Topic: Population, Biotic Community and Succession

Physiognomy and stratification: A community is first noticed by its physiognomy. Physiognomy refers to the external appearance or “look” of the community. The external appurtenance is the total effect created by the combination of vertical structure and architec-ture of dominant species of vegetation. For instance, the high physiognomy of forest differs markedly from a low physiognomy of a grassland. However, several communities may have similar physiognomy, yet they differ sharply on the basis of species composition and dominants (e.g., different forests types).

Stratification of a community depicts vertical layering of the vegetation. Different layers are occurred by different species. The vertical stratification provides physical structure to the plant community, in which many forms of plants and animal life are adapted to live in a well developed forest eco-system exhibits a highly stratified structure, consisting of several layers of vegetation. These layers include the canopy, the understory tree layer, the shrub layer and the herb layer. Similarly, a pond community has surface dwellers and bottom dwellers. Vertical stratification lead to increase in number of species and to efficient use to resources of a habitat by different types of plants. In aquatic ecosystems, stratification from surface to bottom is determined by light penetration from temperature profile and oxygen profile.

Species diversity: Some communities, such as tropical rain forest and coral reef community, show high species diversity with many different kinds of species living at each trophic level. In other communities, like a desert, there may be relatively few species in the entire community. Species diversity includes the total number of species present in a community and the relative abundance of these species. Diversity is recognised as an important functional attribute of biotic community. You will study several diversity-related aspects in Chapter 20.

Keystone and link species: The species having much greater influence on community characteristics, relative to their low abundance or biomass, are called keystone species. These species play a vital role in controlling the relative abundance of other species. Removal of keystone species causes serious disruption in the functioning of the community. For example, in the tropical rain forests, the different species of figs are the keystone species as the produce large quantity of fruits. During the time of food scarcity, these fruits are eaten by monkeys, birds, bats and other vertebrates. Thus, by protecting the fit trees, the animals dependent on them are also conserved.

Only a few species work as keystone species, and several other work as critical link species. Mycorrhizal fungi in soil are critical link species as they establish essential links in the absorption of residues. Some critical link species may also provide food for the network species; other play important roles as pollinators of flowers or dispersal agents of seeds and fruits. Tropical rain forests are rich in critical link species due to high degree of animal-dependent pollination and dispersal.

Ecotones and edge effect: The transition zone between two communities is known as ecotone. For example, the ecotone between grassland and forest. Ecotone contains few species from both communities. The total number of species is often greater in the ecotone than in the adjoining communities. The tendency of increased variety and density of some organisms at the community border is known as edge effect. The organisms which occur primarily, or most abundantly, or spend the greatest amount of their time in junctions between communities, are called edge species.

Analysis of Plant Communities

Analysis of community characters is generally done for: (a) recording variation within and between communities, and (b) naming and classifying communities. Community analysis involves measurements of various characters in sample plots (also called quadrates) located randomly within the community. Measure-ments made in sample plots are appropriately processed to reflect the characteristics of the entire community. Various community characters can be categorised as:

• Analytic characters, which are directly observed or measured in sample plots.

• Synthetic characters, which are derived from the measurements of analytic characters.

The analytic characters may be either qualitative or quantitative. Qualitative analytic characters are based on non-quantitative observation; for example, the species composition and stratification of vegetation. On the other hand, quantitative analytic charac-ters, as the name suggests, are measured. The major quantitative analytic characters, as the same suggests, are measured. The major quantitative analytic characters are:

• Frequency (based on percentage of plots in which a species is present, indicating its dispersion in space).

• Density (number of individuals per unit area, indicating the relative abundance of a species).

• Diversity (Total number of species in a unit area, including plants, animals and microbes).

• Cover (percentage land area occupied by a species, indicating the influence zone of a species; cover is expressed as basal cover, area occupied by stem bases, or crown cover, the area covered by canopy).

• Biomass (quantity of living materials per unit area, indicating the growth of a species; see more details on biomass and productivity in Chapter 18).

• Leaf size (percentage of species having different leaf sizes, indicating the adaptation of the vegetation to the prevailing environment).

Synthetic characters (e.g., presence and constance) reflect the pattern of distribution and performance of difference species through all the locations where the community occurs.

