Ecosystems PDF

Ecosystems 1 ❖ In this unit, we will discuss: Origin and characteristics of life Ecosystem and Ecology Structure and functions of ecosystem Functional aspects of an ecosystem Material/nutrient cycling in ecosystem...

Ecosystems 1 ❖ In this unit, we will discuss: Origin and characteristics of life Ecosystem and Ecology Structure and functions of ecosystem Functional aspects of an ecosystem Material/nutrient cycling in ecosystem Ecosystem productivity Ecological succession Ecosystem-examples The first law of ecology is that everything is related to everything else Barry Commoner, 1971 2  1. Introduction Life is a characteristic combination of physical entities having biological processes (signaling and self-sustaining processes). Various forms of life exist as - plants, animals, fungi, protists, archaea, and bacteria. Human beings stand at the top in the hierarchy of life. Biology is the primary science concerned with the study of life, although many other sciences are also involved. Suppose, imagine a cat sitting on a table in your room and staring at you, or a flower fresh and fragrant in a gamla. You know that the cat and the flower are alive where as the table and the gamla aren’t. But if you examine the cat and the table or the flower and the gamla at atomic and molecular levels, you will find that the differences between them blur. 3  Cats, tables and all other things are made of atoms and molecules which behave according to the same physical laws. There is something that differentiates living organisms from non living things though both are composed of atomic and molecular assemblages. This something is ‘life’. Cell is the basic unit of life. It is composed of biochemical molecules. However, an organism is taken as the first living system. Living organisms must gather energy and materials from their surroundings to build new molecules, grow in size, maintain and repair their parts and produce offsprings. Some living organisms such as bacteria and protozoan consist of a single cell (they are unicellular) whereas others such as fungi, plants and animals, consist of large number of cells (they are multicellular). 4  A group of similar organisms (i.e. belonging to the same species) in a particular area at a particular time is called Population. Number of populations (organisms of different species taken collectively) in an area are called a community. An identical or distinguishable portion of the earth containing several communities is called a biome. Entire zone on earth consisting of living organisms is known as Biosphere. Biosphere along with its environment is called Ecosphere. 5  2. Origin and Characteristics of Life Extraterrestrial life: Earth is the only known planet to have life. It is because of its unique environment which is also known as the life-support-system. Other locations within our Solar System that may host life include subsurface Mars, the atmosphere of Venus and subsurface oceans on some of the moons of the gas giant planets. Astro ecology experiments with meteorites show that Martian asteroids and cometary materials are rich in inorganic elements and may be fertile soils for microbial, algal and plant life, for past and future life in our and other solar systems. Some people believe that the life originated somewhere else and then transferred to Earth in the form of spores via meteorites, comets, or cosmic dust. 6  Origin and Characteristics of Life Owing to its unique environment, the earth is the only planet to harbor life: On the earth we can find living organisms from the poles to the equator, from the bottom of the sea to several miles in the air, from freezing waters to dry valleys. Over the last 3.7 billion years or so, living organisms on the Earth have diversified and adopted to diverse environmental conditions. The diversity of life is truly amazing, but all living organisms do share certain similarities. All living organisms can replicate, and the replicator molecule is DNA. Earlier, living organisms were divided into two kingdoms: animal and vegetable, or the Animalia and the Plantae. 7  Now the most often used scheme divides all living organisms into five kingdoms: Monera (bacteria), Protista, Fungi, Plantae, and Animalia. This coexisted with a scheme dividing life into two main divisions: the Prokaryotes (bacteria, etc.) and the eukaryotes (animals, plants, fungi, and protists (euglena)). Another type of biological entities, the viruses, are not organisms in the same sense as other living organisms are. However, they are of considerable biological importance. 