An ecosystem is a biotic assemblage or grouping of organisms taken together with their environment. It is a region with a specific and recognizable landscape form. The nature of the biotic grouping in the region is based on its geographical features. There are various kinds of life supporting systems that are found on the surface of the Earth, such as hills, mountains, plains, forests, grasslands, deserts, wetlands, oceans, coastal areas, islands, lakes, rivers, and estuaries.
Introduction
An ecosystem is a biotic assemblage or grouping of organisms taken together with their environment. It is a region with a specific and recognizable landscape form. The nature of the biotic grouping in the region is based on its geographical features. There are various kinds of life supporting systems that are found on the surface of the Earth, such as hills, mountains, plains, forests, grasslands, deserts, wetlands, oceans, coastal areas, islands, lakes, rivers, and estuaries. These geographic features show wide variations in their structural composition and functions. The living part of the ecosystem is referred to its biotic component. Topography, climatic conditions and soil characteristics are the major determinants of its biotic assemblage. These factors together create conditions that support a community of plants, animals and microbes that natural evolution has allowed to live in these specific conditions. This chapter discusses what ecosystems are all about. It also reviews various concepts used in ecology including carrying capacity, ecological pyramid, ecological succession, ecosystem structure, factors affecting structure, food chains- trophic level, food webs, living processes and interactions, and functioning of the ecosystems with reference to major biochemical cycles (e.g. carbon cycle).
Concept
of Ecosystem
What
are Ecosystems?
Definition: An ecosystem is a self-regulating community of different
species (plants, animals and microbes) occupying an explicit unit of space,
interacting with each other and with their non-living environment. Since the flora and fauna of a
given area are functionally related, the interaction takes place within (own
species) and between (with other species), thus forming a biotic community. The biotic community is, in turn, interacts with its physical
environment. Together, they form what is known as the ecosystem. In fact, ecosystem is the basic functional unit in ecology since it
includes all the living organisms in an area interacting with their non-living
environment. The size of an ecosystem is arbitrary; it is defined by the system
we wish to study. For convenience, scientists usually consider an ecosystem
under study to be an isolated unit, but natural ecosystems rarely have distinct
boundaries and are not truly self-contained, self-sustaining systems. Instead,
one ecosystem tends to merge with the next in a transitional zone called an Ecotone- a region containing a mixture of species from adjacent regions. A species is all the organisms of the
same kind that are genetically similar enough to breed in nature and produce
live, fertile offspring. The populations
of different species that live and interact within a particular area at a
given time make up a biological
community. An ecosystem is, thus, composed of a biological community
together with all the biotic and abiotic factors that make up the environment
in a defined area.
At global level,
ecosystems are divided into terrestrial- land-based, and aquatic- water based
ecosystems. These form the two major habitat conditions for the Earth’s living
organisms. Major ecosystem types are called biomes. Among the major terrestrial
biomes are deserts, tundra, grassland, temperate forests- deciduous and
coniferous- and tropical rain forests. Soil, moisture, and temperature are
generally the most critical determinants for terrestrial biomes. Aquatic
ecosystems include oceans and seas, rivers, streams, and lakes, estuaries,
marshes, swamps, bogs, fens and reefs. At a regional level, this is divided
into biogeographical realms
e.g. South and
Basic
Components of the Ecosystem
Ecology: How do different
ecosystems- like a hot desert, a tropical forest or a shallow lake- differ in
their composition of flora and fauna, how do they drive their energy and
nutrients to live together, how do they influence each other and regulate their
stability are the basic questions that are studied in Ecology (a term derived from the Greek words Oikos- home + logos- study, was first coined by Earnst Haeckel in 1869). So, ecology
deals with the study of organisms in their natural habitat interacting with
each other and with their non-living surroundings- environment. Specifically,
ecology seeks to understand interactions among organisms, species, populations,
communities, ecosystems, and the ecosphere. Here, an organism is any
form of life on earth, which can be classified into groups (species) that
resemble one another in appearance, behaviour, reproductive systems, body
chemistry, and in the genes they contain. A species include all the
members of a specific kind of plant, animal and microbe. It is difficult to know how many species exist on Earth. Estimates range
from 5 million to 100 million; most of them are insects and microorganisms
scientists believe. Biologists so far have identified and named only about 1.4
million species, and they know a fair amount about approximately one-third of
them, and the detailed roles and interactions of only a few). A population consists of all members of the same species occupying a given area at
the same time e.g. all sunfish in a pond, white oak trees in a forest, and
people in a country. In most natural population, individuals vary slightly in
their genetic makeup- something calls genetic diversity. Populations are thus
dynamic groups that change in size, age distribution, density, and genetic
composition as a result of changes in environmental conditions. The place where
a population or a single organism typically lives is known as habitat. Populations of all the different species occupying and interacting in a
particular place make up a community or biotic community.
