Development Team. Department of Zoology, University of Delhi. Department of Zoology, University of Delhi

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1 Paper No. : 12 Module : 33 Development Team Principal Investigator: Co-Principal Investigator: Paper Coordinator: Content Writer: Content Reviewer: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Dr. Kapinder and Dr. Haren Ram Chiary Kirori Mal College, University of Delhi Prof. K.S. Rao Department of Botany, University of Delhi 1

2 Description of Module Subject Name Paper Name Module Name/Title Module Id Keywords Zool 12 Ecosystem M33 Ecological succession, mechanism of succession, theories of succession, climax characteristics Contents: 1. Learning Outcomes 2. Introduction 3. Causes of succession 4. Trends in succession 5. Types of Succession 5.1 Autotrophic succession 5.2 Heterotrophic succession 5.3 Primary succession 5.4 Secondary succession 5.5 Autogenic succession 5.6 Allogenic succession 5.7 Progressive succession 5.8 Retrogressive succession 6. Causes of Succession 7. Mechanism of Succession 8. Climax characteristics 9. Theories of Climax 9.1 Monoclimax theory 9.2 Polyclimax theory 9.3 Climax Pattern hypothesis 9.4 Information Theory 10. Summary 2

3 1. Learning Outcomes After studying this module, you shall be able to: Understand the term Ecosystem. Learn the concept Ecological succession. Know different types of succession. Understand the causes and mechanism of succession. Learn the concept and theories of climax. 2. Introduction Succession is the ointment of time to the wounds of nature. F. Clement Organisms and the environment are interrelated and interact with each other so that flow of energy clearly defines the trophic structure, biotic diversity and material cycle within the system. The Term Ecosystem was coined by A.G. Tensley in 1935 who defined ecosystem as integration of living organisms in given area with the physical factors of the environment. It results in the flow of energy which causes development of distinctive trophic structure in the given system. An ecosystem is not a static system but it is dynamic in nature. Its structure and function changes with time in a sequential and orderly manner that can be predicted. This gradual and orderly change in the composition of species and processes of communities over time is called as ecological succession. The requirement of time for the process of succession differs with the nature of the climate, the structure and quality of soil. The term succession is usually applied for the directional and continuous change in composition and abundance of species following disturbance at a site or creation of a bare area. The process of succession generally results in a sequence of a number of communities which finally develops into a relatively stable and steady-state community. The directional changes of the communities results in biomass increase and change in the character and life form of species. 3

4 The whole sequence of communities in which one replaces the other in a given area is called sere. The first species established in the area form the pioneer stage exhibiting higher growth rate, short life span, smaller size and over reproduction. The final mature and stabilized stage of community is called climax which persists in the area until it is disturbed by some major natural or anthropogenic disturbance. Odum in 1963 defines ecological succession by explaining three parameters: 1. It is an orderly process in the community development which is directional and its result is predictable. 2. The process of ecological succession takes place due to change in environmental conditions caused by community itself. 3. The succession finally enters into steady state where maximum biomass and interaction of different organisms are continued per unit of energy flow in the system. 3. Causes of succession There are three main primary causes of succession: 1. Initial or Initiating causes. The initial cause of succession can be abiotic or biotic in nature. The abiotic factors include soil erosion and deposits, wind, temperature and fire. The biotic causes include activities of various living organisms. All these biotic or abiotic causes either produce bare areas or demolish the existing organisms in an area. 2. Ecesis or Continuing causes. Ecesis or continuing causes includes migration, aggregation, competition, reaction, etc. It results in successive waves of different populations due to various changes, primarily due to edaphic (soil) features of the area. 3. Stabilizing causes. These causes include several factors like climate of the area which result in the stabilization of the community. 4. Trends of succession Various trends which occur during the process of succession from initial (pioneer) to final (climax) stages include the following features: 4

