Biotic Community Organisation

Nature has a large number of habitats each with its characteristic environmental conditions. Depending upon concurrence of tolerance ranges, a number of species come together to live in a biotic community. The composition and ecological characteristics of each biotic community are specific. They differentiate it from other biological communities. However, with two different areas with similar habitat conditions may have similar biological communities.

Species Composition A number of plants, animals and other organisms occur in a biological community. They constitute species composition. Species composition of communities depends upon the climate, topography, size of the area, habitat diversity and soil. Within a community species composition differs from one stratum to another and from one season to another.

Dominance It is the ability to influence other members of the community. Dominant species have increased abundance, height, cover and biomass relative to others. In a forest, trees that form the uppermost stratum constitute the dominant species. A community has one or a few dominant species. Population of other species become subordinate, suppressed or reduced in number. The dominant species determine the distribution of animals in that community. Biotic community is named either after the dominant species (e.g., Pinus in pine forest, grass in grassland) or after the habitat, e.g., desert community, marine community.

Growth Forms The visible structure or appearance of plants in a community is called growth form, e.g., tree, needle or broad leaves, deciduous or evergreen nature, lianas, shrub, epiphyte, herb, etc. Growth forms found in a region indicate environmental conditions, e.g., small leaf size and small height in deserts, evergreen needle leaved trees in temperate forest or evergreen broad leaved trees in tropical rain forest.

Physiognomy It is external appearance of a community based on vertical structure and architecture of dominant species of vegetation, e.g., forest, savannah, grassland, desert. However, several communities may have similar physiognomy though they differ in species composition and dominants, e.g., forest type.

Stratification It is vertical layering or the phenomenon of having more than stratum formed by different plants and other organisms in the same place. A forest may have 5-7 strata—over storey trees or canopy trees, medium trees, short trees, shrubs, herbs, ground flora, etc. A pond may have plankton at the surface, benthos at the bottom and intermediate forms in the middle. In a deeper water body, stratification depends upon light penetration, temperature profile and oxygen profile. Stratification is useful as it can accommodate a large number of organisms of different types and more efficient use of resources.

Species Diversity It is an important attribute of biotic community which is determined by total number of species and their relative abundance. Greater species diversity indicates higher number of niches and greater stability to the community, e.g., coral reef and tropical rain forest communities. Tropical rain forests hardly occupy 8% of the land but they have more than 50% of all flora and fauna. A hectare of forest may have upto 200 species of tree. Coral reef is marine aquatic ecosystem formed of calcareous skeleton of corals and forminiferans. It has very high degree of biodiversity with every marine life form being present over here. There is decrease in species diversity if the habitat conditions are adverse, e.g., desert.

Keystone Species It is a species which has a significant and disproportionately large influence on the community structure and characteristics. It has often considerably low abundance and biomass as compared to dominant species. Removal or decrease in number of keystone species causes serious disruption in structure and function of community. In inter¬tidal regions starfish feeds on mussels. Removal of Starfish leads to dominance of mussel population that excludes algae and browsing species (limpets, whelks). In tropical rain forests, figs function as keystone species as they provide fruit to a number of animals (monkeys, birds, bats, etc.) during the periods of food scarcity.

Critical Link Species They are species which play an important role in supporting network species by functioning as pollinators, dispersal agents, absorption or circulation of nutrients, etc. Mycorrhizal fungi help the vascular plants in obtaining inorganic nutrients from soil and organic residues. Similarly bees pollinate a large number of flowers or birds disperse the seeds of a number of plants. Tropical rain forests have a number of critical link animals (for pollination and dispersal).

Ecotone and Edge Effect Ecotone is the transition zone between two communities, e.g.. forest and grassland. Its may be narrow or broad. Ecotone has an admixture of species belonging to both the communities. Some specific species also occur exclusively in ecotone. They are called edge or ecotone species. As a result ecotone possesses higher number of species than either of the two communities. The increased number (and even density) of species in the region of ecotone or community border is called edge effect. The species which are found primarily, most abundantly or spend most of their time in ecotone or community boundary are known as edge species

ANALYSIS OF PLANT COMMUNITIES

Analysis of community characters is generally done for recording variations within and between communities, and naming and classifying communities. Community analysis involves measurements of various characters in sample plots (also called quadrats) located randomly within the community. Measurements 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 observations; for example, the species composition and stratification of vegetation. On the other hand, quantitative analytic characters, as the name 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).
• Diversity (number of individuals per unit area, indicating the relative abundance of a species).
• 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).
• Leaf size (percentages 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 different species through all the locations where the community occurs.