Carrying Capacity of the Ecosystem
Each type of
population in the ecosystem has its own reproductive potential. This is the
rate at which the population will grow given its habitat had unlimited
resources. Under an ideal condition, the population of organisms grows exponentially.
This suggests more and more individuals are added to the population over time.
An exponential growth would be possible if there were no limiting factors in
the ecosystem. In reality, population cannot grow exponentially indefinitely.
Eventually, there are too many individuals competing for food and other
resources, and the ecosystem cannot support this large number. The carrying capacity of the ecosystem for a particular species is the maximum size of its population that the
ecosystem can provide for an indefinite period. If population exceeds that
carrying capacity, the consequence will be destructive, as resources will be
consumed faster than the ecosystem can produce them.
Structure
of an Ecosystem
Ecosystems show
large variations in their size, structure and composition. However, all the
ecosystems are characterized on the basis of their communality- certain basic
structural and functional features. Structure refers to parts and the way they
fit together to make a whole system. There are tow key aspects to every
ecosystem: the biotic (living) community of a specific area, and its abiotic
(non-living) environmental factors. The way different categories of organisms
fit together is referred to as the biotic community, which is consists of different
plants animals, and microbes each having its specific functional position with
regards to other biological units with which they interact. Composition and organization of biological-
biotic communities (plants, animals and microbes) and abiotic components (land,
water and air) constitute the structure of an ecosystem
Structural Features
1. Biotic Structure
A biotic community represents all the populations of different
plants, animals and microbes occupying a given area. The grouping or assemblage of plants (e.g. trees, algae), animals (e.g.
large mammals, birds, tiny insects) and microbes (microscopic bacteria) we
observe when we study a natural forest, a grass land, a pond, a coral reef or
some other undisturbed area is refer to as the area’s biota or biotic
community. These organisms have different behaviour and nutritional status in
the ecosystem. Hence, on the basis of
food collection, the biotic
community is further divided
into two sub-components: Autotrophic
and Heterotrophic. The living organisms of a biotic
community which can produce their own food from non-living environment are
called autrophs. On the other hand, the heterotrophs include all animals that take food from autotrophs, as they can not
produce food from their own. Heterotrophs are of two different kinds- consumers and decomposers including detritus feeders. Thus, each biotic community is comprises of (i) Producers, (ii)
Consumers, and (iii) Decomposers.
(i)
Producers: Producers (sometimes also called autotrophs-
self feeders) are organisms such as plants, algae and some bacteria that make
their food from compounds obtained from their environment. In most land
or grazing ecosystems, green plants are the producers; in aquatic ecosystems,
most of the producers are phytoplankton- floating and drifting bacteria and
protests, most of them microscopic. Only producers make their own food; all
other organisms depend directly or indirectly on food provided by producers
e.g. rooted plants, phytoplankton etc.
Photosynthesis: The source of energy
in the ecosystem is the sun. Most producers use sunlight to make complex
compounds by a process called photosynthesis. In this process, leaves of green plants use
water, carbon dioxide (CO2) and minerals to make carbohydrates in the presence
of sunlight. In
most green plants, chlorophyll, a pigment molecule that gives green plants
their colour, traps solar energy for use in photosysthesis. Although a
sequence of hundreds of chemical changes takes place during photosysthesis, the
overall reaction can be summarized as: Carbon dioxide+water+solar energy=
glucose+oxygen. Thus, radient energy is transformed into chemical
energy. The process convert CO2 and water to organic
matter such as glucose and then release oxygen as a by product- a process of
energy conversion.
However, there is a
huge amount of energy loss from the ecosystem as some radiant energy is always
escaped or dispersed into unavailable heat energy. During energy transfer in
the food web, not all of the radiant energy is transformed into energy of
usable form as most of it is dissipated or lost in the process. This happens at
each step of food chain (primary, secondary and tertiary), simply because of
their variable physical growth, different metabolic activity, or reproduction.
There is also a heat loss through respiration and decomposition at each trophic
level. Since energy flow is
unidirectional and is lost at each trophic level, any loss has to be
compensated from an external source such as the sunlight. If solar radiation is
not available to replenish energy in the ecosystem, the world’s bio-physical
system would eventually collapse.
