Ecosystem

Chapter 3

Ecosystem

INTRODUCTION

No life exists in a vacuum. Materials and forces which constitutes its environment and from which it must derive its needs surround every living organism. Thus, for its survival, a plant, an animal, or a microbe cannot remain completely aloof in a shell. Instead, it requires from its environment a supply of energy, a supply of materials, and a removal of waste products. For various basic requirements, each living organism has to depend on and also to interact with different nonliving or abiotic and living or biotic components or the environment.

•  Abiotic: The abiotic environmental components include basic inorganic elements and compounds such as water and carbon dioxide, calcium and oxygen, carbonates and phosphates besides such physical factors as soil, rainfall, temperature, moisture, winds, currents, and solar radiation with its concomitants of light and heat.

• Biotic: The biotic environmental factors comprise plants, animals, and microbes; They interact in a fundamentally energy-dependent fashion. In the words of Helena Curtis “The scientific study of the interactions of organisms with their physical environment and with each other, is called ecology”. According to Herreid II “It mainly concerns with the directive influences of abiotic and biotic environmental factors over the growth, distribution behaviors and survival of organisms.

Ecology Defined

• Ernst Haeckel (1866) defined ecology “as the body of knowledge concerning the economy of nature-the investigation of the total relations of animal to its inorganic and organic environment. (2) 

• Frederick Clements (1916) considered ecology to be “the science of community.

• British ecologist Charles Elton (1927) defined ecology as “the scientific natural history concerned with the sociology and economics of animals.” 

• Taylor (1936) defines ecology as “the science of the relations of all organisms to all their environments.”

• Taylor (1936) defined ecology as “the science of the relations of all organisms to all their environments.” 

• Allee (1949), considered ecology as “the science of inter-relations batwing living organisms and their environment, including both the physical and biotic environments, and emphasizing inter-species as well as intra-species relations.

• G.L. Clarke (1954) defined ecology as “the study of inter-relations of plants and animals with their environment which may include the influences of other plants and animals present as well as those of the physical features.” 

• Woodbury (1955) regarded ecology as “the science which in investigates organisms in relation to their environment: a philosophy in which the world of life is interpreted in terms of natural processes.

•  A. Macfadyen (1957) defined ecology as “ a science, which concerns itself with the inter-relationships of living organisms, plants and animals, and their environments.” 

• S.C. Kenleigh (1961, 1974) defined ecology as “the study of animals and plants in their relation to each other and to their environment.” Certain modern ecologists have provided somewhat broader definitions of ecology.

ECO-SYSTEM

• At present ecological studies are made at Eco-system level. At this level the units of study are quite large. This approach has the view that living organisms and their non-living environment are inseparably interrelated and interact with each other. A.G. Tansley (in 1935) defined the Eco-system as ‘the system resulting from the integrations of all the loving and non-living actors of the environment’. Thus he regarded the Eco-systems as including not only the organism complex but also the whole complex of physical factors forming the environment.

HISTORICAL BACKGROUND

The idea of Eco-system is quite an old one. We find in literature some such parallel terms as:

• Bioeconomic (Karl Mobius, 1977), (ii) microcosm (S.A. Forbes, 1887), 
• Geobiocoenosis (V.V. Doduchaev, 1846-1903); G.F. Morozov; see Sukachev, 1944), 
• Hlocoen (Frienderichs, 1930),  
• biosystem (Thienemann, 1939), 
• Bioinert body (Vernadsky, 1994), and ectosome etc. use for such ecological systems.

The terms ecosystems are most preferred, where ‘eco’ implies the environment, and ‘system’ implies an interacting, inter-dependent complex. In this way, it can be said that any unit that includes all the organisms i.e., the communities in a given area, interact with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity and material cycle (i.e., exchange of materials between living and non-living components) within the system, is known as an ecological system or eco-system.

Eco-system may be visualized as 3-dimensional cutouts from the ecosphere. All primary and secondary producers composing the ecosystem are its essential elements. The unique feature of eco-systems is the maintenance of their chemical state and of their environment.

