Ecological Succession

Ecological Succession When stripped of its original vegetation by fire, flood, or glaciation, an area of bare ground does . not remain devoid of plants and animals. Beginning with plants, area is rapidly colonized by a variety of both plant and animal species that subsequently modify one or more environmental factors. This modification of the environment may in turn allow additional species to become established. The tnsitional series of communities which develop in a given area are called sere or seral stages, while the final stable and mature community is called the climax.The development of the community by the action of vegetation on the environment leading to the establishment of new species is termed succession. Succession is the universal process of directional change in vegetation during ecological time. It can be recognized by the progressive change in the species composition of the community. Retrogression in community development does not occur unless succession is disturbed or halted by fire, grazing, scraping or erosion. CAUSES OF SUCCESSION Since succession involves a series of complex processes, so there exist many causes of its occurrence. Ecologists have recognized the following three primary causes of succession : 1. Initial or Initiating causes. These are climatic as well as biotic in nature. The climatic causes include factors such as erosion and deposits, wind, fire, etc., which arc caused by lightening or volcanic: activity. The biotic causes include various activities of organisms. All these causes produce the bare areas or destroy the existing populations in an area. 2. Ecesis or Continuing causes. These are processes as migration, ccesis, aggregation, competition, reaction, etc., which cause successive waves of populations as aresult of changes, chiefly in the edaphic (soil) features of the area. 3. Stabilising causes. These include factors such as climate of the area which result in the stabilisation of the community. TRENDS OF SUCCESSION (Functional Changes) Trends of changes during ecosystem development from a young to mature stages include the following features: 1. A continuous change occurs in the kinds of plants and animals. 2. An increase in the diversity of species takes place. The general appearance of the community or the physiognomy keeps on becoming more and more complex as the succession proceeds. 3. There is a progressive increase in the amount of living biomass and dead organic matter. Such an increase occurs in gross as well as net primary production in the initial and seral stages. Thus, there is more biomass accumulation, gradually reaching a huge biomass structure in the climax.

4. Green pigment (Chlorophyll) go on increasing during the early phase of primary succession. The ratio of yellow/green pigments remains around 2 in the early stages and increases to 3 to 5 in the climax stage. Pigment diversity also increases. 5. The community respiration increases but the P/R (i.e., Production/Respiration) ratio remains more than 1 in the sera stages. The huge living biomass respires a lot in the climax stage and the P/R ratio equals 1 (i.e.. P/R = 1). Thus, in the early stages P>R and in the climax stage. P = R. ' 6. The food chain relationships become more complex as succession proceeds. 7. Nutrients in the young stage are allocated mostly in the soil, but as the seral stages advance, nutrients get allocated more in the vegetation and less in soil. Further the nutrient cycling becomes more closed or intrabiolic with an efficient cycling mechanism whereas in the young stage the nutrients easily leak out from the system, i.e.. the cycling is more of an open type. 8. The role of detritus becomes progressively more and more important. 9. The quality of the habitat gets progressively modified to a more mesic condition from either too dry or too wet condition, in the early seral stage. 10. The niche specialization increases, i.e., different functions arc more effectively performed by specialist species in mature serai stage, whereas in early stage many functions arc performed but less efficiently by a few species. 11. The life cycle of mature community species are longer and more complex. 12. The importance of macroenvironment becomes less in later stages. 13. Relationship among component species becomes more and more mutalistic or helpful even though there exist enough competition and allelochemic activities to prevent invasion of outside elements. 14. Dispersal of seeds and propagules is by wind in young stage, while by animals in mature stage. BASIC TYPES OF SUCCESSION Based on different criteria, there are following kinds of succession: 1. Primary succession. If an area in any of the basic environments (such as terrestrial, fresh-water or marine) is colonized by organisms for the first time, the succession is called primary succession. Thus, primary succession begins on a sterile area (an area not occupied previously by a community), such as newly exposed rock or sand dune where the conditions of existence may not be favourable initially. 2. Secondary succession. If the area under colonization has been cleared by whatsoever agency (such as burning, grazing, clearing, felling of trees, sudden change in climatic factors, etc.) of the previous plants, it is called secondary succession. Usually the rate of secondary' succession is faster than that of primary succession because of better nutrient and other conditions in area previously under plant cover.