5 1. There is gradual change in the form of plants and animals in the area where succession is going on. It leads to increase in the species diversity. As the succession proceeds, the community structure and function become more complex. 2. As the succession continues, there is gradual increase in the living biomass and accumulation of dead organic matter. 3. The amount of chlorophyll continuously increases during the early stages of primary succession. Initially green pigment (chlorophyll) dominates but later diversity of other pigment also increases (carotenoid, xanthophylls etc). 4. In the early seral stage, production/respiration (P/R) ratio found more than 1 (production is more than respiration) but later in the climax stage, P/R ratio reaches to 1 (P/R = 1, production is equal to respiration). 5. In the beginning of succession, nutrients are allocated mostly in the soil, but in the later stage, maximum nutrients get allocated in the biomass and small amount present in the soil. As a result, biogeochemical cycle becomes more closed and effective. Food chain and food web also became more complex in the later stages of succession. 6. As the interrelation between different organisms became more complex, the role of detritivores becomes more important to recycle the material in the system. 7. The habitat for the community gets progressively modified into mesic condition from either dry or aquatic condition in the beginning of the succession. 8. The life cycle in the early seral stage was small and simple but in the later seral stages, it became more complex and longer. 9. The importance of macroenvironment becomes low as the succession proceeds toward climax stage. 10. Relationship among different species becomes mutalistic, however, other interactions such as competition, predation and amensalism occur to either regulate the population or prevent the invasion of outside elements. 11. In the early stage, dispersal of seeds and propagules occurs by wind, whereas, in the later stage it is mostly by animals. 5. Types of succession 5

6 Succession process is self propelled because each seral stage change the environment of the area in which it cannot renew itself. According to Margalef (1963) with progressive changes the succession attains maturity. However, Odum (1971) explained it as ecosystem strategy according to which there is a tendency of homeostasis in any ecosystem during unfavorable environment. On the basis of energy and nutrition relation, succession can be categorized into two types: 5.1: Autotrophic succession: It is the type of succession in which autotrophs (producers) dominate heterotrophs (consumers). This type of succession occurs in the area where the medium is rich in inorganic substances, thus the area is dominated by plants. As the producers are more in number, the rate of production becomes more the than rate of respiration. In the beginning, producers are present in majority but later the biomass of organism increase, as a result, the ratio of production and respiration becomes one. The diversity of organisms increases as the content of organic matter increases in the climax community. 5.2: Heterotrophic succession: In the early stages of heterotrophic succession, various heterotrophic organisms such as bacteria, fungi and actinomycetes are present in larger number. This type of succession starts in organic rich medium such as the river or streams which are highly polluted with organic matter (sewage), or in the small pools having large quantities of leaf litter. As the heterotrophic succession is initially dominated by heterotrophs, so the rate of respiration becomes greater than production. It can be well studied in the heterotrophic communities of the organisms which are involved in the decomposition processes such as dead parts of the plant, animal remains and wastes etc. In these communities, plants and animals succeed each other in colonization and replacement process. In this type of succession, fungi and some invertebrates dominate the area and they feed on dead and decaying organic matter. High amount of energy and nutrients are available in the beginning of the succession and gradually decline as succession proceeds. In addition to the autotrophic and heterotrophic succession, following types of succession is also present: 5.3: Primary succession: 6

7 It is the establishment of plants on the land that has not been previously occupied by other life forms. It starts in bare area where no other community existed previously and the environmental conditions are elementary such as new volcanic flow, bare rocks, islands, deltas etc. (figure 1). Fig.1: Primary Succession. In a terrestrial site, the first pioneer species which establish in the area are often lichens, microbes and mosses. The pioneers species grow and interact with the environment in the area and over a few generations modify the environmental condition which provides suitable conditions for the establishment of additional organisms which subsequently arrive at the site. The pioneer species when die, they leave patches of organic matter and facilitate the process of soil formation. The pioneer species disappear as the condition of the habitat changes and invasion of new species occurs, leading to the replacement of the preceding community. 5.4: Secondary succession: Secondary succession occurs when the existing vegetation is abruptly removed. It is caused by either natural disaster (flood, fire etc.) or by human activities like deforestation etc. 7

8 Secondary succession also occurs on abandoned farmlands, in overgrazed area and construction projects. The mature complex biological communities in the area are replaced by simpler and earlier seral stages. Secondary succession is the development of biotic communities when existing community is completely or partial destroyed. The plants which are initially colonized In the area are annual plants. These plants produce large number of small seeds which can be easily dispersed by the wind and can effectively colonize the area. These plant species can tolerate high intensity of light and grow faster. These annual species are then replaced by herbaceous perennial plants, which grow and modify the conditions for shrubs and then trees. Thus, an abandoned farmland over a period of time around 30 to 40 years becomes dominated by trees and eventually transformed into climax community i.e. forest (figure 2). Fig.2: Secondary Succession. The secondary succession occurs much faster than primary succession which takes hundreds of years to reach at climax stage. It occurs much faster because the area has already sufficient nutrients and other conditions accumulated by previously occupied community which accelerates the process of succession. The secondary succession was studied by David 8