Thus, an eco-system is an integrated unit, consisting of interacting plants and animals whose survival depends upon the maintenance of abiotic i.e., physicochemical environment and gradients such as moisture, wind and solar radiation with its concomitants of light and heat, as well as biotic structures and functions. The integrated unit may or may not be isolated, but it must have definable limits within which there are integrated functions. The physiologist's study various functions in individual plants or animals, but the ecologists study them at the eco-system level. A real ecologist endeavors for maintaining holistic or eco-system perspective of the process being studied by him

ASPECTS OF ECO-SYSTEM

The eco-system can be defined as any spatial or organizational unit including living organisms and non-living substances interacting to produce an exchange of materials between the living and non-living parts. The eco-system can be studied from either structural or functional aspects.

• Structural Aspect: The structural aspects of ecosystem include a description of the arrangement, types and numbers of species and their life histories, along with a description of the physical features of the environment.

• Functional: The functional aspects of the ecosystem include the flow of energy and the cycling of nutrients.

• Habitat: The non-living part of the eco-system includes different kinds of habitats such as air, water and land, and a variety of abiotic factors. Habitat can be defined as the natural abode or locality of an animal, plant or person. It includes all features of the environment in a given locality. For example, water is used as habitat by aquatic organisms, and it comprises three major categories-marine, brackish and freshwater habitats. Each of these categories. may be subdivided into smaller unit, such a freshwater habitat may exist as a large lake, a pond, a puddle, a river or a stream. The land is used as a habitat for numerous terrestrial organisms. It includes many major categories of landmasses, which are called biomes. Biomes are distinct large areas of earth inclusive of flora and fauna, e.g., deserts, prairie, tropical forests, etc. Soil is also used as a habitat by a variety of microbes, plants and animals.

Abiotic Factors: Among the main abiotic factors of the ecosystem are included the following:

• The climatic factors as solar radiation, temperature, wind, water currents, rainfall.
• The physical factors as light, fire, pressure, geomagnetism.
• Chemical factors as acidity, salinity and the availability of inorganic nutrients needed by plants.

Biotic or Biological Factors: The biological (biotic) factors of ecosystem include all the living organisms-plants, animals, bacteria and viruses. Each kind of living organism found in an ecosystem is given the name a species. A species includes individuals which have the following features:

• They are genetically alike. 
• They are capable of freely inter-breeding and producing fertile off springs.

Relationships: In an ecosystem, there exist various relationships between species. The relationship may be as under:

Effects: 

• They may have a negative effect upon one another (competition).
• They may have a neutral effect (neutralism). 
• They may have beneficial effect (protocol-operation and mutualism)

 Other kinds of Relationship

• The species may aggregate, or separate, or show a random relationship to one another.

Population:

• A population is a group of inter-acting individuals, usually of the same species, in a definable space. In this way we can speak of population of deer on an island, and the population of fishes in a pond. A balance between two aspects determines the size of a population of any given species:
• Its reproductive potential
• Its environmental resistance. 

In this way population size is determined by the relative number of organisms added to or removed from the group.

Factors Regulating Population: Following factors does population regulation:

• Physical attributes of the environment (e.g., climate)
• Food (quantity and quality)
• Disease (host-parasite relationships)
• Predation
• Competition (inter-specific and intra-specific)

An ecosystem contains numerous populations of different species of plants, animals and microbes; all of them interact with one another as a community and with the physical environment as well. A community or biotic community, thus, consists of the population of plants and animals living together in a particular place.

Division of Ecosystem

The ecosystem can be divided from the energetic viewpoint into three types of organisms: producers, consumers, and reducers. These can be explained as under:

• Producer: Photosynthetic algae, plants and bacteria are the producers of the ecosystem; all other organisms depend upon them directly or indirectly for food.
• Consumers: Consumers are herbivorous, carnivorous, and omnivorous animals; they eat the organic matter produced by other organisms.
• Reducers Reducers are heterotrophic organisms like animals; they are fungi and bacterial that decompose dead organic matter.