Chemosynthesis: A few producers,
mostly specialized bacteria, however, can convert simple compounds from their
environment into more complex nutrient compound without sunlight (produce
organic matter to some extent through oxidation of certain chemicals), a
process called chemosynthesis. In one such case, the source of energy is heat
generated by the decay of radioactive elements present deep in earth’s core and
released at hot-wave vents in the ocean’s depths. In the pitch-darkness around
such vents, specialized chemosynthetic organisms use the heat to convert
dissolved hydrogen sulfide (H2S)
and carbon dioxide (CO2) into organic
compounds- nutrient molecules.
(ii) Consumers (heterotrophs- other-feeders): all organisms
which get their organic foods (nutrients) by feeding upon other organisms (the
tissues of producers) are called consumers. There are several types of
consumers, depending on their food sources: Herbivores (plant
eaters) are feed directly on producers and hence also known as primary consumers e.g, rabbit, insect
etc. Carnivores (meat eaters): are feed on other consumers. When
they are feed on herbivores they are called secondary
consumer e.g. fox, frog etc. Omnivores: Cconsumers that feed
on both plant and animals) are called tertiary, e.g. rats, humans etc).
(iii)Decomposers
(detritivores- detritus feeder): The organisms that produce
simple basic elements as food from dead and decomposed organic matters are
called decomposers. They derive their nutrition by breaking down the complex
organic molecules to simple organic compounds, and ultimately into inorganic
nutrients. These are micro-consumers including different types of bacteria and
fungi. These organisms break down complex compounds of dead and living cells
through the process of decomposition and release them into the environment e.g.
carpenter ants, termites etc. These make up the nutrient pool. Thus, the
circular flow of matter is complete. However, as will be discussed further on,
only the material flows complete the cycle; energy flow is unidirectional due
to entropy.
2. Abiotic Structure
The physical and
chemical components of an ecosystem constitute its abiotic (non-living)
structure. It includes factors such as climatic, edaphic or soil, topographic,
energy balance, nutrients supply and toxic substances.
Factors Affecting the Structure of an
Ecosystem
Physical factors affecting
ecosystems are duration of sunlight, shade, average temperature and temperature
range, average precipitation and its timing, wind flow, latitude, altitude,
frequency of fire, nature of the soil, velocity of water and amount of
suspended solid materials to name a few. Climate plays a
very important role in the physiological processes of the ecosystem. Radiant
energy from the sun is transformed into various forms of energy when it reaches
the earth. Various physiological processes of green plants such as
photosysthesis, transpiration and their vegetative growth depend on solar
energy. Sun light affects respiration rates in animals and also pigmentation of
the skin. Temperature affects metabolic processes through regulating the
enzymes and the chemical reactions within the body of organism. It also affect
plant growth- warmer vs cooler. Plant and animal life also vary according to
the temperature of the region. Wind, temperature and rainfall may affect the
level of pollution in an area. Atmospheric gases such as oxygen, nitrogen and
carbon dioxide are also vital to the life processes of plants and animals.
Oxygen supports life. Carbon dioxide in the atmosphere is necessary for
photosysthesis. Nitrogen is the basis of protein and vital to all cell
development. The topography of an area determines the plant and
the animal life within that area. Temperature decreases with altitude; wetland
flora and fauna will differ from that of desert areas. Water is vital
to life. The availability and quality of water determine the kind of flora and
fauna found on an area. Edaphic factors: Soil and water support
plant growth. Thus, the composition of soil, its mineral and water content, its
texture and organic content will determine the type of vegetation growth.
Chemical Factors: Important chemical
factors affecting ecosystems are supply of water and air in the soil, supply of
plant nutrients (carbon, nitrogen, phosphorus, potassium, hydrogen, oxygen, and
sulphur) dissolved in soil moisture and in aquatic habitats, level of toxic
substances dissolved in soil moisture and in aquatic habitats, and salinity and
level of dissolved oxygen (aquatic ecosystems).
All the biotic components of an ecosystem are influenced by the abiotic
components and vice versa, and they are linked through energy flows
(unidirectional) and material flows (cyclic).
