Concept of

Introduction:

In , the living (plants, animals and ) and nonliving environment (e.g. water, air, , etc.) are inseparably interrelated and interact with each other. No living can exist by itself, or without an environment. Every organism uses energy, nutrients and water from its surrounding environment in various life activities. 1. The plants obtain the energy directly from the sun, and, in case of animals and microorganisms, energy is taken from other organisms through feeding on plants, , and/or . 2. The terrestrial plants obtain water mainly from soil, while animals get it from free standing water in the environment or from their food. 3. The plants obtain most of their nutrients from the soil or water, while animals get nutrients from plants or other organisms. Microorganisms are the most versatile, obtaining nutrients from soil, water, food, or other organisms.

As a result, the organisms interact with one another and with their environment in a number of ways. These fundamental interactions among organisms and their non-living/physico-chemical environment constitute an interrelating and interdependent ever-changing system known as an ecological system or ecosystem. The ecosystem has been considered as the basic functional unit of and ecology as study of . The togetherness of organisms and environment has been expressed in history by different ecologists. However, the formal terminologies began to appear in different parts of the world in late 1800s. Karl Mobius, a German scientist, in 1877 gave the term ‘biocoenosis’ to a of organisms in oyster reef; in 1887, S. A. Forbes, an American scientist, described lake as ‘microcosm’ and Russian ecologist, Sukachev in 1944, expanded it to ‘geobiocenosis’. Although the roots of ecosystem concept can be traced in 19th century, it is largely a twentieth century construct. A. J. Lotka came up with the idea of ecosystem and wrote in his book (entitled Elements of Physical (1925): “the organic and inorganic worlds function in a single system to such an extent that it is impossible to understand either part without understanding the whole.”

However, the term ‘Ecosystem’ was first coined in 1935 by the British ecologist Sir Arthur G. Tansley as part of a debate over the nature of biological communities: “Our natural human prejudices force us to consider the organisms as the most important parts of these systems, but certainly the inorganic “factors” are also parts - could be no systems without them, and there is a constant interchange of the most various kinds within each system, not only between the organisms but between the organic and the inorganic. These ecosystems, as we may call them, are of the most various kinds and sizes.”

Definitions of Ecosystem:

Tansley described the most fundamental nature of ecosystems – as a system in which biotic and abiotic components of environment are interrelated. The main focus is on the organisms in the definition and the nature of the “constant interchange of the most various kinds” is not made clear.

The great ecologist, E. P. Odum (1971) defined ecosystem as “Any unit that includes all of the organisms (i.e. the “community”) in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e. exchange of materials between living and nonliving parts) within the system is an ecological system or ecosystem” Thus, Odum describes explicitly that ecosystem is a geographical unit and plays a central role in defining structural and functional features of the ecosystem.

Allen and Hoekstra (1992) stated ecosystem as “The functional ecosystem is the conception where biota are explicitly linked to the abiotic world of their surroundings. Systems boundaries include the physical environment. Size is not the critical characteristic, rather the cycles and pathways of energy and matter in aggregate form the entire ecosystem.” They defined it as “functional ecosystem” and emphasized on the functional features such as nutrient cycling or trophic dynamics as much as what it contains or its size.

Characteristics of Ecosystem:

Though there may be differences in the definitions given by different authors, all have three common characteristics – , abiotic environment and interactions between these two. The biotic component of ecosystem generally consists of communities of organisms, and abiotic component includes the physico-chemical environment surrounding them. Interactions may be numerous including food webs, trophic dynamics, nutrients cycling, flow of energy, etc. It has held a central position in modern ecology and environmental sciences. Modern ecology is now defined as “the study of structure and functions of ecosystems.” Now a day, most of the environmental management strategies include recognition of ecosystems as a way of ordering our perception of nature. Ecosystems differ greatly in their composition - in the number and kind of species, the type and relative proportions of non-living constituents, and in the degree of variations in time and space. A forest, a grassland, a pond, a coral reef, a part of any field and a laboratory culture can be some examples of ecosystem. The size of ecosystems varies tremendously. An ecosystem could be an entire rain forest, covering a large geographical area, or it could be a single tree inhabiting a large no. of birds and/or microorganisms in its leaf litter. It could be a termite’s gut, a lake or the biosphere as a whole with an entire intertwined environment of earth. The number of ecosystems on earth is countless and each ecosystem is distinct.

All ecosystems have the following common characteristics as given by Smith (1966):

1. The ecosystem is the major structural and functional unit of ecology. 2. The structure of an ecosystem is related to its ; the more complex ecosystems have high species diversity. 3. The function of ecosystem is related to energy flow and material cycling through and within the system. 4. The relative amount of energy needed to maintain an ecosystem depends on its structure. The more complex the structure, the lesser the energy it needs to maintain itself. 5. Ecosystems mature by passing from less complex to more complex stages. Early stages of such succession have an excess of potential energy and a relatively high energy flow per unit . Later (mature) stages have less energy accumulation and its flow through more diverse components. 6. Both the environment and energy fixation in any given ecosystem are limited and cannot be exceeded without causing serious undesirable effects. 7. Alterations in the environment represent selective pressures upon the population to which it must adjust. Organisms which are unable to adjust to the changed environment disappear ultimately.