FOOD CHAINS OF FOOD WEB

Species are related by their feeding behaviors in food chains or food webs. There are two basic types of food chains as under-

• The consumer food chain includes the sequence of energy flow from producer+herbivore+carnivore+reducer
• The detritus food chain pypasses the consumers, going from producer+reducer

Basic Theme of Ecosystems

 Relationship: The first and foremost theme of an ecosystem in that everything is somehow or other related to everything else, the relationships include interlocking functioning of organisms. among themselves besides with their environment. Biocoenosis and bioecocoenois are roughly equivalent to community and ecosystem respectively. Biotopes are the physical environment in which such communities exist. According to Lamotte (1969), it is this network of multiple interactions that permits us to define the ecosystem completely. Many ecologists regard Interdependence as the first basic theme of ecology. Ecosystem includes interacting and interdependent components that are open and linked to each other.

Limitation: The second basis theme is Limitation which means that limits are ubiquitous, and that no individual or species goes on growing indefinitely. Various species control and limit their own growth in response to overcrowding or other environmental signals and the total numbers keep pace with the resources available.

Complexity: Complexity is a third characteristic of any eco-system. The three-dimensional interactions of the various constituent elements of an ecosystem are highly complex and often beyond the comprehension on the human brain.

GENERAL CHARACTERISTICS OF AN ECO-SYSTEM

1. The ecosystem is a major structural and functional unit of ecology. (2) 
2. The structure of an eco-system is related to its species diversity as such the more complex ecosystem has high species diversity. 
3. The relative amount of energy required to maintain an ecosystem depends on its structure. The more complex the structure, the lesser the energy it requires to maintain itself. 
4. The function of the ecosystem is related to energy flow in material cycling through and within the system. 
5. Ecosystems mature by passing from less complex to more complex states. Early stages of such succession have an excess of potential energy. Later (mature) stages have less energy accumulation.  Both the environment and the energy fixation in any given ecosystem are limited. They cannot be exceeded in any way without causing serious undesirable effect. 
6. Alterations in the environments represent selective pressures upon the population to which it must adjust. Organisms, which fail to adjust to the changed environment, must vanish.

To conclude the eco-system is an integrated unit or zone of variable size, it comprises vegetation, fauna, microbes and the environment. Most ecosystems process a well-defined soil, climate, flora and fauna and their own potential for adaptation, change and tolerance. The functioning of any ecosystem involves a series of cycles. These cycles are driven by energy flow, the energy being the solar energy

STRUCTURE OF ECO-SYSTEMS

Meaning of Structure

 By structure of an eco-system, we mean as under:

• The composition of biological community including species, numbers, biomass, life history and distribution in space etc.
• The quantity and distribution of the non-living materials, such as nutrients, water etc. 
• Structure of an ecosystem the range, or gradient of conditions of existence, such as temperature.

Natural And Function of Structure of Eco-system

The structure of an ecosystem is in fact, a description of the species of organisms that are present, including information on their life histories, population and distribution in space. It guides us to know who’s who in the ecosystem. It also includes descriptive information on the non-living features of ecosystem give us information about the range of climatic conditions that prevail in the area. From structural point of view all ecosystems consist of following two basic components:

Autotrophic Component of Producers: These are the components in which fixation of light energy use of simple inorganic substances and buildup of complex substance predominate.

• The component is constituted mainly by green plants, including photosynthetic bacteria. 
• To some lesser extent, chemosynthetic microbes also contribute to the buildup of organic matter
• Members of the autotrophic component are known as eco-system producers because they capture energy from non-organic sources, especially light, and store some of the energy in the form of chemical bonds, for the later use. 
• Algae of various types are the most important producers of aquatic eco-systems, although in estuaries and marshes, grasses may be important as producers. 
• Terrestrial ecosystems have trees, herbs, grasses, and mosses that contribute with varying importance to the production of the eco-systems.

