with their large leaves floating on the water surface colonize the habitat with their rhizomes. The habitat changes chemically as well as physically. The dead remains of plants are deposited at the bottom. The water depth in the pond decreases due to evaporation of water and deposition of organic matter, as a result, the concentration of the nutrients increases. Some free-floating species such as Azolla, Lemna, Wolffia, Pistia, Spirodella, Salvinia, etc. also become associated with rooted plants (Fig 20.4). These species increase in their number due to high availability of nutrients and their dead parts fill up the pond ecosystem gradually, resulting in the further build up of the substratum.

  1. Reed-swamp stage

 

Pond margins, because of good environmental conditions of high moisture, enough light and aeration soon gets covered by emergent hydrophytes such as Scirpus, Typha, Sagittaria and Phragmite. Although their root system is completely under water and anchored in soil but their shoots are partly or completely exposed to air, so are like amphibian plants (Fig 20.5). They have well developed rhizome and form very dense vegetation. These plants start from margins and then cover the pond. They facilitate considerable decrease in water level resulting in change in substratum which in turn change the aquatic habitat into marshy land or swamp. This stage is often known as ‘Red swamp stage’.

  1. Sedge-meadow stage

 

Further decrease in water level changes the nature of substratum. Species of some Cyperaceae and Gramineae such as Carex, Juncus, Cyperus and Eleocharis colonise the area (Fig. 20.6). They form a mat like vegetation towards the centre of the pond and give the pond a marsh or swamp like appearance. As a result of high rates of transpiration, there is much loss of water and nutrient availability increases due to addition of more and more organic matter. Thus, mesic conditions start approaching the area and gradually marshy vegetation disappear.

  1. Woodland stage

 

As the area is becoming dry due to exposure to sun, terrestrial plants like shrubs (Salix, Cornus) and trees (Alnus, Populus) form open vegetation or woodland (Fig 20.7). These plants can tolerate bright sunlight as well as water logged conditions. They cast the shade and make the area drier due to rapid transpiration. By this time, the area is rich in humus with rich flora of microorganisms, thus the conditions favour the arrival of new tree species.

  1. Forest stage

 

Many trees whose seedlings are shade loving invade the area. They cover the whole area when they grow and develop (Fig 20.8). This is the climax community. Depending upon the climate, the climax will be rain forest, temperate forest or tropical forest. In tropical climate, with heavy rainfall, there develop tropical rain forests. In temperate regions, mixed forest of Ulmus, Acer and Quercus are formed. Thus, an area once under the deep water gets finally converted into a forest.

20.2.2 Lithosere- A Xerosere on rock

 

Lithosphere is a type of Xerosere succession which originates on rock surface deficient in water and organic matter. The rock surface is in unweathered state. The pioneers to colonize this primitive type of substratum are lichens and finally xerosere ends with a climax community (Fig 20.9). The various stages are described below:

  1. Crustose-lichen stage

 

Initially, the rock surface is dry and hard as well as soil is absent. Under such conditions, blue-green algae and lichens can grow and multiply. They form crusts on the dry rock and remain in a dormant condition for very long time. In cooler climates, crustose lichens like Rhizocarpon, Rinodina and Lecanora are the common pioneers (Fig 20.10). They produce some carbonic acids which bring about weathering of rocks in the form of cracks or roughen the rock surface. They may absorb water from rain and dew drops which wet their external surface. The dead organic matter of algae and lichens become mixed with the small particles of rocks to form a thin layer of moist soil on the rocks. The soil formed by weathering of the organic matter make the substratum suitable for the growth of foliose lichens.

  1. Foliose-lichen stage

It appears on the substratum partially built up by the crustose lichens (Fig 20.11). It includes species of Parmelia, Umbilicaria and Dermatocarpon. Their large leaf-like thalli overshadow the first stage and gradually the crustose lichen die and decay. The foliose lichens can absorb and retain more water. They not only add organic matter but accumulate dust particles and help in the creation of humus. The weathering of rocks mixed with humus form a thin soil layer on rock surface and thus there is a change in the habitat.