SUCCESSION

The successive replacement of communities in area over a period of time is known as Biotic or Ecological succession.The natural development of a series of biotic communities at the same site, one after the other till a climax community develops which does not change further became it is in perfect harmony with the environment of the area.

Succession is the universal and unidirectional change in vegetation during ecological time. It is the law of nature. Any barren land or area becomes successively colonized by wide varieties of communities depending upon by another due to modification in environment by community environment interactions, refers to succession.

The concept of succession was developed by Warming (1896), Cowles (1899),Clements (1907, 1916) and others.The term succession was coined by Hult (1885).

Succession is a long term and unidirectional cumulative changes in the structure and composition of community occurring at the same site or habitat. Succession is a non-cyclic changes in the community over time. Diagrammatic representation of succession through autogenic processes.

These changes can be noticed only after a decade or sometimes even after a century. Some of these changes are related with origin, migration or extinction of taxa and are thus have historical importance. Odum (1969) defined ecological succession as follows :

Succession is an orderly sequence of changes in vegetation growing on a barren site with time. It is reasonably directional and thus predictable. It results from modification of physical environment by the communities, i.e. it is community controlled, even though the physical environment determines the pattern, the rate of change often sets limits as to how far development can go. It culminates in a stablized ecosystem in which maximum biomass and symbiotic functioning between organisms are maintained per unit of available energy flow.

TYPES OF SUCCESSION

Depending upon the type of nudity of the area, ecological or biotic succession is of two types

• primary succession
• secondary succession

Primary succession (= Prisere)— The initial invasion and colonization of pioneer community on bare rocks or other surfaces lacking organic soil (sand dunes or glaciated surfaces) is called as primary succession.

Secondary succession (= subsere)— It takes place when the surface is completely or largely denuded of vegetation but has already been influenced by living organisms and has an organic component. There is a gradual invasion of higher organisms that takes place in such areas where organic components are present and influenced by living organisms e.g., a cleared forest or a previously burned or flooded area in which seeds, spores and organs of vegetative reproduction (rhizomes) are present in the ground and then development of herbs, shrubs and trees take place.

In both primary and secondary succession, the flora and fauna of surrounding areas are major factors influencing the types of plants and animals entering the successions through chance dispersal and migration.

FOUR FACTS OF ECOLOGICAL SUCCESSION

The four major structural and functional attributes of ecological succession are:

• Increase in species diversity.
• Increase in structural complexity.
• Increase in organic matter.

Tendency towards metabolic stability, which are both causes and effects of very process of change, and eventual stability that characterize ecosystem in general.

UNDERLYING FEATURES OF ECOLOGICAL SUCCESSION

The first biotic community which develops in a bare area is called pioneer community. It has very little diversity. This stage takes the longest time to change the environment for invasion of the next community.

Climax community is the stable, self perpetuating and final biotic community that develops at the end of biotic succession and is in perfect harmony with the physical environment. It is also termed as climatic climax community. Climax community has maximum diversity and niche specialisation.

The various biotic communities that develop during biotic succession are called as seral or transitional communities.

The entire sequence of development stages of biotic succession from pioneer to a climax community is known as sere.

The series of developmental stages of biotic succession in an arid area is called as Xerosere, while biological succession on an arid area is called Xerarch.

Xerosere is of further two types

• Lithosere— Sequence of successional stages on a bare rock.
• Psammosere— Sequence of successional stages on sand.

The various stages of biotic succession taking place in a water body are collectively called as hydrosere while such a succession is known as hydrarch succession.

Ecological dominance : Successions always progress towards a climax in which one species is dominant or a few species are codominant. These impose restriction on the species that can grow in association with them. Species composition first changes rapidly and then more gradually. The number of species of autotrophs increases in primary and some time early in secondary succession but it may decrease in older stages. The number of heterotrophic species continues to increase until fairly late in the sere. Species diversity increases initially, then becomes constant and finally may or may not decline in older stages.

Productivity and biomass : The succession involves increasing productivity until a climax community is reached in which the maximum efficiency of energy conversion takes place. The amount of energy flowing through an ecosystem is a major limiting factor in determining the number and biomass of the organisms it can support. The later stages of successions do become more productive but there is a decline in gross productivity associated with the climax community. The accumulation of nutrients in the increasing standing crop biomass may lead to a reduction in nutrient recycling.