Heterotrophic Component or Consumers: These are the components in which utilization; rearrangement and decomposition of complex materials predominate. The organisms involved are known as consumers, as they consume autotrophic organisms like bacterial and algae for their nutrition, the amount of energy that the producers capture, sets the limit on the availability of energy for the ecosystem. Thus, when a green plant captures a certain amount of energy from sunlight, it is said to produce the energy for the ecosystem. The consumers are further categorized as:

• Macroconsumers: Marcoconsumers are the consumers, which in a order as they occur in a food chain are, herbivores, carnivores (or omnivores).Herbivores are also known as primary consumers. (b) Secondary and tertiary consumers, if preset, are carnivores of omnivores. They all phagotrophs that include mainly animals that ingest other organic and particulate organic matter.

•  Microconsumers: These are popularly known as decomposers. They are saprotrophs (=osmotrophs) they include mainly bacteria, actinomycetes and fungi. They breakdown complex compounds of dead or living protoplasm, they absorb some of the decomposition or breakdown products. Besides, they release inorganic nutrients in environment, making them available again to autotrophs.

Standing Corp

The amount of living material in different trophic levels or in a component population is known as the standing corp. This term applies to both, plants as well as animals. The standing crop may be expressed in terms.

• Number of organisms per unit area.
• Biomass i.e., Organism mass in unit area, we can measure it as living weight, dry weight, ash-free dry weight of carbon weight, or calories or any other convenient unit suitable. 

Decomposers

• In the absence of decomposers, no ecosystem could function long. In their absence, dead organisms would pile up without rotting, as would waste products, It would not be long before and an essential element, phosphorus, for example, would be first in short supply and then gone altogether, the reason is the dead corpses littering the landscape would be hoarding the entire supply. The decomposers tear apart organisms and in their metabolic processes release to the environment atoms and molecules that can be reused again by autotrophic point of view. Instead, they are important from the material (nutrient) point of view. Energy cannot be recycled, but matter can be. Hence it is necessary to feed Energy into ecosystem to keep up with the dissipation of heat or the increase in entropy. Matter must be recycled again and again by an ecological process called biogeochemical cycle.

An Illustration

Abiotic Part 

The abiotic or non-living parts of a freshwater pond include the following:

• Water
• Dissolved oxygen
• Carbon Dioxide
• Inorganic salts such as phosphates, nitrates and chlorides of sodium, potassium, and calcium 
• A multitude of organic compounds such as amino acids, holmic acids, etc. according to the functions of the organisms, i.e., their contribution towards keeping the ecosystem operating as a stable, interacting whole.

 Produces: In a freshwater pond there are two types of producers.

• First are the larger plants growing along the shore or floating in shallow, water.
• Second are the microscopic floating plants, most of which are algae.

These tiny plants are collectively referred to as phytoplankton. They are usually not visible. They are visible only when they are present in great abundance and given the water a greenish tinge. Phytoplanktons are more significant as food producers for the freshwater pond ecosystem than are the more readily visible plants.

ECOLOGICAL PYRAMIDS

The main characteristic of each type of Ecosystem in Trophic structure, i.e., the interaction of food chain and the size metabolism relationship between the linearly arranged various biotic components of an ecosystem. We can show the trophic structure and function at successive trophic levels, as under:

• Producers 
• Herbivores 
• Carnivores

It may be known by means of ecological pyramids. In this pyramid the first or producer level constitutes the base of the pyramid. The successive levels, the three make the apex. Ecological pyramids are of three general types as under:

• Pyramid of numbers: It shows the number of individual organisms at each level, 
• Pyramid of energy: It shows the rate of energy flow and/or productivity at successive trophic levels. 
• Pyramid of energy: It shows the rate of energy flow and/or productivity at successive trophic levels.

The first two pyramids

• That is the pyramid of numbers and biomass may be upright or inverted. It depends upon the nature of the food chain in the particular ecosystem; However, the pyramids of energy are always upright.

• The energy pyramid gives the best picture of overall nature of the ecosystem. Here, number and weight of organisms at any level depends on the rate at which food is being produced. If we compare the pyramid of energy with the pyramids of numbers and biomass, which are pictures of the standing situations (organisms present at any moment), the pyramid of energy is a picture of the rates of passage of food mass through the food chain. It is always upright in shape.