All ecosystems have a feeding hierarchy which starts with an energy source (e.g. the sun) and then followed by producers, consumers and . These components are dependent on one another. One of the important features is presence of grazing or and webs which become the lifeline of ecosystems. In grazing food chain and webs, green plants (i.e. producers) synthesize food from non-living nutrients with the help of the sunlight in the process of . Animals (i.e. consumers) consume plants and other animals to get the nutrients. When plants and animals die and decay or when animals excrete waste, bacteria and fungi (i.e. decomposers) feed on the dead or waste materials and release the nutrients back into water and/or soil for reuse by the producers. In a detritus food chain or web, the energy comes from dead organic matter (i.e. detritus) instead of green producers. One example of a detritus is the ecosystem of a deciduous forest floor. Ecosystems are sustained by the presence of . Each organism in an ecosystem has a purpose (i.e. niche), as a result, the loss of one species can alter both the size and stability of ecosystems. In a whole, the ecosystems are open systems – depicting that things are entering and leaving the system, even though the general appearance and basic functions may remain constant for long periods of time.

Structure of Ecosystem:

The ecosystem is largely divided into two components - Abiotic and Biotic components. Ecosystem structure is created due to interaction between abiotic and biotic components, varying over space and time.

1. Abiotic Components:

The abiotic components of an ecosystem refer to the physical environment or the non-living factors. The organisms cannot live or survive without their abiotic components. They mainly include: i) Inorganic substances required by organisms such as , water, nitrogen, calcium, phosphorus, etc. that are involved in material cycles. The amount of these inorganic substances present at any given time in ecosystem is called as standing state or standing quality of ecosystem. ii) Organic compounds like proteins, carbohydrates, amino acids, lipids, humic substances and others are synthesized by the biotic counterpart of an ecosystem. They make biochemical structure of ecosystem. iii) Climatic factors including mainly rain, light, temperature, humidity, wind and air and iv) Edaphic and other factors such as minerals, soil, topography, pH, etc. greatly determine the functions, distribution, structure, behavior and inter-relationship of organisms in a .

2. Biotic Components:

The biotic components of the ecosystems are the living organisms including plants, animals and microorganisms. Based on their nutritional requirement, i.e. how they get their food, they are categorized into three groups – i) Producers are mainly the green plants with chlorophyll which gives them the ability to use solar energy to manufacture their own food using simple inorganic abiotic substances, through the process of photosynthesis. They are also called as photoautotrophs (photo-light, auto-self, troph- nutrition). This group is mainly constituted by green plants, herbs, shrubs, trees, phytoplanktons, algae, mosses, etc. There are some chemosynthetic bacteria (sulphur bacteria) deep beneath in the ocean which can synthesize their food in absence of sunlight, thus known as chemoautotrophs (chemo-chemical, auto-self, troph-nutrition). ii) Consumers lack chlorophyll, so they depend on producers for food. They are also known as . They mainly include herbivorous (feed on plants), carnivorous (feed on other animals), omnivorous (feed on both plants and animals) and organisms (feed on dead parts, waste, remains, etc. of plants and animals,). iii) Decomposers (saprotrophs) are the microorganisms, bacteria and fungi, which break down complex dead organic matter into simple inorganic forms, absorb some of the decomposition products, and release inorganic nutrients that are reused by the producers.

All ecosystems have their own set of producers, consumers and decomposers which are specific to that ecosystem. The nutritional relationship among different biotic components of an ecosystem is shown in Fig 1.

Fig 1: Nutritional relationship among different biotic components of an ecosystem

Function of Ecosystem:

Ecosystem functions are the physical, biological and geochemical processes that take place or occur within an ecosystem. Or simply, we can say ecosystem functions relate to the structural components of an ecosystem (e.g. plants, water, soil, air and other living organisms) and how they interact with each other, within ecosystem and across ecosystems. Every ecosystem performs under natural conditions in a systematic way. It receives energy from sun and passes it on through various biotic components and in fact, all life depends upon this flow of energy. Besides energy, various nutrients and water are also required for life processes which are exchanged by the biotic components within themselves and with their abiotic components within or outside the ecosystem. The biotic components also regulate themselves in a very systematic manner and show mechanisms to encounter some degree of environmental stress.

The structure and function of ecosystems are very closely related and influence each other so intimately that they need to be studied together. Despite the broad spectrum and great variety of functions in nature, the simple –– classification is a good working arrangement for describing the ecological structure of a biotic community. Production, consumption and decomposition are useful terms for describing overall functions. These and other ecological categories pertain to functions and not necessarily to species as such, because a particular species population may be involved in more than one basic function. For example, individual species of bacteria, fungi, protozoa and algae may be quite specialized metabolically, but collectively these lower phyla organisms are extremely versatile and can perform numerous biochemical transformations. Table 1 represents various structural and functional aspects of an ecosystem.

Structural Aspects Functional Aspects a) Biotic components a) Food chains and food webs Producers, consumers and decomposers. b) Energy flow b) Abiotic components c) Nutrient cycling Inorganic substances (C, H, O, N, P, S, d) Ecosystem processes to explain etc.), organic compounds (proteins, amino interactions among the components acids, lipids, carbohydrates, humic of ecosystem substances, etc.), climate and its e) Ecosystem development components (temperature, humidity, f) Ecosystem regulation and stability moisture, sunlight, rainfall, wind, air etc.), g) Ecosystem services edaphic and other factors (minerals, soil, topography, pH, etc.)