GENERAL PROCESS OF SUCCESSION

The whole process of a primary autotrophic succession is actually completed through a number of sequential steps, which follows one another.

These steps in sequence are as follows :

Nudation : This is the development of a bare area without any form of life. The area may develop due to several causes such as landslide, erosion, deposition or other catastrophic events.

The, causes of nudation are as follows:

• Topographic : Due to soil erosion by gravity, water or wind, the existing community may disappear. Other causes may be deposition of sand etc., landslide, volcanic activity and other such factors.
• Climatic : Glaciers, dry period, hails and storms,. frost, fire etc. may also destroy the community.
• Biotic : Man is most important cause, responsible for destruction of forests, grasslands for various purposes such as industry, agriculture, housing etc. Other factors are disease epidemics due to fungi, viruses etc. that destroy the whole population.

Invasion : This is the successful establishment of a species in a bare area. The species actually reaches this new site from any other area. This whole process is completed in following three successive stages :

• Migration (dispersal) : The seeds, spores or other propagules of the species reach the bare area. This process, known as migration, is generally brought about by air, water, etc.
• Ecesis (establishment) : The process of successful establishment of the species after reaching new areas, as a result of adjustment with the conditions prevailing there, is known as ecesis. In plants, after migration, seeds or propagules germinate, seedlings grow, and adults start to reproduce. Only a few of them are capable of doing this under primitive harsh conditions, and thus most of them disappear. Thus as a result of ecesis, the individuals of a species become established in the area.
• Aggregation : After ecesis, as a result of reproduction, the individuals of the species increase in number, and they come close to each other. This process is known as aggregation.

Competition and co-action : After aggregation of a large number of individuals of the species on the limited place, there develops competition (inter- as well as intra specific) mainly for space and nutrition. Individuals of a species affect each other’s life in various ways and this is called co-action. The species, if unable to compete with other species, if present, would be discarded. To withstand competition; reproductive capacity, wide ecological amplitude etc., are of much help to the species.

Reaction : This is the most important stage in succession. The mechanism of the modification of the environment through the influence of living organisms on it, is known as reaction. As a result of reactions, changes take place in soil, water, light conditions, temperature etc., of the environment. Due to all these the environment is modified, becoming unsuitable for the existing community which sooner or later is replaced by another community (seral ommunity). The whole sequence of communities which replaces one another in the given area is called a sere, and various communities in the sere, as seral communities, seral stages or developmental stages. The pioneers are likely to have low nutrient requirements, more dynamic and able to take minerals in comparatively more complex forms. They are small sized and make less demand from environment.

Stabilization (climax) : Finally, there occurs a stage in the process, when the final terminal community becomes more or less stabilized for a longer period of time and it can maintain itself in equilibrium with the climate of the area. This final community is never replaced, and is known as climax community and the stage as climax stage. The species which colonize the bare area in the beginning of succession arc called pioneers.

Retrogressive succession : It is one in which continuous biotic influences have some degenerating influence on the process. Due to destructive effects of organisms, sometimes the development of disturbed communities does not occur and the process of succession instead of progressive becomes retrogressive. As for example forest may change to shrubby or grassland community. This is called retrogressive succession.

Deflected succession : Sometimes due to changes in local conditions as soil characteristics or microclimate, the process of succession becomes deflected in a different direction than that presumed under climatic condition of the area. Thus the climax communities are likely to be different from the presumed climatic climax community. This type of succession is called deflected succession. For example, in India, with a monsoon type of climate, in some habitats like temporary ponds, pools etc., it is common to observe each year the development of different kinds of communities in different seasons of the year.

SUCCESSION ON A BARE ROCK (XERARCH)

Xerosere begins on barren or naked rocks or rocky substratum. Rock surface is devoid of soil without which rooted plants can not grow and colonize. No free minerals and nutrients are available on the surface of rocks. Moisture is also not retained and the environment is xeric (scarcity of water). The process of transformation of a barren rock into a forest site refers to xerosere (Lithosere).