9 Duggins, an ecologist at University of Washington, after disturbance in the subtidal kelp forests of Torch Bay, Alaska. He found that when kelp forest was removed, after one year, a mixed canopy of kelp species (Nereocystis luetkeana and Alaria fistulosa) was formed and an understory of Costaria costata and Laminaria dentigera developed. After that, continuous stands of Laminaria setchellii and Laminaria groenlandica species was formed during the second and third years, and finally the community had restored its original composition. 5.5: Autogenic succession: This is a type of succession in which communities of an ecosystem grow, utilize the resource and modify their environment by their growth, death and decay. This changed condition facilitates new species from outside the system to enter into the area and leads to the establishment of new species in that area. The new established species causes replacement of the previously existing species. The whole process continues and there is replacement of one kind of community with another kind of community. This type of succession is known as autogenic succession. Autogenic succession can be easily understood by the example of alteration of the light environment. When light falls on the leaves, it reflects and seizes solar radiation to create a vertical profile of light within a plant community. The intensity of light from canopy of plant to the bottom level decreases, and less light reaches the plants, at the ground level, to perform the process of photosynthesis. In primary succession, the light does not show this kind of effect because the newly exposed site has not been initially occupied by higher plants. But in case of secondary succession, higher plants are removed by some natural or anthropogenic disturbance. Under these conditions, the light available at the ground is high, and seedlings are able to establish themselves in the area. When these plants grow, their leaves interrupt sunlight and reduce the amount of light falling on the ground for the shorter plants. The reduced amount of light available for shorter plants at the ground causes decrease in photosynthetic rate which in turn reduces the growth of these shaded plants. However, all plants cannot photosynthesize at the same rate, so those plants which grow faster can take the benefits and grow fastest to take greater access to the light resource. As a result, small plants are not able to get sufficient light and grow slower. The fast growing species of plant take the advantage and outcompete the other species and dominate the site. However, less availability 9

10 of light below canopy provides suitable environment for species adapted to grow in shady environment, will later displace the plants which are presently dominating the area. 5.6: Allogenic succession: This type of succession occurs where replacement of existing communities occurs due to changes in the environment which are brought about in the habitat by some external agencies and not by the existing vegetation itself. Allogenic succession generally occurs in the area which is highly disturbed or eroded. It can also occur in the pond or aquatic systems where the nutrients and other pollutants are released from outside and causes modification of the environment resulting in replacement of communities, e.g. allogeneic succession has occurred over most of North America and Northern Europe due to climate change, following the retreat of last Pleistocene ice sheet about 10,000 years ago. The ice sheet has expanded around thrice and retreated causing a similar advance and retreat in biota. In a forest or grassland, annual fluctuations of temperature and precipitation can affect relative growth of different species inhabiting a forest or grassland. However, the effect on relative growth of different species plays little influence on the secondary succession. Moreover, if changes in the environmental condition occur for longer time than the organisms survival are likely to affect the succession process. For example, change in temperature, light intensity and photoperiod causes seasonal succession of dominant phytoplankton in freshwater lakes that is repeated every year with small variation. Competition and seasonal patterns of predation by herbivorous zooplankton also influences temporal patterns of species composition. 5.7: Progressive succession: In this type of succession, the community becomes complex and contains more species and biomass over time. It results in formation of higher community with mesic conditions. For example, Woodland stage to climax forest stage (figure 3). 10

11 Fig.3: Progressive Succession. 5.8: Retrogressive succession: It is a type of succession in which the community becomes more simplistic and contains fewer species and less biomass over time (figure 4). In certain cases, a situation occurs in which a community depreciate and a new simpler community develops. Some retrogressive succession is allogenic in nature. When the depletion of glaciers occurred on the slopes and flat terrains in Alaska, succession proceeds from the formation of simpler community to complex communities (herb and shrub stage) and finally reaches to climax community, the spruce forest. Along with spruce tree, some mosses and small trees were also found in the forest. But during the course of time there was sudden invasion of sphagnum mosses. It has the ability to hold large amount of water. As a result it absorbed large amount of water from the soil and leads to death of spruce tree. Another example of retrogressive succession is the introduction of grazing animals into grassland resulting in degenerated rangeland. 11

12 Fig.4: Retrogressive Succession. 6. Causes of succession The succession is the process of gradual and orderly replacement of one community by another due to disturbance in the environmental conditions caused by either external factor or by community itself. Disturbances are abrupt events which bring about significant changes in the area distinguished by opening up new opportunity for new species to establish in the area and removal of existing species. The process of succession is caused by two main factors: (a) Allogenic factors: flood, Climate, landslide and fires. (b) Autogenic factors: death and decay of individuals of existing species, Immigration of new species, reaction between community and environment. 7. Mechanism of succession 12