Table 1: Structural and Functional aspects of an Ecosystem a) Food chains and Food webs:

The flow of energy is mediated through a series of feeding relationships in a definite sequence or pattern i.e. from producers to primary consumers to secondary consumers and to tertiary consumers. Nutrients too move in along this food chain. The sequence of eating and being eaten in an ecosystem is known as food chain. All organisms, living or dead are potential food for some other organism and thus, there is essentially no waste in the functioning of a natural ecosystem.

Some common examples of food chains are:

Grass → Grasshopper → Frog → Snake → Hawk (Grassland ecosystem)

Plants → Deer → Lion (Forest ecosystem) Phytoplanktons → Zooplanktons → Small fish → Large fish (Pond ecosystem)

Each organism in the ecosystem is assigned a feeding level or depending on its nutritional status. Thus, in grassland food chain, grass occupies 1st trophic level, grasshopper the 2nd, frog the 3rd, snake the 4th and hawk 5th trophic level. At each trophic level some energy is lost as heat and respiration, as a result available energy decreases moving away from the first trophic level. Therefore, the number of trophic levels in a food chain is limited. The decomposers consume the dead organic matter of all these trophic levels.

The food chains can be of two types –

1. Grazing food chain:

The food chain that starts from green plants and ends in a .

Some examples are:

Grass → Insect → Sparrow → Eagle

Tree → Bird → Snake → Hawk

Plants → Deer → Tiger

2. Detritus food chain:

In many cases, the principal energy input is not green plants but dead organic matter. These are called detritus food chains. The detritus food chains are commonly found in forest floors, salt marshes and the ocean floors in very deep areas.

Example of detritus food chain is as follows:

Leaf litter → Bacteria → Protozoa → Small fish → Large fish

It is very important to know about the food chains, because certain animals eat only particular type of animals or plants. A balance is maintained in the entire ecosystem through these feeding relationships. The food chains keep a check on the of organisms. If in a grassland ecosystem, the deer population increases, grass reduces; but when grass reduces, the deer that feed on it will also be checked in their population size. So, the grass and other plants will get time to grow again.

The food chains also exhibit the property of or biological magnification. Certain chemicals, heavy metals or pesticide are either slowly degradable or non-biodegradable in nature. As they enter the food chain in low concentrations, they tend to accumulate at each trophic level and as a result, an increase in their concentrations is exhibited with increase in trophic level of a food chain. In this way, the top trophic level is worse affected due to high accumulation of the chemicals in organisms.

The real world is more complicated than a simple food chain. While many organisms specialize in their diets (e.g. anteaters), other organisms do not. Hawks don’t limit their diets to snakes, snakes eat things other than mice, and mice eat grass as well as grasshoppers. A more realistic representation of who eats whom is called a food web. A food web is defined as a network of interwoven food chains with numerous producers, consumers and decomposers operating simultaneously at each trophic level so that there are a number of options of eating and being eaten at each trophic level.

Food webs can get quite complex with several interconnected food chains. They give greater stability to an ecosystem. In a linear food chain, if one species become extinct or one species suffers, then the species in the subsequent trophic levels are also affected. In a food web, on the other hand, there are a number of options available at each trophic level. So if one species is affected, it does not affect other trophic levels so seriously. b) Energy Flow:

Everything that organisms do in ecosystems (breathing, running, burrowing, growing) all require energy. So, how do they get it? In an ecosystem, there is a continuous interaction between plants, animals, and their environment to produce and exchange materials. The energy needed for this material cycling comes from the sun. Sun is the ultimate source of energy, directly or indirectly, for all other forms. The green plants capture the solar energy and convert it through the process of photosynthesis into chemical energy of food (organic matter) and store it into their body. This process is called as . The rate of total organic matter production by green plants (primary producers) is known as gross primary . The green plants use some of the energy in the process of respiration. Rest amount of energy is called as net primary production, the amount of energy left for the heterotrophic organisms. In this stored form, other organisms take the energy and pass it on further to other organisms. During this process, a reasonable proportion of energy is lost out of the living system. At the consumer level, the rate of assimilation of energy is called secondary productivity. The whole process is called as flow of energy. The most important feature of this energy flow is unidirectional or one-way or non-cyclic flow. It flows from producer to to organisms; it is never reused back in the food chain unlike the nutrients which move in a cycle (Fig 2). As the flow of energy takes place, there is a gradual loss of energy at each level.

Primary productivity of an ecosystem depends upon the solar radiations, availability of water, nutrients and upon the plants and their chlorophyll content. Productivity of tropical rainforest and estuaries is highest. The greater productivity of tropical rainforests to a large extent is due to the favourable combination of high incident solar radiation, warm temperatures, abundant rainfall, and rich diversity of species. These factors result into longer, almost year-round growing season. In estuaries, the natural wave currents bring lots of nutrients with them congenial for growth. On the other hand, ecosystems have limitations of adequate water supply while tundra ecosystems have low water temperature as and hence show low primary production.

Fig 2: Schematic diagram showing unidirectional flow of energy and nutrients cycling in an ecosystem. c) Nutrient Cycling:

We have already seen that while energy does not recycle through an ecosystem, nutrients do. Since the inorganic elements move through both the biological and geological world, we call them biogeochemical cycles. Of the 30 to 40 elements necessary to life, six rank as the most important: carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus. These nutrients move from non-living to the living and back to the non-living again in a cyclic manner (Fig 2). Each element has its own unique cycle, but all the cycles do have some things in common. Reservoirs are those parts of the cycle where the chemical is held in large quantities for long periods of time. In exchange pools the chemical is held for only a short time. The length of time a chemical is held in an exchange pool or a reservoir is known as residence time. The oceans are the reservoir for water, while a cloud is an exchange pool. Water may reside in an ocean for thousands of years, but in a cloud for a few days only. The biotic community includes all living organisms. This community may serve as an exchange pool and serve to move chemicals from one stage of the cycle to another. For example, the trees of the tropical rain forest bring water up from the forest floor to be evaporated into the atmosphere. The energy from most of the transportation of chemicals from one place to another is provided either by the sun or by the heat released from the mantle and core of the earth.