Various stages of xerosere are as follows :

• Pioneer Community : Only the lichens have the capacity to grow over barren rocks. Initially the crustose lichens like Graphis, Haemotomma, Lecanora, Lecidia, Rhizocarpon, Strigula or Verrucaria appear on the surface of bare rocks.Their soredia, cephalodia or isidia are dispersed by wind (air borne) and wherever they find bare rocks, settle and grow into the thalli of crustose lichens They develop inconsipicous, thin, flat crust like hard thalli closely adhered to the substratum by their rhizines. Only their fruiting bodies are seen above the substratum. They are the first to survive on dry bare rocks. Their dead and decaying thalli produce a thin layer of humus over the rock surface and in this process of decomposition, certain acids are produced to bring about chemical weathering of rock. Gradually the environment of rock changes and a thin layer of soil is produced with enough humus which invites foliose and fruticose lichens to form seral community by replacing pioneers.
• Seral Communities : Several communities appear and disappear during xerosere and this process continues till the climax is attained.

The dominant seral communities are as under in order of their appearance :

• Foliose lichen stage : The thin soil layer over the rock surface provides suitable environment to foliose lichens like Parmelia, Physcia, Peltigera, Xanthoria, Gyrophora or Dermatocarpa. They produce large sized and leaf-like lobed crustose forms from the substratum by their dominance. They produce organic matter for humification and mineralization at a much faster rate and bring about decomposition of rocks through chemical weathering. As the thickness of soil increases, the foliose lichens are replaced by mosses and liverworts.
• Moss stage : Mosses and liverworts both need a thin layer of soil for attachment of their rhizoids. Mosses grow in pure patches and develop cushion like or carpet like dense growth. Funaria, Pogonatum, Polytrichum, Grimmia, Tortula appear on such substratum and they replace the lichens altogether by competing for space, nutrition and light. Certain fruticose lichens may also be seen at this stage intermingled with mosses. Thick carpet of moss contribute organic matter to the substratum and its decomposition helps in further weathering of rock to increase thickness of soil layer. Liverworts are next to invade this stage they produce large thalli and form an interwoven net of super imposed thalli. Marchantia, Pellia, Plagiochasma, Riccia are the common liverworts which invade moss stage.
• Herb Stage : Mosses and liverworts interact with the environment very efficiently and help in formation of a thick soil ayer over the rock (bed-rock) which invites herbaceous vegetation for invasion. They establish firm attachment with the soil as they produce roots for attachment as well as absorption. At this stage, xeric environment is converted to mesic condition because the soil layer retains enough moisture in their fine capillaries. Humus also retains moisture. The lower plants have altogether being replaced by higher plants at this stage and rate of accumulation of organic matter further increases. Initially, species diversity increases but later declines. Tridax, Justicia, Evolvulus,Convolvulus, Trifolium, Desmodium, Lindenbergia, Ageratun, Vernonia like herbaceous plants colonize well during this stage. They have drought resistance capacity also and often form pure patches.
• Shrub Stage : Gradually the herbaceous plants are replaced by woody shrubs from this area as they are more efficient and aggressive. Seeds and propagules of shrubs migrate from nearby area and gradually, establish themselves by replacing herbaceous vegetation. They prefer open sun and capable of growing under diverse moisture conditions. They prevent wind erosion by minimizing wind velocity. They conserve soil moisture and improve soil fertility. Their roots help in chemical and biological weathering of rocks and their fallen leaves, flowers, fruits or twigs add enough organic matter in soil. Capparis, Zizyphus, Clerodendron, Cassia, Zygophyllum, Urena, Crotalaria, Tephrosia, etc. are the common shrubs which appear at this stage.

Fig. : Diagrammatic representation of different plant communities of a lithosere appearing on a rock

• Climax Community : Shrub stage is replaced by woodland stage which finally develops into a forest with unique biodiversity but domination of trees and shrubs. Since the soil is quite thick by now, tree seedlings establish well. They exhibit very slow growth in the beginning but later on, they exhibit vigorous growth to form a woodland. In temperate region, evergreens and conifers predominate the forest vegetation (Pines, Abies, Picea, Taxus, Cephalotaxus, Oak, Birch or Maple) whereas, in tropical region, deciduous forms are common (Acacia, Prosopis, Anogeissus, Boswellia, Capparia, Shorea, Dalbergia or Tectona). Types of climax community depends upon the cliamte. Therefore, it is also called climatic climax community.

SUCCESSION IN AQUATIC ENVIRONMENT (HYDRACH)

This is biotic succession in newly formed pond/lake. Hydrosere begins from a pond by pioneer community belonging to phytoplanktons and ends into a climax community represented by forest when the pond dries up completely by interaction of several seral communities. Hydrosere is a type of primary succession and is usually autogenic in nature. In hydrosere, successive changes are well marked in various plant communities rather than the animal communities and thus communities are designated on the basis of dominant life-forms.