Functional Attributes: Living Processes and
Interactions
Functioning of the Ecosystem
Every ecosystem
performs in a systematic way under natural conditions. Ecosystems have inputs
of matter and energy that are used by plants and animals to grow, reproduce and
maintain life, and they tend to maintain equilibrium (achieve a set of
balances) of the various activities and processes that occur within it. In the
process, the producer receives energy from the sun and passes it on through various
consumers. Various nutrients and water are also required for life processes in
addition to energy, which are exchanged among themselves (biotic) and with
their non-living (abiotic) components. The biotic components are regulated in
an orderly manner and mechanisms to encounter or withstand some degree of
environmental stress. Although many of
these balances are built on self-regulatory mechanisms, some are quite
sensitive and can destroy the system. Moreover, the living and non-living components of the ecosystem interact collectively
in a way that makes it difficult to separate each of these factors; they are
interwoven through the flows of energy and material. The major functional
attributes of an ecosystem are as follows:
(i) Food chains, food webs and trophic level;
(ii) Ecological pyramid;
(iii) Ecological succession;
(iv) Energy flow;
(v) Material flow;
(vi) Bio-geochemical cycles
The structure and
functions of ecosystems are very closely related. The producers and consumers
are arranged in the ecosystem in such a manner that their interactions along
with population size are expressed collectively as trophic structure. Each feeding level is known as trophic level. It indicates an
organism's position in the food chain. As all organisms whether dead or alive
are potential sources of food for other organisms, ecologists assign every
organism in an ecosystem to a tropic (feeding) level (derived from the
Greek word trophos meaning
“nourishment”), depending on whether it is a producer, or a consumer or
decomposers. Thus, the major feeding levels constitute the trophic levels. All producers belong to the first trophic level; all primary consumers
(all herbivores) belong to the second trophic level; organisms (all carnivores)
feeding on these herbivores belong to the third level, and so on. Decomposers
are fed by all trophic levels. For example, a
caterpillar eats a leaf; a bird (say robin) eats the caterpillar, a hawk eats
the robin. Once plant, caterpillar, robin, and hawk are all die, will be
consumed by decomposers.
Food
Chains
Food chains provide the path through which
the flow of energy and materials (nutrients) take place in the ecosystem. There
is a transfer of energy from producers to consumers through a recurring process
of eating and being eaten. This is how
energy transformations in the ecosystem take place by means of a series of
steps or levels. The sequence of
organisms, each of which is a source of food for the next (each such pathway)
is called a food chain (Figure-3.1). In the grazing food chain, green plants are eaten
by herbivores- the primary consumers. These are, in turn, eaten by carnivores-
the secondary consumers. Omnivores are tertiary consumers, which eat plants as
well as animals and are at the top of trophic level. To be more specific, for
example, a caterpillar eats plant leaf, a frog eats the caterpillar, a snake
eats the frog, and an owl eats the snake.
There is another type of food chain, known as the detritus food chain. The two food chains are, however, interlinked. Detritus food chain begins where the grazing food chain ends. When plants and animals die their remains are returned to the soil environment. Microorganisms feed on dead organic matters. These decomposers break down complex organic matters through biochemical processes and release the minerals into the soil, water and atmosphere, which make up these minerals, and with the help of photosysthesis make up the nutrient pool. Green plants take up these minerals and with the help of photosysthesis produce food.
Figure- 3.1:
The Food Chain; Source: Google image
Food
Web
Food web is a
network of food chains where different types of organisms are connected at
different trophic levels. Figure 3.2 illustrates an example of food web in a
typical aquatic ecosystem. While it is interesting to trace these pathways, it
is equally important to recognize that food chains seldom exist as isolated
entities. In
fact, food chains are numerous, and none of these occur in isolated sequences.
All of these are interlinked and form an interlocking pattern of organisms
known as food web (Figure 3.2). Real ecosystems are more complex than this simplistic pathway. Most
consumers eat and are being eaten by two or more types of organisms. Some
animals feed at several trophic levels. For instance, herbivore population
feeds on several kinds of plants, and is preyed upon by several secondary
consumers- carnivores, or omnivores. Thus, the organisms in most ecosystems
form a complex network of feeding interactions. Virtually all food chains are
interconnected and form a complex web of feeding relationships- called the food web. Trophic levels can also be assigned in food webs as in food chains.
This determines how energy moves from one organism to another through the
ecosystem. Energy typically flows one way through land ecosystems by passing
through two interconnected types of food webs i.e. grazing food webs and detrital food webs. In grazing food webs, the energy flows from plants to herbivores (grazers), then through an
array of carnivores and eventually to decomposers. In detrital food webs, organic
waste material or detritus is the major food source, and energy flows mainly
from plants to decomposers and detrivores. In many terrestrial ecosystems (such
as forests) and in aquatic ecosystems (such as streams and marshes), detrital
pathways predominate. In the deep ocean, most of the energy flows through
grazing food webs.