The biogeochemical cycles are of two basic types: – i) gaseous cycles - such as nitrogen and carbon, the reservoir is in the atmosphere or hydrosphere (ocean); ii) sedimentary cycles – such as , the reservoir is in the lithosphere. The nutrients are first taken up by the producers, bound in the organic compounds (carbohydrates, proteins, lipids, etc.) and move along the food chain to heterotrophic level and ultimately from alltrophic levels, with the detritus, to the decomposers. The decomposers break down the complex organic compounds and release the nutrients back to the soil from where they are again taken up by the plants, thus making the cycle complete. These biogeochemical cycles give an insight in how human activities lead to eutrophication in water bodies and global climate change. i) Carbon Cycle:

Carbon, the basic building block of life molecules, is circulated through the carbon cycle.

This cycle shows that carbon may be present as gaseous atmospheric CO2, dissolved in groundwater as HCO3 or molecular CO2 (aq), in underlying rock strata as limestone (CaCO3), and as organic matter, represented in a simplified manner as (CH2O). Photosynthesis fixes inorganic carbon as biological carbon, which is a constituent of all life molecules. An important aspect of the carbon cycle is that it is the cycle by which energy is transferred to biological systems. Organic or biological carbon, (CH2O), is an energy-rich molecule that can react biochemically with molecular oxygen, O2, to regenerate carbon dioxide and produce energy. This can occur in an organism as shown by the “decay” reaction or it may take place as combustion, such as when wood is burned.

Fig 3: Showing Carbon cycle ii) :

The oxygen cycle involves the interchange of oxygen between the elemental form of gaseous O2 in the atmosphere and chemically bound O in CO2, H2O, and organic matter. Elemental oxygen becomes chemically bound by various energy-yielding processes, particularly combustion and metabolic processes in organisms. It is released during photosynthesis.

Fig 4: Showing Oxygen Cycle iii) :

Nitrogen, though constituting much less of biomass than carbon or oxygen, is an essential constituent of proteins. The atmosphere is 78% by volume elemental nitrogen, N2 and constitutes an inexhaustible reservoir of this essential element. The N2 molecule is very stable so that breaking it down to atoms that can be incorporated in inorganic and organic chemical forms of nitrogen is the limiting step in the nitrogen cycle. This does occur by highly energetic processes in lightning discharges such that nitrogen becomes chemically combined with hydrogen or oxygen as ammonia or nitrogen oxides. Elemental nitrogen is also incorporated into chemically bound forms or fixed by biochemical processes mediated by microorganisms. The biological nitrogen is returned to the inorganic form during the decay of biomass by a process called mineralization.

Fig 5: Showing Nitrogen Cycle iv) Phosphorus Cycle: The phosphorus cycle is crucial because phosphorus is usually the limiting nutrient in ecosystems. There are no common stable gaseous forms of phosphorus, so the phosphorus cycle is strictly sedimentary. In the geosphere phosphorus is held largely in poorly soluble minerals, such as hydroxyapatite, a calcium salt. Soluble phosphorus from these minerals and other sources, such as fertilizers, is taken up by plants and incorporated into the nucleic acids of biomass. Mineralization of biomass by microbial decay returns phosphorus to the salt solution from which it may precipitate as mineral matter.

Fig 6: Showing Phosphorus Cycle v) Cycle:

The sulfur cycle is relatively complex. It involves several gaseous species, poorly soluble minerals, and several species in solution. It is involved with the oxygen cycle in that sulfur combines with oxygen to form gaseous sulfur di oxide (SO2) an atmospheric pollutant, and soluble 2- sulfate ion, (SO4 ). Among the significant species involved in the sulfur cycle are gaseous hydrogen sulfide, H2S; mineral sulfides, such as PbS; sulfuric acid, H2SO4, the main constituent of acid rain; and biologically bound sulfur in sulfur-containing proteins.

Fig 7: Showing Sulfur Cycle d) Ecosystem Processes:

Ecosystem processes include the processes by which transfer of energy and nutrients take place in between biotic and abiotic components of ecosystem. The storage and fluxes are two main paths of ecosystem processes. Storage means accumulation of chemical compounds in the ecosystem components and flux indicates the conversion or movement of stored chemicals from one component to another e.g. via food chains and food webs. These processes mainly include synthesis of food by the producers, transfer of energy and nutrients contained in food through different levels of food chains and food webs, returning back of nutrients to the soil by decomposers. The various ecosystem processes and their controlling factors are briefly described as follows: i) Photosynthesis:

The primary function of photosynthesis is to convert solar energy into chemical energy, mediated by the plants. The planet’s living systems are powered by this process. In the presence of sunlight, green plants take carbon, hydrogen and oxygen from carbon dioxide and water, and then recombine them into glucose molecule and O2 as a byproduct.