• The pyramids of biomass are comparatively more fundamentalism; as the reason is they instead of geometric factor; show the quantitative relationships of the standing crops. The pyramids of biomass in different types of ecosystems may be compared as under: In grassland and forest there is generally a gradual decrease in biomass of organisms at successive levels from the producers to the top carnivores. In this way, the pyramids are upright. However, in a pond the producers are small organisms, their biomass is least, and this value gradually shows an increase towards the apex of the pyramid and the pyramids are made inverted in shape.

FUNCTION OF AN ECO-SYSTEM

• For a fuller understanding of ecosystems, a fuller understanding of their functions besides their structures is essential. The function of ecosystems includes the process how an eco-system works or operates in normal condition. From the operational viewpoint, the living and non-living components of ecosystem are interwoven into the fabric of nature. Hence their separation from each other becomes practically very much difficult. The producers, green plants, fix radiant energy and with the help of minerals (C, O, N, P, L, Ca, Mg, Zn, Fe etc.) taken from their soil and aerial environment (nutrient pool) they build up complex prefer to call the green plants as converters or transducers because in their opinion the terms ‘producer’ form an energy viewpoint which is somewhat misleading. They contend that green plants produce carbohydrates and not energy and since they convert or transducer radiant energy into chemical form, they must be better called the converters or transducers. However, the term’ producer’ is so widely used that it is preferred to retain it as such.

• While considering the function of an ecosystem, we describe the flow of energy and the cycling of nutrients. In other words, we are interested in things like how much sunlight plants trap in a year, how much plant material is eaten by herbivores, and how many herbivores carnivores eat.

Functions of Eco-system

• The solar radiation is major source of energy in the ecosystem. It is the basic input of energy entering the ecosystem. The green plants receive it. And is converted into heat energy. It is lost from the ecosystem to the atmosphere through plant communities. It is only a small proportion of radiant solar energy that is used by plant to make food through the process of photosynthesis. Green plants transform a part of solar energy into food energy or chemical energy. The green plants to develop their tissues use this energy. It is stored in the primary producers at the bottom of trophic levels. The chemical energy, which is stored at rapid level one, becomes the source of energy to the herbivorous animals at trophic level two of the food chain. Some portion energy is lost from trophic level one through respiration and some portion is transferred to plant-eating animals at trophic level two.

• It is seen that in the various biotic components of the ecosystem the energy flow is the main driving force of nutrient circulation. The organic and inorganic substances are moved reversibly through various closed system of cycles in the biosphere, atmosphere, hydrosphere and lithosphere. This activity is done in such a way that total mass of these substances remains almost the same and is always available to biotic communities.

• The organic elements of plants and animals are released in the under mentioned ways: Decomposition of leaf falls from the plant's dead plants and animals by decomposers and their conversion into soluble inorganic form. Burning of vegetation by lighting, accidental forest fire or deliberate action of man. When burnt, the portions of organic matter are released to the atmosphere and these again fall down, under the impact of precipitation, on the ground. Then they become soluble inorganic form of element to join soil storage, some portions in the form of ashes are decomposed by bacterial activities. The waste materials released by animals are decomposed by bacteria. They find their way in soluble inorganic form to soil storage.

• In the biogeochemical cycles are included the uptake of nutrients of inorganic elements by the plants through their roots. The nutrients are derived from the soil where these inorganic elements are stored. The decomposition of leaves, plants and animals and their conversion into soluble inorganic form are stored into soil contributing to the growth and development of plants. Decompositions are converged into some elements. These elements are easily used in development of plant tissues and plant growth by biochemical processes, mainly photosynthesis.

DECOMPOSERS

• In this world all living organisms require a constant supply of nutrients for growth. The death and decomposition of plants and animals, with release of nutrients constitutes an essential link in the maintenance of nutrient cycles. When an organism dies, an initial period of rapid leaching takes place and populations of macromolecules. The dead organism is disintegrated beyond recognition. Enzymic action breaks down the disintegrating parts of the litter. Animals invade and either eat the rapidly recolonized by micro- organisms, and the litter biomass decreases. It becomes simpler in structure and chemical composition.