8  Only about 1.4M species have so far been enlisted and only 2.5 to 12% of the total number of species on the earth are described. No one knows the exact number of species present on the earth. Scientists, however, believe it to be somewhere between 10M to 80M 9 Levels of organization: The entire world around us is made up of Space, Matter, and Energy Space is expressed in terms of area etc. and is present everywhere around. Everything around us is made up of some substances or materials called matter. This matter is expressed in terms of mass. Energy is present in the universe as an important ingredient that makes this world functional. Space, matter and energy are the subject matter of various Physical Sciences. 10  Atom is the lowest level of organization in the living world and non-living world while biosphere is the highest level of organization in the living world. Atoms, molecules and chemical compounds are nonliving but are very important for life. Life starts at the level of cells which form tissues and organs. An organism is the first living entity for ecological studies. Populations, communities, ecosystems and the entire biosphere are the study area of ecology/environmental studies. Each level in this hierarchy of biological organization is unique in its structure and function and shows additional properties than those of its lower level. At every level there emerge some unique properties, also known as emergent properties, which are always more than the properties of its constituent parts taken together. 11 Life on the earth: Levels of organization 12  3. Ecosystem Life does not exist in space or isolation. It needs a substratum which provides space, necessary substances and favourable conditions for living organisms. In an area, the community of living organisms interacts with its physical environment to form a definite structural and functional system. This structural and functional unit of life in nature is called an Ecological System or simply an Ecosystem. 13  The term ecology has been derived from a Greek word ‘oikos’ (Oikos=household/habitat; logos=study). The word “ecology” (“Ökologie”) was coined in 1866 by the German scientist Ernst Haeckel. It relates to the scientific study of organisms or groups of organisms in their natural habitat. The science of ecology is often categorized as a branch of biosciences that studies the interactions among organisms and their environment, such as the interactions organisms have with each other and with their abiotic environment. Earlier, two other terms viz. ethology and hexicology were also used for such studies but the newer term ecology predominated. 14  Different ecologists have defined Ecology in different ways: Ecology is the scientific study of the distribution and abundance of animals. (Andrewartha,1961) Study of interactions of form, function and factors is ecology. (R. Mishra, 1967) Study of structure and function of nature may be defined as ecology. (Odum, 1971) The Scientific study of the interactions that determine the distribution and abundance of organisms is known as ecology. (Krebs,1985) Ecology is the study of the relationships between organisms and the totality of the physical and biological factors affecting them or influenced by them. (Pianka, 1988) 15  Ecology can be divided into various subdivisions such as: On the basis of taxonomic affinities Plant Ecology- Study of interrelationships of plants with their environment. Animal Ecology- Study of interrelationships of animals with their environment Based on habitat: Habitat ecology-Study of habitats and their effects upon the organisms. Based on levels of organization: Autecology- ecological study of one species of organisms. Synecology-ecological studies of more than one species of organisms. Population ecology-Study of interactions between individuals of same species. Community ecology-Study of interactions between individuals of different species. Biome ecology-Study of interaction between different communities of a biome. Ecosystem ecology-Study of interactions between the biotic and abiotic components of an eco-system. 16 Based on specialized fields of ecology: Freshwater ecology: Study of interactions among freshwater organisms. Marine ecology: Study of interactions among marine organisms. Zoogeography: Geographic distribution of animals. Phytogeography: Geographic distribution of plants. Statistical ecology: Statistical studies on population, sampling techniques and community problem. Estuarine ecology: Study of interactions among estuarine organisms. Terrestrial ecology: Study of interactions among terrestrial (land) organisms. 17  Now, coming back to the ecosystem. An ecosystem is a spatial and organizational unit which is formed by the interactions of living organisms with each other and with their physical environment. Thus, the community of living organisms (plants, animals and micro- organisms) in any area taken together with their non-living environmental components (such as soil, air and water) forms an ecosystem. A pond, grassland, garden, forest, etc are the common examples of ecosystem. The earth’s living organisms interacting with their physical environment (i.e., biosphere) may be considered as a giant and vast ecosystem. On the contrary a small pool of water containing certain forms of living organisms (such as plants, insects, microorganisms, etc) may also be regarded as an ecosystem. 