Figure- 3.2: The Food Web; Source: Google images
Ecological Pyramid
Pyramid of Numbers: Trophic levels of
an ecosystem are often represented by "ecological pyramids". These
are graphic representation of relationship between producers and different
types of consumers in a food chain. More specifically, graphic representation
of trophic levels and function, starting with producers at the base and
successive consumers forming the apex is known as ecological pyramid. The number of individual organisms at each
trophic level is represented by pyramid of numbers. Figure 3.3 shows
an upright pyramid of numbers for grassland ecosystem. The producers in a grass
land are grasses, the herbivores are insects, while tertiary carnivores are
hawk or other birds which are gradually less and less in number, and hence the
pyramid apex becomes gradually narrower. It decreases at successive levels from
base to apex e.g. the total biomass of producers is more than that of the
consumers (total biomass of herbivores). Similarly, the total biomass of
secondary consumers will be less than that of herbivores and so on. Usually, in
a food chain, the number of individuals decreases at each trophic level with
huge number of tiny individuals at the base, and a few large individuals at the
top.
Figure 3.3: Pyramid of Numbers
Pyramid of Biomass: Each trophic level
in a food chain contains a certain amount of biomass (combined net dry weight
per unit area or volume or the total weight of all organic matter contained in
its organisms.). It is more likely that the further a trophic level is from its
source (producer), the less biomass it will contain. A pyramid of biomass represents
the total biomass of organisms at each feeding level in a food chain at a given
place in time. However, the pond ecosystem shows an inverted pyramid of biomas.
The total biomass of producers (phytoplanktons) is far less as compared to
primary consumers (insects), secondary carnivores (small fish) and tertiary
carnivores (big fish). Thus, the pyramid takes an inverted shape with narrow
and broad apex. Say, for example, about a million of phytoplankton in a small
pond may support some 10,000 zooplankton, which in turn may support 100 fish of
a particular kind (say perch), which might feed a couple for a month or so.
This formation is called ecological pyramid.
Ecological Successions
Ecological
succession is a gradual process by which ecosystems change and develop over
time. In a given ecosystem, biological communities usually have a history. The
process by which organisms occupy a site and gradually change environmental
conditions so that other species can replace the original inhabitants is called
ecological succession or development. The bottom line of ecological succession is that in the
process, the species present in a landscape will gradually change; succession
takes place because of the changes in the environmental conditions in a
particular area over a long period of time. Species is adapted to thrive and
compete against other species under a very specific set of environmental
conditions. If these conditions do not remain constant, then the existing
species will be replaced by a new set of species which are well adapted and
better suited to the new conditions. Primary succession occurs when a community begins to develop on a site of previously
unoccupied by living organisms, such as an island, or a body of water. Secondary succession occurs when an existing community is disrupted and a new one
subsequently develops at the site. Ecological succession may also occur when
conditions of an environment change suddenly and drastically. The disruption or
drastic modification may be caused by some natural catastrophe, such as forest
fire, wind flows, storms, flooding and even human activities like agriculture,
deforestation or mining often greatly alter the normal conditions of an
environment. Both forms of succession,
especially in the structure and function of communities, usually follow an
orderly sequence of stages.
Energy flow
Flow of energy in
an ecosystem usually takes place through the food chain; it is the energy flow
which keeps the ecosystem running. The most important feature of this energy
flow is that it is unidirectional- one way flow, and as such energy is not
reused in the food chain. Further, the law of energy follows the laws of
thermodynamics: (i) The first law of
thermodynamics is the is the law of energy conservation, which states that
energy can neither be created nor be destroyed, but it can be transformed from
one form to another. Thus, all energy entering the ecosystem must maintain a
balance with the outgoing and the amount staying in the ecosystem; and (ii) the
second law of thermodynamics is the entropy law, which states that no
transformation of energy is 100 percent efficient as some energy is always
dissipated into unavailable heat energy. That means, each time it is
transformed, some useful energy is lost.