Process of Decomposition

The process of decomposition involves three interrelated components, viz.

•  Leaching  
• Catabolism
• Comminution

Comminution to make small to reduce to power or minute particles. Comminution means the reduction in particle size of detritus. During the course of feeding, the decomposer animals' community detritus physically. And utilize the energy and nutrients for their own growth (secondary production). In due course, the decomposers themselves die and contribute to the detritus.

Function of Decomposition

The two major functions of decomposition within ecosystems are as under:

• The mineralization of essential elements
• The formation of soil organic matter to inorganic form. 

The formation of soil organic matter in nature is a slow process. The decomposition of any piece of plant detritus may take hundreds of years to complete. However, some residues of decomposition within this period do contribute to the formation of soil organic matter

 Community of Decomposer Organisms

• The community of decomposer organisms includes several bacteria, fungi, protests and invertebrates. The different species in such a community function in an integrated manner. For example, a fungus decomposes plant litter and is eaten by an animal. Upon death, bacteria decompose the animal, and protozoa may eat the bacteria

• Fungi and bacteria are the principal organisms that break down organic matter. Certain protozoa, nematodes, annelids and arthropods strongly influence their functioning (i.e., of fungi and bacteria) due to their feeling activities. Microarthropod fauna, comprising mainly of oribatid mites besides other mites and collembolans, are abundant in most forest, grassland and desert ecosystem. 

• In the same way, the interactions of micro-arthropods with soil fungi are also quite important in nutrient cycling. Studies of this aspect are made in mycorrhizal fungi and themicro-arthropods which feed upon these fungi:

1. It is found that Mycorrhizal pump massive amounts of nutrients form detritus and represent a sizable nutrient reservoir themselves. 
2. The orbited mites and other micro-arthropods feed on myocardial fungi they act like herbivorous pests and can alter nutrient relations/cycling in terrestrial ecosystems.
   

DECOMPOSERS WITH VARYING RELATIONS

Some decomposer organisms cannot be assigned a rigid or fixed position in the food web. Their trophic relations can vary from time to time.

 Nectroph: Some decomposers are nectrophs. They cause rapid death of the food source because they have a short-term exploitation of living organism. Nectrophs include may plant parasitic microbes as well as some herbivores, predators, and microtrophs (organisms which feed on living bacteria and fungi.).

Biotrophs: On the other hand, biotopes resort to a long-term exploitation of their living food resource. For example, root-feeding nematodes and aphids, obligate plant parasites, e.g., and mycorhizae and root nodules, etc.

Saprotophs: The apostrophes utilize food already dead, and most of the decomposers belong to this category.

Decomposers occupying different trophic levels

There are some such organisms causing decompositions as can occupy various trophic levels under different conditions. For instance, the root parasites like Fusarium and Thizoctonia are necrotrophs, which often show a saprotrophic tendency. In the same way, the predators (foxes and kites) sometime behave as saprotrophs. Biotrophs sometime act as necrotrophs or as saprotrophs.

Soul Invertebrates and Termites

There are some soil invertebrates e.g., earthworms and collembolans distribute organic matter throughout the soil whereas others e.g., termites and ants, concentrate it at localized sites around or near the royal chamber or in mounds. The following table shows the estimated activities of major groups of soil animals.

ENERGY-ITS FLOW IN ECOSYSTEM

Energy-Defined: Energy can be defined as the capacity to do work, whether that work be on a gross scale as raising mountains and moving air masses over continents, or on a small scale such as transmitting a nerve impulse from one cell to another.

Kinds of Energy: There are two kinds of energy, potential and kinetic. They can be explained as under: 

• Potential Energy 
• Kinetic Energy

Law of conservation of energy: The law of conservation of energy states that energy is neither created nor destroyed. It may change forms, pass from one place to another, or act upon matter in various ways. In this process no gain or loss in total energy occurs. Energy is simply transferred from one form or place to another.