18  The term “ecosystem” was first used by a British ecologist Arthur Tansley in 1935. He explained the concept of ecosystem which can be summarized as: When both, biotic and abiotic components are considered, the basic structural and functional units of nature are ecosystems. There exist varying degrees of positive or negative or even neutral interactions among organisms at both interspecific and intraspecific levels (within the members of same species or between the members of different species). Energy is the driving force of this system. Energy flow is unidirectional and noncyclic. There operate biogeochemical cycles in the ecosystem. This movement of nutrients within an ecosystem is always cyclic. The limiting factors of environment govern the successful growth of organisms. Under natural conditions, different kinds of populations undergo succession. 19 A. Types of ecosystems: Biosphere is the largest and an all encompassing ecosystem. However, it is difficult to handle this huge system for ecological studies. It is for our convenience that we have divided this big ecosystem into smaller ecosystems based on our own spatial considerations. Ellenberg (1973) classified the world into a hierarchy of ecosystems. After biosphere, next lower level is mega-ecosystem such as marine ecosystems (seas, oceans, lakes, etc.), limnic ecosystems (fresh water ecosystems) semi terrestrial (ecosystems of wet soil and air), terrestrial and urban-industrial ecosystems (cropland, city, etc.). Lower to mega ecosystems is, macro ecosystems as forest within a mega- ecosystem. 20  Meso-ecosystems (such as a deciduous broad leaved forest), micro ecosystems and nano-ecosystems are still lower levels of ecosystem. These lower level ecosystems are spatially contained within another (higher level) ecosystem but show certain individuality of their own. 21 In a simpler way ecosystems can broadly be categorized as under: I. Terrestrial ecosystems: all the ecosystems on land such as a forest, a desert, grassland, a cropland, etc. II. Aquatic ecosystems: all the ecosystems where the dominant factor is water such as a pond, a lake, a river, a spring, a lake, a sea, an ocean, etc. III. Natural ecosystems: all the naturally occurring ecosystems where man’s interruption is thought mainly to be unwarranted such as a forest, grassland, river, etc. IV. Artificial ecosystems: all the ecosystems which are created and managed by man such as a cropland, a garden, a pond, etc. 22  e.g. Garden, crops, aquarium, Farmland etc., Various types of ecosystems 23  4. Structure and functions Ecosystem A system is a collection of interdependent parts and/or events that make up a whole. A computer for example has its constituent parts such as CPU, monitor, input devices, etc. which together make it a system. Similarly, a radio set, a mobile phone, a watch, a car or any other machine are examples of system. A computer works only when all of its constituent parts are in proper order and function appropriately. Every part has its specific functioning but all parts function together in the system, say a computer, radio or car to make it a functional system. The whole system fails to function unless there is some kind of input from the outside. 24  System having received some input acts to produce some output. For the computer, radio or car inputs are electric power and petrol and output are a printout, sound and speed. Like any other system, ecosystem too has its structural and functional attributes. It too requires input and produces output. Energy from the sun forms the input for an ecosystem. It works and produces output called ecosystem productivity. 25 A. Structural aspects of an ecosystem The physical components of an ecosystem, their inter-relationships and the resultant configuration constitute the structure of an ecosystem. This structural framework can be expressed in simple categorization (abiotic, biotic, producers, consumers, etc) of various components or in graphical representations such as food chains, food web, ecological pyramids, etc. ❖ As an ecosystem is not only a biological entity. It is composed of following types of general components Abiotic Component Biotic Component 26 Schematic representation - Structural aspects of an ecosystem 27 I. Abiotic Components It is the portion of an ecosystem that is non living. Rocks, soil, gases, water, temperatures, winds, other forces, etc. are abiotic component of ecosystem. It is, thus, also said to be the aggregate of environmental factors and includes: Climatic factors such as rainfall, humidity, temperature, light. Topographic factors such as altitude, slope, direction of mountain ranges etc. Edaphic factors such as soil composition, soil texture, soil biota etc. 28  These abiotic components/environmental factors in an ecosystem are capable of bringing marked distributional, structural and functional changes in organisms. An organism requires harmonious relationship with its immediate environment for its proper growth, reproduction, etc. The difference between the types of vegetation or consumers of a desert and a rain forest indicates the role of environmental factors on the distribution and survival of organisms in different ecosystems. These environmental factors exhibit diurnal, seasonal, annual and cyclic variations to which the organisms are subjected. 