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Concept of Ecosystem

Introduction:

In nature, the living organisms (plants, animals and microorganisms) and nonliving

environment (e.g. water, air, soil, etc.) are inseparably interrelated and interact with each other. No

living organism 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, predation, parasitism and/or decomposition. 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 ecology and ecology as study of ecosystems. 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 community 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 Biology (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

energy flow 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 – biotic component, 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 species diversity; 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 biomass. 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 decomposers. These components are dependent on one

another. One of the important features is presence of grazing or detritus food chain 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

photosynthesis. 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 food web is the

ecosystem of a deciduous forest floor. Ecosystems are sustained by the presence of biodiversity.

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 carbon dioxide, 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 habitat.

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

heterotrophs. They mainly include herbivorous (feed on plants), carnivorous (feed on other

animals), omnivorous (feed on both plants and animals) and detritivores 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 autotroph–heterotroph–decomposer 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

Producers, consumers and decomposers.

b) Abiotic components

Inorganic substances (C, H, O, N, P, S,

etc.), organic compounds (proteins, amino

acids, lipids, carbohydrates, humic

substances, etc.), climate and its

components (temperature, humidity,

moisture, sunlight, rainfall, wind, air etc.),

edaphic and other factors (minerals, soil,

topography, pH, etc.)

a) Food chains and food webs

b) Energy flow

c) Nutrient cycling

d) Ecosystem processes to explain

interactions among the components

of ecosystem

e) Ecosystem development

f) Ecosystem regulation and stability

g) Ecosystem services

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 trophic level 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 consumer.

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 population size 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 biomagnification 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 primary production. The rate of total organic matter production

by green plants (primary producers) is known as gross primary productivity. 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 herbivores to carnivores 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, desert ecosystems have limitations of adequate water supply while

tundra ecosystems have low water temperature as limiting factor 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 phosphorus cycle, 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) Oxygen Cycle:

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 Cycle:

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) Sulfur 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

sulfate ion, (SO42-). 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:

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2

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).

Chlorophyll

Sunlight

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 autotrophs 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 methane 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

soils. 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 herbivore 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

reproduction. 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 ecological

succession. 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 disturbance 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 resistance 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 pollination 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) Commensalism, (ii) Proto co-operation and (iii) mutualism.

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 symbiosis. 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) Competition,

(ii) Predation, (iii) Parasitism and (iv) Antibiosis.

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 aquatic ecosystem:

Marine Ecosystem

Freshwater Ecosystem

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 abundance 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, insectivores 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 deserts 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 habitats 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.