The diagram
(Figure- 3.3) shows the manner in which energy and materials move in the
ecosystem, where the source of energy is the Sun. Energy enters most ecosystems
as high-quality sunlight The producers (autotrophs)- mostly the green plants
take up minerals such as carbon, oxygen, nitrogen, hydrogen, calcium,
potassium, phosphorus, sulphur etc. from the environment and fixed solar
radiation to build organic matters carbohydrates, protein, fat etc.. Thus,
incoming radiant energy is transformed into chemical energy (converted to
nutrients by photosynthesizing producers), which is then transferred from
producers to primary and secondary consumers through a recurring process of
eating and being eaten. The energy is than passed on to tertiary consumers and
eventually to decomposers. However, not all the radiant energy is transformed
into useful energy, as part of it is dissipated. This happens at each step of
the food chain. As each organism uses the high-quality chemical energy in its
food to move, grow and reproduce, some of it is converted to low-quality heat
that flows into the environment in accordance with the second energy law (each
time energy is transformed some useful energy is lost). There is a heat loss
through respiration and decomposition. Thus, there is a huge amount of energy
loss from the ecosystem as some energy is always dispersed into unavailable
heat energy during energy transfer in the food web. This loss needs to be
recovered from an external source- the sunlight. These concepts of energy flow
and material flow (recycling chemical nutrients) are further discussed below
(biochemical cycles).
Figure-
3.3: Diagram Showing Energy and Material Flow in an Ecosystem
Material Flow
Besides energy
flow, the other functional attribute of an ecosystem is material flow (some
prefer nutrient cycling). The prominent feature of material flow is that it is
cyclic. As organic matter (carbohydrate) is broken down into chemical energy,
its chemical elements are released back to the environment, where in the
inorganic state they may be reabsorbed by producers (autotrophs). Thus, there
is a continuous cycle of nutrients flow from living organisms into the abiotic
environment then back into the living organisms of an ecosystem. Living
organisms need large quantities of carbon, oxygen, hydrogen, nitrogen,
phosphorous and sulphur. Some of these, for example, oxygen, nitrogen and
carbon are present in the atmosphere while minerals such as phosphorous,
sulphur etc. occur in soil and rocks. The cyclic movement of these elements
through the biosphere is known as the bio- geochemical cycle.
The biosphere is a
source of large quantities of essential elements. In a given ecosystem, these
elements are constantly used and reused by living organisms. Water, carbon,
nitrogen, sulfur and phosphorous, for instance, are recycled in ecosystems
through complex biochemical cycles. The
ecosystem dynamic are governed by physical laws, including the law of
conservation of matter and the first and second law of thermodynamics. The recycling
of matters is the basis of the cycles of elements that occur in ecosystem.
Matter and energy are processed through the trophic levels of an ecosystem via
food chains and food webs. The relationships between producers and consumers in
an ecosystem, often depicted as ecological pyramid.
Importance: One vital element of organic matters is
carbon which is essential to all forms of life on Earth. Life has a significant
role in the carbon cycle as all living organisms contain carbon. It is especially
important as it is the basic building block of the carbohydrates, proteins,
fats, and nucleic acids such as DNA and RNA, and other organic compounds
necessary for life. Organisms higher up in the food chain take these by eating
green plants or other animals. Green plants and bacteria use atmospheric carbon
dioxide (plus some dissolved carbonates in the case of aquatic organisms) to
create the molecules of life. Plants and animals respire in order to stay
alive. The products of respiration include carbon dioxide, which is released
into the atmosphere. When organisms die, their remains decompose, also
releasing carbon back into the atmosphere and the soil. Further, carbon is a key component of nature’s thermostat. If
the carbon cycle removes too much CO2 from the atmosphere, Earth will cool; if the
cycle generates too much, Earth will get warmer.
Sources of Carbon and Regulation: Carbon is the 4th most abundant element in the universe. It is present in sedimentary rocks as carbonate. But most of the carbon
involved in the carbon cycle is dissolved in rivers, lakes and oceans as
carbonates, and in the atmosphere as carbon dioxide. In fact, most of the earth’s carbon- 10,000 times that in the total mass
of all life on earth- is stored in ocean floor sediments and on continents. Therefore,
the oceans
play a major role in regulating the level of carbon dioxide in the atmosphere.
Some carbon dioxide gas, which is readily soluble in water, stays dissolved in
the sea, some is removed by producers through the process called
photosynthesis. Some reacts with seawater to form carbonate and bicarbonate
ions. As water warms, more dissolved CO2 returns to the atmosphere. In marine ecosystems, some organisms take up
dissolved CO2, carbonate or bicarbonate
ions from ocean-water. These ions can then react with calcium to form calcium
carbonate (CaCO2), to build shells and the
skeletons of marine organisms. When these organisms die, tiny particles of
their shells and bones slowly sink to the ocean depths and are buried for eons
(as long as 400 million years) in bottom sediments, where under immense
pressure they are converted to limestone rocks. The image above
shows the carbon cycle. The green numbers next to each label represent the
carbon reservoirs, in units of billions of tons (gigatons). For example, the
atmosphere contains about 750 gigatons of carbon, mostly in the form of carbon
dioxide, but also trace amounts of other gases, such as methane. The soil
contains about 1580 gigatons, in the form of organic matter, bacteria, etc.