3. Autogenic succession. After the succession has begun, in most of the cases, it is the community itself which, as a result of its reactions with the environment, modifies its own environment and, thus, causing its own replacement by new communities. This course of succession is known as autogenic succession. 4. Allogenic succession. In some cases replacement of one community by another is largely due to forces other than the effects of communities on the environment. This is called allogenic succession and it may occur in a highly disturbed or eroded area or in ponds where nutrients and pollutants enter from outside and modify the environment and in turn the communities. 5. Autotrophic succession. It is characterized by early and continued dominance of autotrophic organisms such as green plants. It begins in a predominantly inorganic environments and the energy flow is maintained indefinitely. There is gradual increase in the organic matter content supported by energy flow. 6. Heterotrophic succession. It is characterized by early dominance of heterotrophic organ-isms such as bacteria, actinomycctcs, fungi and animals. it begins in a medium which is rich in organic. organisms which are the first to colonize and aggregate are called pioneers. The pioneer communities are likely to be more dynamic and have lownutrient requirements and to take minerals in comparatively more complex forms. They arc small-sized and make less demand from environment. 3. Competition and Coaction Due to aggregation of a large number of individuals of the species at the limited place, there develops competition (i.e., interspecific and intraspecific competition) for space and nutrition. Individuals of a species affect each other's life in various ways and this is called coaction. The species which fail to compete with other species are ultimately discarded. The reproductive capacity, wide ecological amplitude, etc., help the species to withstand the competition. 4. Reaction Reaction in-cludes mechanism of the modification of the environment through the influence of living organismsonit.Due to this very significant stage, changes take place in soil, water, light conditions, tem-perature, etc., of the en-vironment. As a result of reaction, the envi-ronment is modified and become unsuitable for the existing community which sooner or later is replaced by another community (seral community). The whole sequenceof com-munities that replaces one another in the given area is called a sere, and different commu-nities constituting the sere are called seral communities, seral stages or developmen-tal stages. 5. Stabilization (Cli-max) Finally, there occurs a stage in the pro-cess, when the final terminal community becomes more or less established for a longer priod of time and it. Diagrammatic representation of succession through autogenic processes. Seral communities modify the environmental complex which in turn changes the vegetation and the process continues until the dynamic equilibrium is reached. Note the pioneer, scral and climax communities.

can maintain itself in equilibrium with the climate of the area. The final community is not replaced nnd is known as climax community and the stage as climax stage. SOME EXAMPLES OF SUCCESSION 1. Hydroscre. A good example of succession is the hydrarch succession or hydrosere (Fig. 82), in which a pond and its community are converted into a land community. In the initial stage, phytoplankton (eg., someblue green algae (cyanobacteria), green algae (e.g.. Spirogyra, Oedogonium), diatoms and bacteria) are the pioneer colonizers. They are consumed by zooplankton (e.g., protozoans such as Amoeba, Paramecium, Euglena, etc.), fish such as blue gill fish, sun fish, large mouth, etc. Gradually these organisms die and increase the content of dead organic matter in the pond. This is utilized by bacte-ria and fungi, and minerals are re-leased after the decomposition. The nutrient-rich mud then supports the growth of rooted hydro-phytes such as Hydrilla, Elodea, Valtisneria, Ceraiophyllum, etc., in the shal-low water zone. This submerged stage isalso inhab-ited by the animals such as dragon flies, may flies and crustaceans such as Asellas. Gammarus. Daph-nia, Cypris, Cy-clops. etc. The hydrophytes die and are decomposed by mircroorganisms, thus, releasing nu-trients. In addition to this, due to silt-ing, the water depth of the pond is reduced and at the margin of the pond grow rooted floating vegetation (i.e., the plant spe-cies whose leaves reach the water surface and roots. remain in the mud). Plants such as Nelumbo nuclfera, Trapa, Monochorla, etc., grow in these conditions. Jn floating stage faunal living space is increased and diversified. Hydras, frogs, salamanders, gill-breathing snails, diving beetles (Dysticus) and host of new insects capable of utilizing the under surfaces of floating leaves appear. Some turtles and snakes also invade the pond, Gradually die water depth in the pond decreases due to evaporation and the deposition of organic matter and the concentration of the nutrients increases. Free-floating plants such as Azolla, Lemna, Pistia, Wolffia, Spirodella, etc., increase in number because of the high nutrient availability. Gradually their dead parts fill up the pond ecosystem, resulting in die further build up of the substratum. At this stage, die pond becomes a swampy ecosystem. The reed swamp species (such as Scirpus or bulrushes, Typha or cattail, Phragmiies (reed grass), Rumex. etc.) and sedges (e.g., Carex, Juncus, Cyperus, etc.) invade the pond and die latter are gradually replaced by mesic communities as the water depth is reduced greatly. Gradually land plants, such as, shrubs (Salix, Cornus) and trees (Populus, A lima) invade ending in the climax community such as deciduous forest (Pig. 8.2). In association widi the changes in water depth and vegetation, the aquatic fauna also change and ultimately gets replaced by land animals. Thus, possible trend of succession in the aquatic environment is as follows : Climax community ↑ Open scrub land = Deciduous forest ↑ Terrestrial communities