In the process of photosynthesis, carbon dioxide is fixed (or reduced) to carbohydrates

(glucose; C6H12O6) and water is split in the presence of light to release O2 molecule. It is to mention here that O2 released comes from the water molecule and not from CO2. The overall photosynthesis process can be written as:

Carbon Dioxide + Water + Light Carbohydrate + Oxygen

and can be represented by the following chemical equation:

Chlorophyll 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Sunlight

Energy is stored in the bonds of glucose molecule (C6H12O6). This energy is transferred into animals and humans when they consume plants. Further, O2 released in the process is used by plants and animals during respiration. Thus, photosynthesis and respiration go hand in hand (Fig 8). The process of respiration breaks apart the glucose molecule and the released energy is then used for all metabolic processes.

Fig 8: Schematic diagram of Photosynthesis at Trophic level ii) Respiration:

The organic matter produced in the process of photosynthesis is oxidized back to CO2 by all sources including combustion or by the respiration of plants, animals, and microbes. Respiration is the exergonic biochemical process in which glucose and oxygen combines to release

CO2, water, and energy. The released energy in form of ATP is utilized by the cells to perform the physiological activities of the organisms (Ricklefs and Miller, 2000; Taiz and Zeiger; 2006 and Singh et. al., 2014). Respiration may be aerobic (with oxygen) or anaerobic (without oxygen). Aerobic

Respiration is reverse of photosynthesis. It is the process by which organic matter (CH2O) is decomposed back to CO2 and H2O with a release of energy. The equation of aerobic respiration is written as follows:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

All the higher plants and animals obtain their energy for maintenance and for formation of cellular material in this manner. During photosynthesis, solar energy is captured and stored in high energy bonds in carbohydrate (such as glucose, C6H12O6). The later is used by the plants or ingested by the heterotrophs. The energy contained in carbohydrates is released during respiration via glycolysis and the Kreb’s cycle, carbon dioxide and water are also released. In virtually all ecosystems, photosynthetic provide energy for the total system. Thus, the ultimate source of energy in the system is the Sun. A sketch of the ecosystem respiration is illustrated in Fig 9.

Fig 9: Schematic diagram of Ecosystem Respiration

Anaerobic respiration occurs in saprophages, such as bacteria, yeasts, molds, protozoa, etc. The bacteria are examples of obligate anaerobes that decompose organic compounds with the release of methane gas. Desulfovibrio and other variety of sulphate reducing bacteria are ecologically important examples of anaerobic respiration because they reduce SO4 to H2S gas in deep sediments and in anoxic waters. The H2S gas rises to the surface where it can be oxidized by other organisms (for example, photosynthetic sulphur bacteria). Many bacteria, like Aerobacter, are capable of both aerobic and anaerobic respiration, but the end products of these reactions is different and amount of energy released is very less in anaerobic conditions. iii) Decomposition:

Decomposition is a biological process of breakdown of complex organic materials (carbohydrates, proteins, amino acids, nucleic acids, etc.) into the simpler one (CO2, H2O and inorganic nutrients like N, P, S, etc.). It results from both biotic and abiotic processes. For example, forest fire release large quantities of CO2 and other gases into atmosphere and minerals into the . The grinding action of freezing and thawing and water flow break down the organic materials by reducing the particle size. However, by and large, the heterotrophic microorganisms or decomposers or saprophages ultimately act on the dead bodies of plants and animals and release of nutrients is done by bacteria and fungi. In the process, bacteria and fungi obtain food for themselves. Decomposition, therefore, occurs through energy transformations within and between organisms. If it did not occur, all the nutrients will be locked in the dead bodies and no new life could be produced. Decomposition is a process of equivalent importance as photosynthesis and needs to be understood in its full detail (Heal et al., 1997). The process of decomposition keeps the ecosystem going by release of nutrients back in the soil from where they are taken up by the producers again. The decomposers are usually considered as nature’s recyclers.

The microbial cells are having the enzymes necessary to carry out specific chemical reactions. These enzymes are secreted into dead matter; some of the decomposition products are absorbed into the organism as food, whereas other products remain in the environment or are excreted from the cells. There is a wide variety of decomposer species in nature which act upon dead organic matter. The decomposition rate of different organic compounds varies. Fats, sugars and proteins are decomposed readily, while the cellulose of plants, the lignin of wood, the chitin of insects, the bones of animals arendecomposed very slowly. The more resistant products of decomposition result in humus or humic substances.

There are four stages of decomposition: i) initial leaching, the loss of soluble sugars and other compounds that are dissolved and carried away by water; ii) fragmentation of particulate detritus by physical and biological action accompanied by the release of dissolved organic matter; iii) rapid production of humus and release of additional soluble organics by saprotrophs; and iv) the slower mineralization of humus. Mineralization is the release of organically bound nutrients into inorganic forms for the use of plants with the help of ammonifying, nitrifying and other microbes (Smith, 1996; Juma, 1998; Singh et al, 2014). Humus is dark brown or yellowish brown colloidal substance having the nutrients in readily available forms to plants. Detritus, humic substances and other organic matter undergoing decomposition are important for soil fertility.

In aquatic muds, sediments and in the rumen of ungulate herbivores decomposition is carried out by the anaerobic bacteria (facultative as well as obligate anaerobes) through the process of anaerobic respiration or fermentation. In the decomposition process, the role of litter feeding organisms mainly protozoans (microfauna), mites (mesofauna), nematodes (macrofauna) and megafauna comprised of snails, earthworms, millipedes, mollusks and crabs includes the fragmentation of large materials into smaller particles. Their absence in overall decomposition may slow down the rate of process. During the process of litter decomposition, a large proportion of carbon is lost as respiration of decomposer organisms and nutrients are released during mineralization.

