Plant communities 1 Ecotone and edge effect • An ecotone is a zone of junction or a transition area between two biomes (diverse ecosystems). • Ecotone is the zone where two communities meet and integrate. • In nature there is a zone of transition or tension in which the conditions for each of the adjacent communities become more adverse, and there is usually an intermixing of species from both the communities. • For e.g. the mangrove forests represent an ecotone between marine and terrestrial ecosystem. • Other examples are grassland (between forest and desert), estuary (between fresh water and salt water) and riverbank or marshland (between dry and wet). • Such a region presenting a situation of special ecological interest is known as an ecotone or tension zone. Characteristics features of Ecotone • A general characteristic features of the ecotone is that it has sufficiently greater number of species, and moreover, the density of most of the species is higher than that in the neighbouring communities. • This unique general feature of an ecotone is known as principle of edges. • It may be narrow (between grassland and forest) or wide (between forest and desert). • A well-developed ecotone contains some organisms which are entirely different from that of the adjoining communities. • Ecocline is a zone of gradual but continuous change from one ecosystem to another when there is no sharp boundary between the two in terms of species composition. • Some ecologists have introduced the continuum concept in ecology, where there are no distinct communities with well defined boundaries, but there is change in space, soil parameters, altitudes, and other abiotic factors. • There are no sharp borders or changes in species composition in areas where a continuum can be demonstrated. Edge Effect – Edge Species • Edge effect refers to the changes in population or community structures that occur at the boundary of two habitats (ecotone). • Sometimes the number of species and the population density of some of the species in the ecotone is much greater than either community. This is called edge effect. • The organisms which occur primarily or most abundantly in this zone are known as edge species. • In the terrestrial ecosystems edge effect is especially applicable to birds. • For example, the density of birds is greater in the ecotone between the forest and the desert. Succession • The term was first given by Hult (1885). • Community are never stable but keep on changing. • This relatively definite sequence of communities over a period of time in the same area is called ecological succession. • Communities are never found permanently in complete balance with their component species or with the physical environment. • Succession is a universal process of directional change in community composition, on an ecological time scale. Ecological succession • Environment is always changing over a period of time due to • 1) variations in climatic and physiographic factors, and • 2) the activities of the species of the communities themselves. • These influences bring about marked changes in the dominants of the existing community, which sooner or later replaced by the another community at the same place. • Gradual replacement of simple organisms with more complex organisms overtime • Role of succession is to make an The occurrence of relatively definite sequence of communities ecosystem more over a period of time in the same area is known as ecological sustainable/biodiverse succession. • Odum (1971) preferred to designate this orderly process as ecosystem development rather than the ecological succession. Causes of Succession • 1) Initial or initiating Causes: these are climatic as well as biotic. Causes those are responsible for the destruction existing habitat. Such occurrences happen due to the following factors: Climatic Factor: Such as wind, deposits, erosion, fire etc. Biotic Factor: Such as various activity of organisms. • 2) Ecesis or Continuing Causes: Causes those are responsible for changes in population shifting features of an area. Such factors are: • (a) Migration for safety against outside aggregation. • (b) Migration due to industrialization and urbanization. • (c) As a reactionary step against local problems. • (d) Feeling of competition • 3) Stabilising Cause: according to Clements, climate of the area is the chief cause of stabilizaiton and other factors are secondary. • Causes which bring stability to the communities. Such factors are: • a) Fertility of land • (b) Climatic condition of the area • (c) Abundance of availability of minerals etc. • An ecological succession proceeds along the following trends • 1) a continuous change in the kinds of plants and animals • 2) a tending increase in the diversity of species. • 3) an increase in the organic matter and biomass supported by the available energy flow • 4) decrease in net community production or annual yield. Basics types of succession • Primary Succession starts on bare surfaces which have not previously borne vegetation. It usually starts with lower organisms and takes a longer time to reach a climax community. Examples of primary succession are found in ponds, vegetation on rocks, etc. • The first group of organisms establishing there are known as pioneers, primary community or primary colonisers. • Factors which can give rise to primary bare surfaces include: Erosion & Extreme drought, Deposition of sand dunes & Volcanic ash, Mining or construction activities & Hurricanes, Landslides & Earthquakes, Emergence of bare surfaces & Thunderstorms • Secondary Succession starts from previously built up substrata with already existing living matter or starts on already colonized surface. It may start with fairly complex organisms and it takes a shorter time or duration to reach a climax community. Examples of secondary succession are found in abandoned farmlands and grasslands. • Autogenic Succession- When the succession has began the vegetation itself is responsible for replacing itself by changing existing environmental conditions. Community reacts with its own environment and modifies its own environment and thus causing its own replacement by new communities. This course of succession is known as autogenic succession. • Allogenic Succession- Allogenic succession occurs as a result of the changes brought about in the habitat by external agencies not by existing community itself. • On the basis of successive changes in nutritional and energy contents, succession are