Fossil fuel reservoirs hold about 4000 gigaton As can be seen, the bulk of the
carbon is in the deep ocean, around 38,100 gigatons. The numbers in red show
the carbon fluxes (per year) between different carbon pools. The numbers are
also in gigatons.
Figure- 3.4: Carbon Cycle;
Source:
Google images; Essayweb.net
Biological Carbon
Cycle: Carbon is continually being released from carbon sources
and is removed by carbon sinks. The cyclic movement of carbon across these
reservoirs or sources is called the carbon cycle. The biological carbon cycle works over
periods from days to a few thousands of years. The cycle is primarily based on
carbon dioxide gas (CO2). The main source
is atmospheric carbon dioxide that makes up only 0.036% of the volume of the
troposphere, and is also dissolved in water. It enters the living system
through producers, when they remove CO2 from the atmosphere (terrestrial) or water
(aquatic) and use photosynthesis to convert it into complex carbohydrates such
as glucose. It re-enters the atmosphere when it is given off during the process
of respiration by living organisms and during the process of decomposition. The
cells in oxygen consuming organisms then carry out aerobic respiration, which
break down glucose and other complex organic compounds and converts the carbon
back to CO2 in the atmosphere
or water for reuse by producers. This linkage between photosynthesis and
aerobic respiration circulates carbon in the ecosphere and is a major part of
the global carbon cycle (Figure 3.4).
Geological Carbon
Cycle: In addition to the biological turnover of carbon, the
Earth itself has a carbon cycle- geological- with carbon being continually
released from carbon sources and removed by carbon sinks over millions of
years. Carbon dioxide in the
soil exists as carbonic acid, which combines with minerals in the soil to form
carbonates. Over time, these carbonates are eroded and transported by wind and
water back to the sea. Carbonates in the oceans eventually sink to the bottom;
therefore, the oceans are a net carbon dioxide sink. However, at times plate tectonics drives the sea floor deep
underground at the subduction zones. As the sea floor gets buried deeper, it
heats up and eventually releases the carbon dioxide, which makes its way back
to the surface through volcanoes, hotsprings, or gradual seeps. Plate tectonics
also affects the land. Deeply buried carbonate rocks can be pushed upwards,
exposing them on the surface. This is happening in the
Nitrogen
Cycle
Nitrogen is a micronutrient
which influences the rate at which plants and animals grow. The nitrogen is
taken up by plants and used in metabolism for biosynthesis of amino acids,
proteins, vitamins, etc., and passes through the food chain. Organisms use
nitrogen in the form of nitrates to make many organic compounds such as
proteins, DNA and RNA. Nitrogen gas (N2) makes up 78 percent of the volume of atmosphere, but it can not be used
readily (in its original form) by plants and animals. This is a major limiting
factor for the growth of organisms in ecosystems. Fortunately, lightening
transforms a small amount of tropospheric nitrogen into nitrates. Nitrogen is
added to the cycle through volcanic action. But some nitrate is lost when it is
buried in deep-sea sediments. But most of the nitrates originate from nitrogen
fixation. Certain bacteria (mostly cyanobacteria in soil and water and Rhizobium
bacteria living in small nodules on the root systems of a wide variety of
leguminous plants such as peas, beans, lentils etc.) convert nitrogen gas into
compounds (molecular nitrogen) that can enter food webs as part of nitrogen
cycle. The conversion of atmospheric nitrogen gas into chemical forms- mostly nitrate
ions and ammonium ions- that are useful to plants is called nitrogen fixation. Plants, in turn,
convert inorganic nitrate ions in soil water into proteins, DNA and other
nitrogen-containing nutrients. Animals get their nitrogen by eating plants and
plant-eating organisms. Nitrate is also formed from the decay of organic matters.
Birds and fish add to the nitrate pool. Decomposer bacteria convert the
nitrogen-rich organic compounds, wastes and dead bodies of organisms into
inorganic compounds such as nitrate. Certain types of bacteria return nitrogen
to the atmosphere by de-nitrification of nitrates back to nitrogen to begin the
cycle again (Figure 3.5)..