 

  1. Moss-stage

 

Now the built up substratum favours the growth of certain xerophytic mosses like Grimmia (black moss), Tortula (twisted moss), Polytrichum (Hair moss), Bryum, Barbula and Funaria (Fig 20.12). At their successful growth they compete with the lichens and overshow the foliage lichens. The moss also acts like a sponge when wet, in some cases providing a semi-aquatic microhabitat for a variety of

 

  1. Herb-stage

 

The extensive growth of mosses adds more soil and moisture to the surface. Minerals are added to it due to leaching. The soil is now fit for germination of seeds of xerophytic herbs like Aristida, Festuca, Justicia, Poa, Solidago, Tridax etc. First short-lived annuals, then biennial and finally perennial grasses make their appearance. These will shade out and displace the moss, which enabled them to survive in the first place. After their death, there is accumulation of humus in soil, evaporation and temperature decreases; this favors the growth of shrubs.

 

  1. Shrub-stage

Now the habitat with enriched soil become suitable for the growth of shrubs like species of Rhus, Phytocarpus, Zizyphus and Capparis (Fig 20.13). The shrubs overshadow the herbs and replace them. These shrubs check the evaporation from soil, thus humidity increases and make the habitat suitable for the growth of tree seedlings which make up the climax community. At this stage, a number of herbivorous invertebrates that feed on particular plant species also start inhabiting the area. Thenafter, the carnivorous invertebrates, small mammals and carnivores also appear. The interactions among all these plants and animals make the area more habitable and eventually a climax community is reached.

  1. Forest – stage

 

Due to increasing humus content in soil and release of minerals from weathering of rocks, thin layer of soil is formed, which supports small trees like Acacia, Prosopis, Boswellia etc. (Fig 20.14). The first species of the trees are relatively xeric with stunted growth and are widely spaced. Gradually the xeric species may be replaced by mesophytic and may give rise to new herbaceous vegetation in the area.

Summary

 

  1. The climax community is final, terminal and self-perpetuating community in succession, as it is in more or less equilibrium with itself and its environment. It represents a steady state of species composition, community structure and energy flow.
  2. The species of climax community are more or less stable with the climate. This climax community cannot be replaced through competition by any other group of species. In other words, only few dominant species can form climax community in a particular climatic region.
  3. According to Monoclimax theory, each climax is direct expression of its climate – the climate is the cause, climax is the effect, which in turn reacts upon the climate. It is the climate that controls the occurrence of the life forms of the dominant species and which in turn become the characteristic appearance of that climax community. All other communities existing besides the climatic complex are called as subordinate communities. Clements proposed four terms for these communities: subclimax, disclimax, preclimax and postclimax.
  4. The Polyclimax theory was proposed by Tansley. According to this theory, the climax communities in a region form a mosaic of vegetation climaxes and are controlled by a number of factors besides the climate such as soil moisture, soil nutrients, topography, slope, temperature, fire, etc. and various biotic factors.
  5. Whittaker rejected classification approach of describing climax and proposed Climax Pattern theory. He believed that since species composition and the balance of the climax community is determined by the total environment of the ecosystem, any change in the environment will shift the balance among populations. As a result, the climax community represents a pattern of populations that corresponds to the changes of environmental gradients forming ecocline.
  6. Hydrosere is the ecological succession which occurs in a water body like pond, pool or lake. In a newly formed pond (e.g. due to a landslide), the process starts with the colonization of phytoplanktons, the pioneer community and through a number of intermediate seral stages, finally reaches a forest as climax community. The whole process of hydrosere from an open  water body to a forest may take place at least two hundred years (probably much longer). Changes occur both in plant community as well as in animal community during the course of succession.
  1. Lithosphere is a type of Xerosere succession which originates on rock surface deficient in water and organic matter. The rock surface is in unweathered state. The pioneers to colonize this primitive type of substratum are lichens and finally xerosere ends with a climax community.

 

you can view video on 20. Ecological Succession – Part 2

References

 

  • Clements, F.E. (1916). Plant succession: an analysis of the development of vegetation. Washington,Jenny, H., Arkley, R.J. and Schultz, A.M. (1969). The pigmy forest-podsol ecosystem and its dune associates of the Mendocino coast. Madrono. 20: 60-75.
  • Odum and Barrett (2005). Fundamentals of Ecology. Cengage Learning, New Delhi.
  • Tansley, A.G., Sir (1935). The use and abuse of vegetational concepts and terms. Ecology. 16: 284-307.
  • Whittaker, R.H. (1951). A criticism of the plant association and climatic climax xoncepts. Northwest Science. 25: 17-31.