13 The mechanism of succession was explained by F.E. Clement. The following are the various steps of basic process of succession. 1. Nudation- It is the process of formation of new bare area as a result of either appearance of a raw geological formation (primary bare area) or destruction of existing vegetation (secondary bare area). The primary bare area is produced by physiographic processes which include wind, soil erosion, gravity or glaciers, volcanic activity, emergence of new land surface, production of unstable strata and submergence of land surface. However, formation of secondary bare areas occurs due to biotic agents like overgrazing, deforestation, anthropogenic activities, insect out break etc. and abiotic agents such as drought, lightning and wind etc. It may occur due to following reasons: a) Climatic factors like fire, glacier, hail and storms, long dry periods and frost that may destroy community. b) Topographic factors like volcanic activity, land slide, deposition of sand, soil erosion by gravity, wind or water etc. may destroy the existing vegetation. c) Biotic factors like agricultural practices, destruction of forest for housing, industry and several other purposes of human. Overgrazing by animals, disease caused by various pathogens such as protozoa, bacteria, fungi, helminthes and insects also destroy flora and fauna of an area. 2. Invasion- when climatic conditions are changed in the area, it became the potential site for the establishment of various species. The species reaches in the area from other communities of different site and successfully establish itself in a bare area. It has three stages: a. Migration (dispersal): It is the arrival of spores, seeds, disseminules or propagules which are reproductive units to the bare area. This process is usually done by wind and water. These species form the first pioneer stage in the process of succession. b. Ecesis (establishment): When a species enters a new area, it successfully establishes itself in the area as a result of adjustment with the conditions existing in the area. The seeds or propagules germinate, grow into adult forms and reproduce. Only few individuals of the species are able to reach at adult stage in the harsh conditions while others die before reaching the adult stage. 13

14 c. Aggregation: After the establishment of new species in the area, they grow, reproduce and start increasing in number. As a result of increase in number, their population comes close to each other and become denser. 3. Competition: As the number of individuals of species increase in the process of aggregation in the limited area, the nutrients become limiting for them which cause severe competition (inter-specific and intra-specific) amongst members of colonizing species for space and nutrition. If any species which is not able to compete would be eliminated from the area while those who can compete, survive and reproduce in the area. 4. Reaction: It is most important step in the process of succession in which modification of the environment occur due to interaction between colonizing species and the habitat. As a result of reaction of inhabiting species with the habitat, changes occur in the abiotic component such as water, soil, light condition and temperature. These changed conditions of the environment make the area unsuitable for pre-existing species and causes its removal from the area and new species invade and establish in the new environment. This whole sequence of replacement of one community by another in an area is known a sere and different communities which constitute sere is called a seral community or seral stage. Each seral stage has its own species composition and unique characteristic structure. 5. Climax (Stabilization): It is the final stage in the process of succession in which the terminal community becomes more or less stabilized in the area for longer period of time and maintains the equilibrium between itself and the environmental condition. The community structure would not change until there would be a sudden and large disturbance in the area. Thus, they are called as climax stage or climax community. In an ecosystem, both autotrophic and heterotrophic succession function in synchronized manner in which one derive mineral from soil and atmosphere and other returns these minerals into the soil and atmosphere respectively. This dynamic and predictable process leads to stable ecosystem. 8. Climax characteristics 14

15 The climax community of an area during succession is the stabilized community which forms equilibrium with the environment. According to Clement (1935), climax has following characteristics: i) Unity: The climax is considered as unit and index of the climate of area. The climate type is indicated by growth forms of plants and it occurs only when all species are taken as an organized unit. ii) Stability: The climax community is almost stable in the present climatic conditions which cannot be replaced by other species through the process of competition. iii) Origin and phylogenetic relation: The developing stage of climax community has its own characteristics which describe its climatic conditions. As the organisms grow and develop, their characteristics change with the age, similar pattern of changes are shown by community with changing the climatic conditions. 9. Theories of climax The major theories which define, identify and interpret the climax communities are as follow: 9.1: Monoclimax theory: The theory explains that similar type of community is present at climax stage in the given land area. In other words, succession begins in diverse areas like pond, rock and river will finally converge into a same single climax community decided by their regional climate. The stable climate leads to a stable climax community which remains stable indefinitely. But this theory was not accepted by all ecologists. Cowles explained that the succession process cannot reach the equilibrium state. In fact it is a variable approach rather than a constant approach. However, Cooper considered climax as minimum changing state rather than finally changed state of succession. The climax community in the same climate may differ as it depends upon primary stage along with its habitat characteristic. It may be possible that in same climate condition, a lithosere and hydrosere may have different pioneer community and after several intermediate 15