Decomposition is highly regulated by fire, moisture, temperature and by the drivers of global climate change. A moistened and fairly temperature increase the rate of decomposition. Too less as well as too much soil moisture and temperature, severity of climate and global climate change factors reduce the rate of decomposition. iv) Herbivory and Carnivory:

Herbivory refers to ingestion of autotrophs such as plants, algae, and photosynthesizing bacteria by the heterotrophs. Herbivores can be of two types: i) monophagous – are organisms that exclusively eat one plant species, and the survival of these organisms is dependent on the survival of the primary food source. They are immune to the plant’s defenses. For example, Giant Panda feeds on bamboo and Koala Bears who feed on Eucalyptus leaves; ii) polyphagous - feed on more than one type of plant. Most herbivores are polyphagus in nature. Further, there are different subgroups of herbivores animals depending on type of food they consume. Frugivores - eat primarily fruit; folivores - eat leaves; nectarivores - feed on nectar; Granivores - grain eaters; Graminivores - grass eaters; Palynivores - pollen eaters.

Herbivory is a beneficial process for plants as well. Fruit seeds are dispersed over wide areas as the moves. Tough seed coatings are removed in the digestive tract of the herbivore, and its dung fertilizes the soil, providing an ideal environment for seed germination. , low level herbivory can remove aging roots and leaves, allowing new growth of young roots and shoots. The new roots and shoots that grow provide better nutrients for absorption and . The highest rates of herbivory occur in rainforests due to presence of an increased number of young expanding plant life in the understory of rainforests which are more acceptable to insects, pathogens, and folivores mammals. In a typical terrestrial ecosystem, herbivory may remove about 10% of net primary productivity; though percentage varies in different types of ecosystems. For example, the herbivory in different ecosystems amount to 2-3% for desert scrub, 4-7% for forest, 10-15% for temperate grasslands, 30-60% for tropical grasslands and grasslands managed for cattle raising (Singh et al., 2015).

Carnivory is the ingestion of animals or herbivores by other organisms. Organisms that prefer meat over plants are accordingly called carnivores. Carnivores predators kill and eat their prey. For example, a lion or a tiger hunting smaller animals like rabbits or deer, sea otters hunting sea stars or blue whales consuming zooplanktons and fishes. The carnivores habits can occur in plants and fungi that feed on insects or microscopic invertebrates. Studies have indicated that consumers play an active role in the maintenance and regulation of energy flow through the ecosystem and hence contribute to its persistence. e) Ecosystem Development: An ecosystem is not static in nature. It grows and changes in its structure and function with time. These changes are very orderly and can be predicted, the process known as . The ecological succession is defined as an orderly process of changes in the biotic community structure and function with time mediated through modifications in the physical environment ultimately culminating into a stable community known as climax. Clements (1916) while studying plant communities defined succession as “the natural process by which the same locality becomes successively colonized by different groups or communities of plants.” Odum (1969) preferred to call this process as ecosystem development. Ecosystem development may further be defined in terms of following parameters:

1. It is an orderly process of community development that involves changes in species structure with time. It is a directional process and thus predictable.

2. The succession is a community controlled process even though physical environment determines the patterns, rates of change and development.

3. It culminates in a stabilized ecosystem in which maximum biomass is maintained per unit of energy flow.

In the process of succession, the species present in an area will gradually change due to changes in the surrounding environmental conditions over a period of time. Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these conditions change, then the existing species will be replaced by a new set of species which are better adapted to the new conditions. Ecological succession may also occur when environmental conditions suddenly and drastically change. A forest fire, wind storm, and human activities like agriculture all greatly alter the conditions of environment. The process is called as primary succession, if a community starts from the primitive substratum unoccupied by any other community. Secondary succession is referred to if a community starts on a previously built substratum. In such cases, the existing community might have disappeared due to sudden environmental changes such as fire, change in climatic factors, etc.

The process can be understood by an example of a bare land area where succession starts after a long period of rainfall and availability of nutrients.  Changes are noticed when a large bare area of land without any life form is left undisturbed or untilled and exposed to nature for a long time. Initially the soil is bare and totally exposed to sun and any rainwater would run off with little water being absorbed by the soil. In such conditions, such grasses will grow that would love the pounding sun and limited moisture available in soil. Once a blanket of grass thickens, it does not allow the rainwater to run off and increases the moisture content of soil.  Once the grass grows tall, it offers partial shade to plants that may grow between the clumps of grass. Therefore, a second plant species have a more congenial habitat and these plants would grow creating more shade and would retain more water. These changed conditions are not ideal for grass, so it dwindles in quantity.  Now these plants offer shade from the harsh sunlight and maintain adequate moisture level in the soil for another species to grow. Also, the dwindling of grasses provides the required space for this third plant species which may grow into larger bushes. These bushes, once grown tall, have to face sun unimpeded. However, these bushes may leave sufficient space in between which may be a suitable habitat for the growth of fourth plant species, i.e., large trees requiring adequate sunlight and water from soil and stifle the growth of the bushes beneath them.  Thus, the earlier barren land is now full of several tall trees, few bushes, small plants and grass. This may be the equilibrium situation and hence may continue for a long time until any natural or manmade occurs. When this succession stabilizes, it is said to have reached a climax.