Law of Decrease in Energy: The second law of thermodynamics states that on the transformation of from one kind to another, there is an increase in entropy and a decrease in the amount of useful energy. In this way, when coal in burned in a boiler to produce steam, some of the energy creates steam that performs work, but part of the energy is dispersed as heat to the surrounding air.

Single Channel Energy Model: Lindemann (1942) was the first to propose the community energetics approach or the trophic-dynamic model) to ecology, which enables an investigator to compare the relative rates at which different kinds concerning energy flow through forest ecosystems by the application of this kind of approach, e.g. by comparing ratios of leaf fall to litter deposition on the forest floor. His conclusion was that the rates of leaf production are higher and those of litter accumulation lower, in the tropics than at higher latitudes.

• The following conclusion can be drawn from the above figure:

• Out of the total incoming solar radiation (118,872 g cal/cm2 /yr), 118,761 gcal/cm2 /yr remain unutilized. In this way, the gross production (net production plus respiration) by autotrophs comes to be 111 gcal/cm2 /yr with an efficiency of energy capture of 0.10 per sent.

• Again 21 per cent of this energy, or 23 gcal/cm2 /yr (show on the bottom as respiration) is consumed in metabolic reactions of autotrophs for their growth, development, maintenance and reproduction.

•  15 gcl/cm2 /yr are consumed by herbivores that graze of feed on autographs-this figure amounts to 17 per cent of net autotroph production.

• Decomposition is 3 gcal/cm2 /yr which amount to be 3, 4 per cent of net production.

• The remainder of the plant material, 70 gcal/cm2 /yr of 79.5 per cent production, is not utilized. It becomes part of the accumulating sediments. Apparently much more energy is available for herbivory than is consumed.

We may conclude the following conclusions.

• Various pathways of loss are equivalent to and account for total energy capture of the autotrophs i.e. gross production. 

• The three upper ‘fates’ i.e., decomposition, herbivory and not utilized collectively are equivalent to net production. 

• Of the total energy, which is incorporated at the herbivory level, i.e., 15/ gcal/cm2yr, 30 percent of 4.5 gcal/cm2 /yr is used in metabolic reactions. 

• In this way more energy is lost via respiration by herbivores (30 percent) than by autotrophs (21 percent).

• Considerable energy is available for the carnivores, namely 10.5 gcal/cm2 /yr or 70-per cent. It is not entirely utilized, merely 3.0 gcal/cm2 /or 28.6 per cent of net production passes to the carnivores. This utilization of resources is evidently more efficient than the one, which occurs at autotroph-herbivore transfer level.

This is a simplified energy flow diagram:

• The diagram depicts three trophic levels. Boxes numbered 1, 2, 3 in a leaner food chain exhibit these. 
• L. shows total energy input (3000). 
• LA shows light absorbed by plant cover (1500).
• P.G. shows gross Primary production. 
• A shows total assimilation. 
• Pn shows net primary production. 
• P shows secondary (consumer) production. 
• Nu shows energy not used (stored or exported).
• NA shows energy not assimilated by consumers (egested).
• R shows respiration

It is very simplified energy flow model of three trophic levels

Apparently, the energy flows are greatly reduced at each successive trophic level from producers to herbivores and then to carnivores. It is reflected that at each transfer of energy from one level to another, major part of energy is lost as heat or other form. The energy flow is reduced successively. We may consider it in either term as under:

• In terms of total flow (i.e., total energy input and total assimilation)
• In terms of secondary production and respiration components

In this way of the 3,000 Kcal of total light, which falls upon the green plants, approximately 50 per cent (1500 Kcal) is absorbed. Only 1 per cent (15 Kcal) of it is converted at first trophic level. Thus net primary production comes to be at 15 Kcal. Secondary productivity (P2 and P3 in the diagram) is about 10 percent at successive consumer trophic levels in other words at the levels of herbivores and the carnivores. However, efficiency may be sometimes higher as 20 per cent, at the carnivore level as shown (or P3=0.3 Kcal) in the diagram.