16 seral stages finally reach to same climax community. It may also be possible that similar pioneer and seral communities would lead to different climax communities. In various conditions under uniform climate, different climax community can be formed. It occurs due to different soil, topography and other factors. So, climate is not solely responsible for determining the fate of climax. 9.2: Polyclimax theory According to Tansley (1939) climax is controlled by not a single factor but by several other factors. Various climax communities can be expected in the area which is under the control of different abiotic component such as moisture, soil, temperature and activities of different biotic factors. He also recognized that the existence of a number of climax communities, forming a mosaic correspond to the mosaic of habitat. Climate is only one factor of the several other factors, any of which can control the structure and the stability of climax. Tansley recognized following types of climax. 1. Climatic climax: It occurs under normal climatic conditions, soil and topography. 2. Edaphic climax: Self-perpetuating vegetation which differs from climatic climax of the area. 3. Topographic climax: Variation in topography of the area cause variation in microclimates and each variation causes self-perpetuating vegetation. 4. Fire climax: Frequent burning of vegetation causes elimination of those species which are fire sensitive and there is development of self perpetuating vegetation. 5. Zootic climax: it is a self perpetuating community which develops due to various zoological factors such as grazing by herbivores. Out of above described climaxes, Climatic, edaphic and topographic climaxes are example of primary climaxes, whereas, fire and zootic climaxes are the examples of secondary climax. 9.3: Climax Pattern hypothesis Climax pattern theory was proposed by R.H. Whittaker (1953) which states that a community of the ecosystem is adapted to each and every environmental factor and not to a single climatic factor. The major factors which influence community structure are climate, fire, genetic makeup of species, biotic factors, wind and dispersal. The pattern of populations in the climax communities differs according to total environmental change. Hence, in climax pattern, definite number of climax communities will form in which structure and stability of 16

17 the climax community is not determined by single factor. So, in such case, pattern of population are formed which change with the pattern of environmental gradients and integrate to form ecoclines. It is the community which clearly expresses the climate of the area. 9.4: Information Theory This theory was proposed by Leith, Odum and Golley. It states that succession and climax are components of ecosystem development. In autotrophic succession, increase in organic content and biomass results in increase of species diversity which is supported by available energy. Thus, in a climax community, there is increase in available energy and biomass (the information content). However, in the case of heterotrophic succession, rate of respiration is higher than rate of production which results in gradual depletion of energy. In an ecosystem, both autotrophic and heterotrophic successions operate in a synchronize manner. The autotrophs receive minerals from the soil and atmosphere, at the same time heterotrophs return nutrients into the soil and atmosphere by the process decomposition of dead and decaying organic matter. So, succession reaches the climax stage when amount of energy and nutrients received by autotrophs is equal to the amount returned to the environment by heterotrophs (detritivores). 10. Summary The term succession is generally used for directional and continuous change in species composition and abundance following disturbance of a site or creation of a bare area. On the basis of energy and nutrition relation, succession can be divided into autotrophic succession and heterotrophic succession. Primary succession occurs in the area where environmental conditions are elementary and that area is occupied first time by the few simple living organisms. Secondary succession occurs in the area which was previously occupied by some types of community and has been devastated by some natural or human influenced activities. 17

18 Autogenic succession refers to the process of modification of environment by pioneer community results in the replacement of its own community by some new modified community. Allogenic succession refers to serial replacement of communities resulting from external environmental factors other than the effects of communities on the environment. Progressive succession refers to the succession where the community becomes complex and contains more species and biomass over time. Retrogressive succession refers to the succession where the community becomes simplistic and contains fewer species and less biomass over time. The climax community of given area during succession is the stabilized community which form some sort of equilibrium with the environment. Clement explains the characteristics of climax as unity, stability and origin and phylogenetic relationship. There are four important theories of climax. Monoclimax theory states that in a given region all land surface are occupied by same type of community which is called climax community. Polyclimax theory was proposed by Tansley in 1939, according to which climax is controlled not by one factor but it involved many other factors. Climax Pattern hypothesis states that a natural community of the ecosystem is adapted to all the factors of environment and not to only one aspect like climate. According to Information theory, ecosystem development includes succession and climax. In autotrophic succession, diversity of species is likely to increase with an increase in organic content and biomass supported by the available energy. 18