↑ Mesic communities ↑ Reeds and sedges ↑ Free floating and rooted plants ↑ Rooted and aquatic plants ↑ Phytoplankton Pond ecosystem Climax communities vary from place to place. For example, in low lying lands in some parts of Kashmir, the climax community has trees such as Salix (Ambasht, 1988). In Indian upland plateaus, the climax woody species consist of Diospyros. Butea and Zizyphus and ground vegetation of Eragrostis, Sporobolus, Bothriochloa, etc. Climax vegetation of lowlands and valleys which provide a mesic environment includes Termlnalia, Flcus, Sterculia, Salix, etc. 2. Succession in xeric habitat Xerosere or xerarch succession begins on exposed parent rocks (lithosere) or dry sand (psammosere). A lithosere (Fig. 8.3) involves die following stages: crustose lichens stage (pioneers) -» foliose lichens stage → moss stage → herbs stage → shrub stage → forest stage (climax stage). Thus, pioneer plants are lichens, mosses and Selaginella, which help in soil formation by, accelerating erosion. In course of time grasses, annuals and herbaceous vegetation grow on the soils deposited on rocks. Like the hydrosere, the lithosere irvolves successive changes in animal life. The pioneer animals of the lichen stages are few specie' of mites, ants and spiders. These animals are exposed to harsh environment such as extreme fluctuations in temperature. During the moss stage, many new species of mites, small spiders, lardigradcs and springtails invade the community. The herb stage is characterized by nematodes, mites, collembola, ants and various insect larvae. During the shrub and forest stages great qualitative and quantitative modifi-cations occur in the fauna. Thus, there occur numerous kinds of animals such as slugs, snails, wire worms, millipedes, centipedes, mites, ants, sow bugs, spring-tails, amphibians such as salamanders, frogs, etc., reptiles such as turtles, skinks and other lizards, snakes, birds such as flycatcher and grouse, and mammals such as mole, mouse, shrews, squirrels, chip-munk and fox.community are determined by the total environment of the ecosystem and not by one aspect, such as climate alone. Involved are the characteristics of each species population, their biotic interrelation-ships, availability of flora and fauna to colonize the area, the chance dispersal of seeds and animals, and the soils and climate. The pattern of climax vegetation will change as the environment changes. Thus, the climax community presents a pattern of populations that corresponds with and changes with the pattern of environmental gradients, intergrading to form ecoclines. The central and most widespread

community in the pattern is the prevailing or climatic climax. It is the community that most clearly expresses the climate of the area. 4. Information theory. This theory was proposed by Leith, Odum and Golley. It considered succession and climax in terms of ecosystem development. In autotrophic succession (ecosystem development), diversity of species tends to increase with an increase in organic matter content and biomass supported by the available energy. Thus, in a climax community, the available energy and biomass, which is called information content, increase. In contrast to it, in a heterotrophic succession occurs a gradual depletion of energy, because the rates of respiration always exceed production rates. However, in an ecosystem both the autotrophic and heterotrophic successions operates in a co-ordinate manner. The autotrophic individuals derive mineral elements from the soil and atmosphere, while the heterotrophic individuals carry on the return of the nutrients to soil and atmosphere, through decomposition of complex dead organic matter. Thus, succession reaches a stage, the climax stage, when the amount of energy and nutrients received from the environment by the plants is again returned in more or less similar amount to the environment by decomposition through heterotrophs. Certain Recent Models of Succession Connell and Slatyer (1977) have proposed the following three models to accommodate different possible pathways of succession 1. Facilitation model. It is based on the Clements ideas of relay communities in which the seral community is believed to modify. According to this model each new community in course of time prepares suitable ground to facilitate its own replacement by another better suited community. So each community like a 'relay process' delivers the habitat to next or higher status community. This model has not been supported by any proper evidence. 2. Tolerance model. This model is based on the concept of IFC or initial floritic composition which suggests that arrival of new invaders of higher life form types necessarily dees not eliminate pioneers. According to this model, only such higher succession or climax species are able to join which can be tolerated by the early settlers. So, the tolerance model differs from Clement's relay succession model in the sense that as the succession proceeds, more and higher life form plants are tolerated to join and coexist, than necessarily replacing the earlier component species. With the passage of time, species which mutually tolerate each other gain control over the habitat to form the climax vegetation. 3. Inhibition model. According to this model, the early arrived species (populations) on a new habitat may develop counter-mechanism to normal replacement process. For example, the allelopathy may be the common counter-acting adaptation to thwart or inhibit the entry of late arriving species. This kind of highly adaptated early stage communities may not be common on a wide range ofhabitats. In such a case, relay succession gains control only after the death of the allelopathic plants. Since inhibition model lacks universality, so it was also criticised.

