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