Course description
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 South-east Asia falls within the Oriental realm. At a national level, this forms biogeographic regions. At an even more local level, each area has several structurally and functionally identifiable features such as different types of forests, grasslands, river chtchments and mangrove swamps in deltas and coastal areas. Here too, each of these forms a habitat for specific plant and animal species.
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
Trophic Structure and Level
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 and Materials Flow in the Ecosystem
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.
Bio-geochemical Cycles
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.
Carbon Cycle
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 Himalayas, which contain sedimentary carbonate rich rocks that were formed at the bottom of some ancient ocean. Once at the surface, the rocks are once again exposed to weathering and erosion Some CO2 also enters the atmosphere from aerobic respiration and from volcanic eruptions, which free carbon from rocks deep in Earth’s crust. Any excess carbon dioxide in the atmosphere is absorbed by water in the ocean, thus maintaining the balance of atmospheric carbon dioxide This carbon re-enters the cycle very slowly, when some of the sediments dissolved that can enter the atmosphere as CO2. Some carbon lies deep in the Earth in fossil fuels- coal, petroleum, and natural gas- and is released to the atmosphere as carbon dioxide only when these fuels are extracted and burned. Geologic processes can also bring bottom sediments to the surface, exposing the carbonate rock to chemical attack by oxygen and conversion to CO2 gas. Acidic rain falling to exposed limestone rock also releases carbon dioxide back into the atmosphere.
Human Activity: For most part of human history, we had no net impact on atmospheric carbon. However, human interference has led to imbalance in the atmospheric carbon dioxide. Excessive carbon dioxide is added to the atmosphere by burning of fossil fuels in homes, industries, and automobiles. Since 1800, especially 1950 as world population and resource use have soared, we have disturbed the carbon cycle in two ways: i) forest removal has left less vegetation to absorb CO2 through photosynthesis, and ii) burning fossil fuels and wood produces CO2 that flows into the atmosphere. The capacity of the environment to absorb carbon dioxide is being reduced because of the human intervention and over exploitation of forest resources. The fairly large input of CO2 from human activities or change in natural processes can affect climate and ultimately the types of life that can exist on our planet e.g. alter climate patterns, disrupt global food production and wild life habitats, and possibly raise average sea level.
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
Sulphur Cycle
Organisms need this nutrient in the form of sulphate- an essential element of biological molecules (atoms) in small quantities. Sulphur and its compounds are important elements of industrial processes. For example, sulphur dioxide is a bleaching agent and is used to bleach wood pulp for paper and fiber for various textiles. It is also used to preserve dry fruits. Most of the sulphate is found in rocks, although some of it comes from the atmosphere. It also appears as the yellow aspects of soil in many regions. Sulphur also occurs in combination with a number of metals such as lead and mercury. When bacteria digest plant matter, they emit hydrogen sulfide (H2S). Various types of microorganisms present in the soil change one form of sulphuer into another form. Sulphur circulates through the biosphere in the sulphur cycle (Figure 3.7). Much of earth's sulphur is tide up underground in rocks (such as pyrite) and minerals, including sulfate salts (such as gypsum, or hydrous calcium sulfate) buried deep under ocean sediments. Sulpuhr also enters the atmosphere from several natural sources. Hydrogen sulfide (H2S), a colourless, highly poisonous gas with a "rotten egg" smell, is released from active volcanoes and the breakdown of organic matter in swamps, bogs, and tidal flats caused by decomposers that don't use oxygen (anaerobic decomposers. Sulfur dioxide (SO2), a colourlss, suffocating gas, also comes from volcanoes. Particles of of sulphate salts, such as anmonium sulfate, enter the atmosphere from sea spray.
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, Sulphur dioxide reacting with atmospheric moisture precipitates into acid rain. Sulphur is released into the atmosphere through the burning of fossil fuels, especially high sulphur coal and this is a primary constituent of acid rain. Due to its high reactivity sulphur is quickly deposited as sulfates on land and other surfaces.
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.