Resource-Ratio Hypothesis of Succession This model was proposed by Tilman (1985) who was agreed to Clement's idea of replacement of communities. According to him, the resources of the habitat are regarded as the key factor. Limited resource leads to competition. Due to competition a resource ratio is created for each kind of resource. Depending on the newly created ratio level, new species adapted to it succeeds. In conclusion, succession isdirectional and progresses towards the climax. In some cases, it may be non-directional, i.e., it returns to a particular stage and it is cyclic. It take hundreds and thousands of years to complete primary succession, while the secondary succession is quicker (takes 10 to 200 years for its completion). In some cases, the initial stage community may inhibit altogether the process of succession by allclochem ic inhibition of new arrivals. COMMUNITY EVOLUTION Like the responses of communities to changing abiotic conditions, community evolution involves progressive changes in climax communities. Because the evolution is exceedingly slow, it cannot be observed in operation, and few instances from the fossil record are sufficiently complete to show the process in action. The example mat best demonstrates evolution of the basic structure of the community is that of the development of a terrestrial community of a modern type by early reptiles some 250 million years ago. Between the time when vertebrates (amphibians) first became able to lead a predominant terrestrial xistence some 350 million years ago and the establishment of an essentially modern type food web some 100 million years later, the structure of terrestrial community was decidedly different from what it is now. Development of the modern type of community structure required not only a complete rearrangement of the niche structure of the community but also the evolution of new species • that could fill the new niches (Olson, 1961, 1966). Attainment of the adaptations needed for terrestrial life by the first amphibians did not in itself establish a land-based vertebrate community. These early amphibians were carnivores, and the only animals inhabiting the land environment were insects. It is unbelievable that the clumsy locomotor system of early amphibians would have allowed them to prey effectively on animals such as insects. Thus, the first communities inhabited by terrestrial vertebrates are best regarded as extensions of aquatic communities, with the land habit as an adaptation to improve the capabilities of organisms whose prime food supply was aquatic invertebrates and fish. By some 300 million years ago, reptiles had evolved that could feed effectively on terrestrial invertebrates. An entirely land-based community was theoretically possible in which all herbivore niches were assumed by invertebrates and some of the carnivore niches uy vertebrates. However, the palaeoccological evidences suggest that most contemporary carnivorous vertebrates were unable as yet to realize an entirely terrestrial carnivore niche, so that 'he great majority of the energy flow through the community continued to pass through the aquatic route. The typical food chain to the highest terrestrial vertebrate carnivore was plant → aquatic invertebrate → aquatic invertebratefeeding vertebrate → semi-aquatic predator → terrestrial predator.

By 250 million years ago terrestrial herbivorous vertebrates had evolved and a fully terrestrial vertebrates community could come into being. From this time onward the basic structure of the terrestrial community was of an essentially modern sort, with all consumer trophic levels occupied by a wide range of animals, both vertebrates and invertebrates. Such evolutionary changes in the structure of communities are caused by a large number of factors. One factor is changes in the regional climate. It became progressively drier during the period under consideration, and the development of a land-based community reasonably responded in this sort of change. Indeed many evolutionary changes in community structure can be explained on the basis of responses of major changes in the regional abiotic factors of the environment (Axelrod, 1950, 1958). But other chief factors of evolutionary change in community include reorganization of the community's structure in response to the realization of niches that had not previously existed in the community.