Succession

The biotic communities are dynamic in nature and change with the passage of time. The successive replacement of communities in an area over a period of time is known as ecological succession. Both abiotic and biotic components are involved in such change. Succession is a community-controlled phenomenon, which results due to the action and co-action on living organisms. Physical environment often determines the nature direction, rate and optimal limit of change. During succession, change occur both in plant and animal communities. The plant succe-ssion, however, is easily visible. Two basic types of succession can be distinguished. Succession occurring on previously unoccupied sites, such as a rock outcrop or glacial moraine, is called primary succession. The more common type of succession is the secondary succession, which occurs in an area where the natural vegetation has been destroyed or removed. For example, the forests destroyed by fire and excessive lumbering may be reoccupied by herbs in the initial stages. The reappearance and establishment of communities in such areas is called secondary succession. The plant that invade the bare land initially, are called pioneer species. The assemblage of pioneer species forms the pioneer community. Generally, the pioneer species show high rate of growth but short life span. In time, the pioneer community is replaced by another community with different species combination. This second community is replaced by a third community, and so on. The different communi-ties or stages represented by combinations of mosses, herbs, shrub and trees replacing one another during succession are refereed to as seral stages or seral communities. The plant species which get established later, during plants species are low growing and long lived. The terminal stage of succession is represen-ted by the climax community. The climax community is stable and does not show change in species compositions remain the same. The sequence of communities succeeding each other during the course of succession represents the sere. The succession occurring in water bodies like ponds and lakes is called hydrach succession, and the succession taking place in terrestrial areas with low moisture (for example, rock, sand) is known xerarch. These two types or succession are described below.

Succession on a Barr Rock (Xerarch)

Lower plants, like lichens, from a crust over the bare rocks and begin to form soil from their organic remains and by stimulating chemical breakdown of the rocks. Lichens are normally followed by mosses, which speed up the process of soil accumulation by trapping wind-blown particles. Mosses grow in bunch, and together with lichens, make a mat over the substratum. Lichens and mosses, which get established on barren rock, are the pioneer species forming the pioneer community. The accumulation of soil particles in the lichen-moss carpet provides suitable substratum for the germination of seeds of herbaceous plants which are dispersed in it. The seeds of higher plants germinate and grow successfully in pockets of newly formed soil on the rick (Figure). Gradually, more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees. The dead shoots and fallen leaves accumulate and enrich the soil. Passing through the seral stages in course of time. Climax community gets established. Depending upon the climate condition and extent of soil formation, the climax community is generally dominated by trees. The changes in biotic community from the pioneer to the climax stage may take hundreds of years.

Succession in Aquatic Environment (Hydrach)

Ecological succession also occurs in water bodies like ponds and lakes. Water bodies are prone to silting as a result of soil erosion from surroundings area. Blockage of rivers by land-slides and construction dams lead to formation of new lakes on land where hydrach succession sets in due to invasion of aquatic species (Figure). In a pond, the phytoplankton and zooplankton constitute in pioneer community. Submerged aquatic plants, with their roots anchored in the mud, are next to colonise the pond. The dead remains of these organism settle at the pond bottom. Beside, floating plants species invade the pond. With the continued siltation, the pond bottom is gradually raised and water layer becomes shallow and rich in nutrients. As a result, rooted, emergent plants with aerial leaves, such as reeds, are able to colonise the pond. This is accompanied by the invasion of Dragon files, crustaceans and more rooted species of plants. Thus, the species composition of the pond keeps changing with time. With increased setting of silt and deposition of dead organic matter derived from floating and rooted species, the pond becomes shallower until it gets trans-formed into a terrestrial habitat. Ultimately, terrestrial species, like grasses, bushes and trees, colonise the pond area and a climax community is established. The colonization by land plants usually progresses from margins toward the centre of the pond area.

Change in Community Characteristic and Climax

The exact sequence of species and communities that appear during primary of secondary succession, varies with the habitat conditions. The seral stages differ from late successional or climax stages with reference to structure and functions (Table). The average size of individuals generally increase and the community organisation becomes more complex in the climax community as compared to the seral community. The food webs become complex during successional stages. The efficiency of energy use and nutrient conservation increase as the community progresses towards the climax stage.

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