The study of these changes is important to understand the past and predict the future of ecosystem. As succession proceeds, changes occur not only in the biotic community but also in physical environment and overall structural and functional characteristics of ecosystems in a holistic manner. Thus, succession has been considered as ecosystem development that culminates in a stabilized ecosystem in which biomass and symbiotic function between organisms are maintained per unit of available energy flow. f) Ecosystem Regulation and Stability:

All ecosystems regulate and maintain themselves under a set of environmental conditions. While evaluating ecosystem functioning, it is important to know whether an ecosystem is in a state of change or stable. In any ecosystem, any environmental stress tries to disturb the normal ecosystem functions. However, there is always a natural tendency of ecosystem to resist the change and maintain itself in equilibrium with the environment. This self-regulatory mechanism is known as homeostasis. `

An ecosystem is an open system that receives input from the environment and produces an output. The output from one sub - system may become the input for another sub - system in an ecosystem. For instance, food produced by plants becomes input for an autotrophic subsystem, which further provides an input to heterotrophic subsystem. Input from each subsystem controls the output of another subsystem. In open systems, when some portion of the output may be fed back as input to control the functioning, the system is called a cybernetic system (Fig 10).

Fig 10: An Ecosystem as: a) an open system b) a cybernetics system showing negative feedback control

The system shows tolerance or only within a maximum and minimum range, which is its range of tolerance known as homeostatic plateau. It responds to inputs and has outputs, and set two types of system response – negative feedback and positive feedback.

Ecosystem stability can be understood in context of resistance and resilience. The resistance is the ability of the system to resist the forces that tend to disrupt its state of equilibrium. On the other hand, resilience is the ease with which the system returns to its original equilibrium state following any perturbation. For example, a forest ecosystem with large biotic structure may better resist a fire outbreak than a grassland ecosystem with smaller biotic structure. Nevertheless, burnt grassland can quickly recover to its original state than a burnt forest that may take hundreds of years to recover. g) Ecosystem services:

The specific ecosystem functions that are directly or indirectly beneficial to human beings are called ecosystem services. These functions provide life support services to humans and other species. According to Millennium Ecosystem Assessment (MEA), ecosystem services can be categorized into four main types:

1. Provisioning services: These services include the products obtained from ecosystems such as food, fresh water, wood, fibre, genetic resources and medicines.

2. Regulating services: These include the services provided by plants, animals, fungi and microorganisms such as of crops, prevention of soil erosion, maintenance of soil fertility, pest control, water purification, climate regulation, natural hazard regulation, and waste management.

3. Habitat services: The habitat services include the importance of ecosystems to provide habitat for living organisms (plants, animals and micro-organisms) and to maintain the viability of gene- pools.

4. Cultural services: These are the non-material benefits that people obtain from ecosystems such as spiritual enrichment, intellectual development, recreation and aesthetic values, tourism.

Interactions among Biotic Components:

Interactions among living organisms are grouped into two major groups viz.

• Positive interactions

• Negative interactions

I. Positive interactions:

Here the populations help one another, the interaction being either one way or reciprocal. These include (i) , (ii) Proto co-operation and (iii) . i) Commensalism: In this one species derives the benefits while the other is unaffected. eg. (i) Cellulolytic fungi produce a number of organic acids from cellulose which serve as carbon sources for non-cellulolytic bacteria and fungi. (ii) Growth factors are synthesised by certain microorganisms and their excretion permits the proliferation of nutritionally complex soil inhabitants. ii) Proto-cooperation: It is also called as non-obligatory mutualism. It is an association of mutual benefit to the two species but without the co-operation being obligatory for their existence or for their performance of reactions. eg. N2 can be fixed by Azotobacter with cellulose as energy source provided that a cellulose decomposer is present to convert the cellulose to simple sugars or organic acids. iii) Mutualism: Mutually beneficial interspecific interactions are more common among organisms. Here both the species derive benefit. In such association there occurs a close and often permanent and obligatory contact more or less essential for survival of each. eg. (i) Pollination by animals. Bees, moths, butterflies etc. derive food from hectar, or other plant product and in turn bring about pollination. (ii) Symbiotic nitrogen fixation: Legume - Rhizobium . Bacteria obtain food from legume and in turn fix gaseous nitrogen, making it available to plant.

II. Negative interactions:

Member of one population may eat members of the other population, compete for foods, excrete harmful wastes or otherwise interfere with the other population. It includes (i) , (ii) Predation, (iii) Parasitism and (iv) . i) Competition: It is a condition in which there is a suppression of one organism as the two species struggle for limiting quantities of nutrients O2 space or other requirements. eg. Competition between Fusarium oxysporum and Agrobacterium radiobacter. ii) Predation: A predator is free living which catches and kills another species for food. Most of the predatory organisms are animals but there are some plants (carnivorous) also, especially fungi, which feed upon other animals. eg. (i) Grazing and browsing by animals on plants. (ii) Carnivorous plants such as Nepenthes, Darligtoria, Drosera etc. consume insects and other small animals for food. (iii)Protozoans feeding on bacteria. iii) Parasitism: A parasite is the organism living on or in the body of another organism and deriving its food more or less permanently from its tissues. A typical parasite lives in its host without killing it, whereas the predator kills it’s upon which it feeds. eg. Species of Cuscuta (total stem parasite) grow on other plants on which they depend for nourishment. Parasitism may occur even within the species. Hyperparasites are chiefly fungi growing parasitically on other parasites, (i.e.,) Parasite on a parasite. eg. Cicinnobolus cesatii is found as hyperparasite on a number of powdery mildew fungi. iv) Antibiosis: The phenomenon of the production of antibiotic is called as antibiosis. Antibiotic is an organic substance produced by one organism which in low concentration inhibits the growth of other organism. eg. Streptomycin - S.griseus , Penicillin - P. notatum , Trichoderma harzianum inhibits the growth of Rhizoctonia sp.