Humans intervenes
the nitrogen cycle in various ways: First, large quantities of harmful
nitric oxide (NO) are emitted into the atmosphere by industries and motorized
vehicles, resulting from burning of fossil fuel. The NO combines oxygen to form
nitrogen dioxide (NO2) gas, which can react with water vapour to form nitric
acid- a component of acid rain which damage trees and upset aquatic ecosystems.
Second: Heat-trapping nitrous oxide (N2O) gas emitted into the air by
the bacteria on the livestock wastes, and chemical fertilizers applied to the
crop. Third, nitrogen is removed
from earth's crust during mining operation of ammonium nitrate for fertilizers,
deplete nitrogen from topsoil by harvesting nitrogen-rich crops, and leach
water-soluble nitrate ions from soil through irrigation. Fourth, at the
time forests and grasslands are burned, nitrogen is lost from topsoil and
nitrogen oxides are emitted into the atmosphere. Finally, excessive
nitrogen compounds are being added to aquatic ecosystems through agricultural
runoff and municipal sewage discharge. This additional plant nutrients
stimulates rapid growth of algae and other aquatic plants. The subsequent
breakdown of dead algae by aerobic decomposers can deplete the water of
dissolved oxygen and can disrupt aquatic ecosystems.
Figure- 3.5: Nitrogen Cycle; Source: Google images
Phosphorous
Cycle
Phosphorous, in
the form of phosphate ions is mainly a plant food- essential nutrient. Many
animals also need phosphorous for their growth and development, particularly
for shell, bone and teeth. It is a part of DNA molecules that carry genetic
information. Phosphorous neither exists in the gaseous state nor circulated in
the atmosphere, but circulates through water, soil (earth's crust) and living
organisms in the phosphorous cycle. It occurs in rocks as phosphate and moves slowly from phosphate to
living organisms. Some phosphorous reaches the sea through surface runoff. Much
of it is deposited in the deep-sea sediment. Some of it is returned to land
through marine fish and birds Phosphorous that occurs in earth's crust is
gradually released through the processes of erosion and leaching. When
phosphorous (phosphate rock deposits) is released by weathering, is often
dissolved in soil water and then is taken up by plant roots. Wind can also
carry phosphate particles long distances. Animals get phosphorous by eating
producers or animals that have eaten producers. Animal wastes, and the decay of
dead animals and producers recycle back much of this phosphorous to the soil,
to streams, and eventually to ocean bottom as deposits of phosphate rock. Some
phosphate returns to the land as phosphate-rich manure, typically of fish-eating
birds such as pelicans. Phosphorous, returns to the land through gradual
geological processes. Weathering then slowly releases phosphorous from the
exposed rocks and continues the cycle (Figure 3.6).
Humans intervene
in the phosphorous cycle in various ways. A large quantity of phosphate is
released to the environment through widespread use of chemical fertilizers in
croplands. Discharge of municipal sewage containing phosphorous ultimately
finds its way to water bodies. Phosphate bearing mining wastes also discharged
into the aquatic environment. Too much of this causes rapid growth of
cyanobacteria, algae, and aquatic plants, disrupting life in aquatic ecosystems
Figure- 3.6: Phosphorous Cycle; Source: Google
images
Organisms need
this nutrient in the form of sulphate- an essential element of biological
molecules (atoms) in small quantities.
About a third of
all sulfur (including 99 percent of SO2) that reaches the atmosphere comes from the anthropogenic sources. We
intervene the atmospheric phase of sulphur cycle in two ways by i) burning
sulfur-containing coal and oil t produce electric power (responsible for
two-thirds of the human inputs of sulfur dioxide), and ii) refining petroleum;
smelting sulfur compounds of metallic minerals into free metals such as copper,
lead and zinc; and using other industrial processes.
In the atmosphere,
sulphur dioxide reacts with oxygen to produce sulphur trioxide gas (SO3), which
in turn reacts with water vapour to form tiny droplets of sulfuric acid. Sulfur
dioxide also reacts with other chemicals in the atmosphere to produce tiny
particles of sulfate salts. These droplets of sulfuric acid and particulates of
sulfate salts fall on Earth as components of acid deposition, which along with
other air pollutants can harm trees and aquatic life.
Human interference
mostly comes from burning of fossil fuels, which results in the emission of
sulphur dioxide into the atmosphere,
Figure-3.7: Sulfur
Cycle; Source: Google images
Carbon, nitrogen,
phosphorus and sulphur are the major components of the material cycle. Human
interference in these natural cycles has led to disruptions, which have far
reached adverse impacts leading to environmental degradation.