classified as Autotrophic Succession and heterotrophic succession. • Autotrophic Succession - The succession where initially the green plants are much greater in quantity than the animals, is known as the autotrophic succession. it begins in a predominantly inorganic environment and the energy flow is maintained indefinitely. There is a gradual increase in the organic matter content supported by energy flow. • Hetrotrophic Succession- It is characters iced by early dominance of heterotrophs such as animals, fungi, bacteria, actinomycetes. • It begins in a predominantly organic environment and there is a progressive decrease in the energy content. • Progressive Succession- When succession occur in direction of simple community to complex community known as progressive succession. • Retrogressive Succession- when succession occur from complex community to simple community. Depending upon the substratum • 1. Hydrosere or Hydrach- Succession beginning in fresh water/where water is plenty. Ex. Ponds, Lakes, Streams etc. • 2. Xerosere or Xerarch- Succession beginning in dry conditions. It is classified into three classes I. Lithosere- When sucession starts on bare rocks. II. Pasmosere- When succession begins on sand. III. Halosere- When succession starts in saline conditions • 3. Mesarch- when succession begins in mesic conditions General process of succession • The ecological succession is a complex process and it may take thousands of years. • Frederic Clements in 1916 for the first time proposed the sequential phases of an ecological succession. • The process of succession is completed through a series of sequential steps as given below: • (1). Nudation • (2). Invasion • (3). Competition and Co-action • (4). Reaction • (5). Stabilization (climax) Nudation- Nudation is the development of a bare area (an area without any life form). • It is the first step in ecological succession. • The causes of nudation are: • (a). Topographic: Soil or topography related causes such as soil erosion, sand deposit, landslide and volcanic activity results in the formation of a bare area. • (b). Climatic: Destruction of the community due to glaciers, dry period and storm. • (c). Biotic: It includes forest destruction, agriculture and disease epidemics which results in the total destruction of the population in an area. • Invasion: Invasion is the successful establishment of a species in the bare area. • It is the second step in ecological succession. • A new species reaches the newly created bare area and they try to establish there. • The process of invasion is completed in THREE steps: • (A). Migration • (B). Ecesis • (C). Aggregation • (A). Migration (Dispersal): Seeds, spores, propagules of a species reach the bare area due to migration. • The migration can be achieved through air or water medium. • B). Ecesis (establishment): Ecesis is the process of successful establishment of a species in the bare area. • The seeds or spores that reached the new area due to migration will germinate, grow and reproduce. • Only a few progenies will survive due to the harsh environmental condition prevailing in the area. • (C). Aggregation: After ecesis, the individuals of a species increase their number and they stay close to each other. This process is called aggregation Competition and Co-action • Aggregation results in the increase of the number of species within a limited space. • This results in competition between individuals for food and space. • The competition may be intra-specific (individuals within a species) or inter- specific(individuals between species). • Individuals of a species affect each other’s life in various ways and this is called coaction. • Competition and co-action results the survival of fit individuals and the elimination of unfit individuals from the ecosystem. • A species with wide reproductive capacity and ecological amplitude only will survive. Reaction: Reaction is the most important stage in the ecological succession. • It is the modification of the environment through the influence of living organism present on it. • Reaction cause change in soil, water, light and temperature of the area. • Due to these modifications, the present community becomes unsuitable for the existing environmental conditions. • Such communities will be quickly replaced by another community. • The whole sequence of communities that replaces one another in the given area is called sere (sera). • The various communities contributing sere are called seral communities or seral stages. • Stabilization (Climax) : It is the last stage in ecological succession. • The final or terminal community becomes more or less stabilized for longer period of time. • This community can maintain its equilibrium with the climate of the area. • This final community is called the Climax Community (climax stage). • The climax community is not immediately replaced by other communities. • Climax community is determined by the climate of the region. • Example of climax community: Forest, Grassland, Coral Reef • Different Types of Climaxes in an Ecological Succession: • (1). Climatic climax: In this climax, the climax community of the succession is determined by the climate of the region. • The climatic climax will have only one climax community. • (2). Edaphic climax : Here the climax community in the succession is determined by the soil (edaphic factor) of the region. • The edaphic factors may include soil moisture, topography, soil texture and soil nutrients. • (3) Catastrophic climax: Here the climax community is vulnerable to many catastrophic events such as wildfire, snowfall and floods. Characteristics of a Climax Community • The climax community in a succession shows the following characteristics: • The vegetation of the climax community will have high ecological amplitude. • They possess high tolerance towards the environmental conditions. • They show rich diversity in species composition. • The species composition remains constant for many years. • The community possesses a complex food chain system. Characteristics of a Climax Community • The ecosystem will be balanced and self- sustainable. • There will be equilibrium between gross primary productivity and respiration. • The energy used from the sunlight and energy released after decomposition will be balanced. • The uptake of nutrients from the soil and the release of nutrients back to the soil by decomposition will be in equilibrium. • The individuals of the community lost by its death are replaced