Various stages of hydrosere is summerised below :

• Pioneer Community : It is represented by phytoplanktons which comprise a wide assemblage of organisms belonging to Cyanophyceae, Chlorophyceae and Bacillariophyceae. They are capable of multiplying at a faster rate even though the available resources needed for their growth and multiplication are minimum. They have the ability to fix atmospheric nitrogen and multiply rapidly. Their dead organic remains go on depositing in mud and release nutrients through decomposition.
• Seral Communities : After pioneers, several seral communities come one after the other in the following order :

Fig. : Different plant communities appearing at different stages of a hydrosere originating in a pond

• Rooted submerged stage or Submerged stage : Dead organic remains of phytoplanktons develop a thin layer of soft mud at bottom which favours submerged hydrophytes like Hydrilla, Ceratophyllum, Najas, Vallisneria, Potamogeton, Utricularia or Myriophyllum. These plants have the potentiality of producing large amount of soft muds by donating their dead organic remains. They grow faster and gradually occupy the bottom area where light is available for their photosynthesis.
• Rooted floating leaved stage : When the thickness of soft mud at the bottom increases beyond 4 to 6 inches, rooted floating leaved hydrophytes like Nelumbo, Nymphaea, Nymphoides, Eurale or Victoria invade the pond. They have rhizomatous habit or produce tubers which remain burried in the soft mud and produce floating leaves with long petioles (some times 4-5 feet long) to float over the surface of water. They gradually spread throughout the shallow water and their leaves prevent penetration of light below, thus affecting adversely the growth of submerged hydrophytes.They transpire water at a faster rate, resulting into gradually drying of pond.
• Free-floating stage : Plants floating freely over the surface of water invade the pond after rooted floating leaved stage. Eichhornia, Lemna, Spirodela, Wolffia, Pistia, Azolla or Salvinea dominate in this stage of succession. They are capable of changing their place on the mercy of wind. They further eliminate the submerged communities by preventing entry of sun light into water. The water level now becomes very shallow and the pond reaches near drying. They contribute enough amount of organic matter for further built up of the substratum. The floating species now do not find proper place in such shallow pond where depth of water has reduced to less than a feet.
• Reed swamp stage : In this stage, attached emergent like Scirpus, Typha, Sagittaria, Monochoria, Eleocharis etc., predominate and form large pure patches due to having creeping rhizomes burried in mud and abundance of nutrients releasing from rapid decomposition of dead organic remains of previous occupants. The pond gradually converts into a swamp and presents an unfavourable environment for the true hydrophytes.
• Sedge or Marsh Meadow stage : Amphibians like Cyperus, Vitiveria, Juncus, Cymbopogon, Jussiaea, Ranunculus, Dentella, Limnophila, etc. Now start replacing the Reed swamp vegetation gradually and they develop terrestrial look to the pond. Amphibians have the capacity of enjoying aquatic as well as terrestrial environments and they will continue their growth till the shrubs and trees start invading the area. The soil becomes almost dry but possesses high level of fertility.
• Woodland stage : Shrubs and trees start appearing at this stage by gradually replacing amphibians. Salix, Populus (cottonwood), Alnus (Alder), Terminalia, Barringtonia, Eugenia etc., Predominate at this stage and the trace of a pond has altogether disappeared. This is the beginning of climax of the succession.
• Climax community or Climax Forest : In hydrosere, the climax is attained when the forest develops at this area predominated by trees with a few shrubs and ground flora. Forest represents a most stable community capable of maintaining a dynamic equilibrium with the prevailing environment conditions. Nature of forest and its vegetational composition depend upon the climatic and geographical conditions of the area. Deciduous trees predominate under the tropical environment (Shorea, Tectona, Eugenia, Terminalia, Bauhinia, Barrigtonia, Dalbergia, Acacia etc.), whereas, the evergreens under temperature environment (Pinus, Cedrus, Junipers, etc.). The area which was once in the form of a pond with only photoplanktonic population has now representing a dense forest as a result of succession or hydrosere.

CHANGES IN COMMUNITY CHARACTERISTICS AND CLIMAX

The differing habitat conditions during primary and secondary succession, sequence of species and communities varies. The seral stages vary from climax (late successional stage) with reference to structure and functions. Height of individuals usually increases in climax stage when compared with other seral stages.During successional changes, food webs become more complex.While attaining the climax stage, efficiency of energy use and nutrient conservation also increases. Variability and composition of climax stage is determined by prevailing climatic conditions, soil type, topography and water availability.