Types of Ecosystem:

Based on the kind of habitat, there are essentially two types of ecosystems: Aquatic and Terrestrial Ecosystem (Fig 11). Any other sub-ecosystem falls under one of these two types.

Fig 11. Basic types of ecosystems

1. Aquatic Ecosystems:

Aquatic ecosystem is the ecosystem found in a body of water. It encompasses aquatic flora, fauna and water properties, as well. There are two main types of :

a) Marine Ecosystem:

Marine ecosystems are the biggest ecosystems, which cover around 71% of earth’s surface and contain 97% of out planet’s water. Water in marine ecosystems contains high amounts of dissolved minerals and salts. Various marine ecosystems include oceanic (a relatively shallow part of oceans which lies on the continental shelf), profundal (deep or bottom water), benthic bottom substrates, inter-tidal (the place between low and high tides), estuaries, coral reefs, salt marshes, hydrothermal vents where chemosynthetic bacteria make up the food base. Many kinds of organisms live in marine ecosystems: the brown algae, corals, cephalopods, echinoderms, dinoflagellates, sharks, etc. b) Freshwater Ecosystem:

Contrary to the Marine ecosystems, the freshwater ecosystem covers only 0.8% of Earth's surface and contains 0.009% of the total water. Three basic kinds of freshwater ecosystems exist. i) Lentic – slowmoving or still water like pools, lakes or ponds; ii) Lotic - fast-moving water such as streams and rivers; and iii) Wetlands - places in which the soil is inundated or saturated for some lengthy period of time.

2. Terrestrial Ecosystem: The ecosystems on land are called as terrestrial ecosystems. They are broadly classed into: a) Forest Ecosystem:

They are the ecosystems with an of flora, or plants in relatively small space. A wide diversity of fauna can also be seen. A small change in this ecosystem could affect the whole balance and effectively bring down the whole ecosystem. They are further divided into: i) Tropical rainforest: These contain more diverse biodiversity than ecosystems in any other region on earth. They receive a mean rainfall of 80 cm for every 400 inches annually. In these, warm, moisture-laden environments, dense evergreen vegetation comprising tall trees at different heights are present, with fauna species inhabiting the forest floor all the way up to canopy. ii) Tropical deciduous forest: In these ecosystems, shrubs and dense bushes are found along with a broad selection of trees. The trees are mainly which shed their leaves during dry season. The type of forest is found in quite a few parts of the world while a large variety of fauna and flora are found there. iii) Temperate evergreen forest: Those have a few numbers of trees as mosses and ferns make up for them. Trees have developed needle shaped leaves in order to minimize transpiration. iv) Temperate deciduous forest: The forest is located in the moist temperate places that have sufficient rainfall. Summers and winters are clearly defined and the trees shed the leaves during the winter. v) Taiga: found just before the arctic regions, the taiga is defined by evergreen conifers. As the temperature is below zero for almost half a year, the remainder of the months, it buzzes with migratory birds and insects. b) Grassland Ecosystems:

Grassland Ecosystems are typically found in both tropical and temperate regions of the world. They share the common climactic characteristic of semi-aridity. The area mainly comprises grasses with a little number of trees and shrubs. A lot of grazing animals, and herbivores inhabit the grasslands. The two main types of grasslands ecosystems are: i) Savanna: The tropical grasslands are dry seasonally and have few individual trees. They support a large number of predators and grazers. ii) Prairies: It is temperate grassland, completely devoid of large shrubs and trees. Prairies could be categorized as mixed grass, tall grass and short grass prairies.

c) Desert Ecosystems:

Desert ecosystems are located in regions that receive low precipitation, generally less than 25 cm per year. They occupy about 17 percent of land on earth. Some contain sand dunes, while others feature mostly rock. Due to the extremely high temperature, low water availability and intense sunlight, vegetation is scarce or poorly developed, and any animal species, such as insects, reptiles and birds, must be highly adapted to the dry conditions. The vegetation is mainly shrubs, bushes, few grasses and rare trees. The stems and leaves of the plants are modified in order to conserve water as much as possible, for example, succulents such as the spiny leaved cacti. d) Mountain Ecosystem:

Mountain land provides a scattered and diverse array of where a large number of animals and plants are found. At the higher altitudes, under harsh environment, only the treeless alpine vegetation can survive. The animals have thick fur coats for prevention from cold and hibernation in the winter months. Lower slopes are commonly covered with coniferous forests.

Natural and Artificial Ecosystems:

All above ecosystems are Natural ecosystems as these operate themselves under natural conditions without any major interference by man.

Some ecosystems are maintained artificially by human beings where, by addition of energy and planned manipulations, natural balance is disturbed regularly. For example, croplands like maize, wheat, rice-fields etc. where man tries to control the biotic community as well as the physico-chemical environment. These are called as Artificial or Man-engineered ecosystems.

Dr Ankita Assistant Professor B.Ed. (Regular), DDE, LNMU, Darbhanga.