by the individuals of the same species. • The climax community is considered as the manifestation of the climate prevailed in the area. Climax concepts • The final stage of succession is called the climax or climax community (Clements, 1936; Shimwell 1971). • It is the final or stable community in a successional series. • It is self-perpetuating and in equilibrium with the physical and biotic environment. • Climax communities undergo changes in structure as a result of birth, death and growth processes in the community. • There are following theories of the climax: • Mono-climax Theory: • According to the mono-climax theory of succession (Clements, 1936), every region has one climax community toward which all communities are developing. • He believed that climate was the determining factor for vegetation and the climax of any area was solely a function of its climate. • Various terms such as sub-climax, dis-climax, post-climax, and pre-climax are used to describe the deviations from the climatically stabilized climax. • These communities, controlled by topographic, edaphic (soil), or biotic factors are regarded as exceptions by the supporters of the mono-climax view. • Poly climax Theory: • This theory was proposed by Tansley (1939) and later supported by Daubenmire (1966). • The poly-climax theory of succession holds that many different types of vegetation as climax communities may be recognized in a given area. • These will be climaxes, controlled by soil moisture, soil nutrients, activity of animals and other factors. • According to this theory, climate is only one of the several factors, any of which may have a controlling influence on the structure and stability of the climax. • This allows many climaxes in a climate region and is, therefore, called the poly- climax theory. • The difference between this theory and the mono-climax theory is largely a matter of emphasis on which factor is responsible for the stability of a climax. • According to Krebs (1994), the real difference between two theories lies in the time factor of measuring relative stability. • The climate varies on an ecological time scale as well as on a geological time scale. • Succession in a sense, then, is continuous because we have variable vegetation approaching a variable climate. • Climax-pattern Theory or hypothesis : • R.H. Whittaker (1953) emphasized that a natural community is adapted to the whole pattern of environmental factors in which it exists; the major factors are: genetic structure of each species, climate, site, soil, biotic factors (activity of animals), fire, and wind, availability of plant and animal species, and chances of dispersal. • According to this theory, climax communities are patterns of populations varying according to the total environment. • There is thus no discrete number of climax communities and no one factor determines the structure and stability of a climax community. • Whereas the mono-climax theory allows for only one climatic climax in a region and the poly-climax 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. • Hydrosere • Succession takes place in wet habitats • Its first step is the submerged plant stage • Plants of the first stage are- Elodia, Hydrilla, etc. • Succession is limited to six steps such as- Submerged stage, floating stage, reed swamp stage, woodland stage, and climax forest stage. • A hydrosere is a plant succession which occurs in an area of fresh water such as in oxbow lakes and kettle lakes. • The process of Hydrosere aquatic succession completes in the following stages: • Phytoplankton stage: In the initial stage of succession algal spores are brought in the body of water. • Submerged stage: The phytoplankton stage is followed by the submerged plant stage. • Floating stage: When the depth of water reaches about 4 to 8 feet, the submerged vegetation starts disappearing from its original place and then the floating plants make their appearance gradually in that area. • Reed-swamp stages: Under these conditions, the floating plants start disappearing gradually and their places are occupied by amphibious plants which can live successfully in aquatic as well as the aerial environment. • Sedge Marsh or Meadow stage: The filling process finally results in a marshy soil which may be too dry for the plants of pre-existing community. • Woodland stage: In the beginning, some shrubs and later medium-sized trees form open vegetation or woodland. • Climax forest: After a very long time the hydrosere may lead to the development of climax vegetation. • Xerosere • Succession begins with bare rocks, deserts; example: dry places. • Its first step is the blue-green algae as well as thalloid lichen plant stage • Plants of the first stage are Rhizocarpon, Rhinodina, etc. • This succession is ended by six stages, such as – thalluid, licken stage, leafy licken stage, moss stage, herb stage, shrub stage, and climax forest Stage. • The xerarch succession of ecological communities originated in the extremely dry situation such as sand deserts, sand dunes, salt deserts, rock deserts, etc. • Xerosere type of progression that there are no plants only Sandy grounds are present after all wild in the while. In the seed dispersal seats of cactus, euphorbia is consistently increasing through Kangaroo rat and several other animal habitats. • The succession occurs in Xerosere dry condition is called Xerosere. It undergoes the following stages: • Crustose lichen stage: Crustose are land lifeless structure. The crustose may have an external protective layer surface on the rock. • Foliage lichen stage: Now habitat is suitable for foliage lichens. Foliage lichens are leaf-like. • Moss stage: The soil becomes more porous. Thus at this stage moss plants in ade the area. • Herbaceous stage: The soil has a large amount of humus and litter. Thus its water holding capacity is increased. • Shrub stage: Soil conditions now become favorable for shrubs. • Climax stage: The soil is much improved now. So it now allows the growth and establishment of woody plants. • A biome is the complex of the communities maintained by the climate of the region and characterised by a distinctive type of vegetation. It is a major community composed of all plants and animals and smaller communities including the succession stages of the area. • Thus biomes , a bigger unit than community , constitute the great regions of the world distinguished on an ecological basis, such as tundra biomes, forest biomes, grasslands and deserts etc.