ECOLOGICAL SUCCESSION-MEANING AND TYPES

Meaning of Succession

• Biotic communities are not static. Instead, they change through time. This change can be understood on several levels. The simplest level is the growth, interaction and death of individual organisms as they pass through their life cycles, affected by the cycles of seasons and other natural phenomena. Some other levels of community change act over longer time spans and that account for much larger changes in community composition and structure. These include ecological succession and community evolution. 

• It is evident from the above said that the term succession denotes a sequence of changes in the species composition of a community, which is generally associated with a sequence of changes in its structural and functional properties. The term is generally used for temporal sequence (in terms of years, decades or centuries) of vegetation on a site; although only short-term changes can be observed directly, and the long-term ones are inferred from spatial sequences.

• The changes associated with succession are usually progressive or directional. This fact enables one to predict which species are likely to replace other in the course of a succession. Succession tends to continue until the species combinations best suited to the regional climate and the particular site are established.

Historical Background

• The basic idea of succession was in the beginning forwarded by Anon Kerner (1863) in his book “Plant Life of the Danube Basin” during the description of the regeneration of a swamp forest. The term ecological succession was first of all used by Hult (1885) in the study of communities of Southern Sweden. H.C. Cowles held that communities are not static but dynamic. This changed understanding be visualized as an orderly, directional and predictable phenomenon. It was added that succession is autogenic i.e., regulated by biotic interactions within the community. The central foundation of the classical theory was that early communities alter the environment to their detriment and Favour later successional communities. It was revealed by the later studies that allogenesis was perhaps more common and dominant than autogenesis; allogenesis means the control of community dynamics by factors originating outside the community boundaries.

• The succession of animals on these dunes was studied by ‘Shelford (1913). Later on, Olson (1958) restudies the ecosystem development on these dunes and has given us an updated information about it. Federick Clements (1907-1936) elaborates the principles and theory of succession. He proposed the miniclimax hypothesis of succession. During the later years certain other hypotheses were proposed by various ecologists to explain the nature of climax communities: for example, polyplex hypothesis by Braun-Blanquet (1932) and Tansley (1939): climax pattern hypothesis by Whittaker (1953), Mac in tosh (1958) and Selleck (1960): and stored energy theory of information theory by Fosberg (1965, 1967) and Odum (1969).

• Succession is an orderly process of reasonable directional and fairly predictable community development.

• Succession results from modification of the physical environment by a community, i.e., succession is largely community controlled. 

• Succession culminates in a stabilized ecosystem in which maximum biomass and symbiotic function between organisms are maintained per unit of available energy flow. Whittaker (1975) held that through the course of succession community production, height and mass, species-diversity, relative stability, and soil depth and differentiation generally all tend to increase. The culminating point of succession is a climax community of relatively stable species composition and steady-state function, It is adapted to its habitat. It is permanent in its habitat if it is not disturbed.

SUCCESSION: GENERAL PROCESS, CLIMAX

General Process: The process of succession being with a bare area or nudation formed by several reasons, such as volcanic eruption, landslide, following sequential steps

1. Nudation: The process of succession begins with a bare area or nudation formed by several reasons, such as volcanic eruption, Landslide, flooding, erosion, deposit, fire, disease, or other catastrophic agency. Man, also may be reason of formation of new lifeless bare areas for example, walls, stone quarrying, burning, digging, flooding large land areas under reservoirs
2. Invasion The invasion means the arrival of the reproductive bodies or propagules of various organisms and their settlement in the new or bare area. Plants are the first invaders (pioneers) in any area the animals depend on them for food. The invasion includes the following three steps:

• Dispersal or migration: The seeds, spores or other propagules of the species reach the bare area through air, water or animals.
• Ecesis: Ecesis is the successful establishment of migrated plant species into the new area. It includes germination of seeds or propagules, growth of seedlings and starting of reproduction by adult plants
• Aggregation: In this stage, the successful immigrant individuals of a species increase their number by reproduction and aggregate in large population in the area. As a result, individuals of the species come close to one another.