29 II. Biotic Components Biotic components are the living entities in an ecosystem. They include all the microbes, animals, plants and their products. They can be categorized as: a) Producers or Autotrophic components: These are the producers which convert simple inorganic substances into complex organic substances with the help of solar energy. They are of two types: (i) Photosynthetic: They manufacture food with the help of chlorophyll in presence of sunlight so energy utilized is radiant energy. These constitute the major proportion of autotrophic components. It includes green plants, green algae and photosynthetic bacteria. (ii) Chemosynthetic: They manufacture food with the help of chemical energy evolved during chemical reactions. They contribute to lesser extent to the production of food in an ecosystem. 30  In ecosystems we generally consider only green plants as producers as they manufacture their food by using energy from the sun. In the sea these include tiny algal forms to large seaweeds. b) Consumers or Heterotrophic components: They consume the food produced by the producers. They are of following types. (i) Macro consumers: These are the consumer organisms which are of larger size and feed upon the producers. Based upon their position in the food chain they can be categorized in primary, secondary and tertiary consumers. Primary consumers (Herbivores): Eat producers such as green plants eg. Deer, goat, grasshopper, etc. Secondary consumer (Smaller carnivores): eats herbivores (animals) eg. Snake, eagle, lizard, large fish, etc. Tertiary consumers (Larger carnivores): Eats smaller carnivores eg. Lion, hawk, tiger, man, etc. 31 (ii) Micro-consumers or Decomposers: Decomposers are a group of organisms consisting of small animals like worms, insects, bacteria and fungi, which break down dead organic material into smaller particles and finally into simpler substances that are used by plants as nutrition. Decomposition thus is a vital function in nature, as without this, all the nutrients would be tied up in dead matter and no new life could be produced. In simpler terms biotic component of ecosystem consists of producers (green plants), consumers (herbivore and carnivore animals) and decomposers (microorganisms). 32 Trophic structure: The assemblage of various living components of an ecosystem organized in an orderly manner is called its Trophic Structure. In a trophic structure, the producers and consumers are arranged together in various levels in accordance with their inter-relationships (or simply their food- relationship) in an ecosystem. Each level in this structure is known as a trophic level. The structure and functions of an ecosystem are closely related and influence each other so intimately that they need to be studied together. The flow of energy takes place through a series of feeding relationships in a definite sequence, known as Food Chain. Nutrients too, move along the food chains only. Usually an ecosystem may have two to six trophic levels through which energy and nutrients flow. 33  Simply, all the green plants which are the primary producers of organic substances constitute one trophic level in an ecosystem. Similarly, all animals which obtain food or in other words energy by consuming green plants, such as grasshoppers, cattle, rodents, etc (ie. Primary consumers or herbivores) shall be at the same trophic level. And all those animals or predators which live on primary consumers or herbivores (i.e secondary consumers or carnivores) are said to be at a higher but same trophic level. 34 A. 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. A caterpillar eats a plant leaf, a sparrow eats the caterpillar, a hawk eats the sparrow, and when they all die, they are all consumed by microorganisms like bacteria or fungi. Food chains usually have two to six links (or trophic levels in an ecosystem). In nature, we come across two major types of food chains as below: 35 (i) Grazing food chain: It starts with green plants and culminates in carnivores. Some examples are grass → grasshopper → frog → snake → hawk(grassland ecosystem) grass → rabbit → fox phytoplanktons → waterfleas → small fish → tuna (pond ecosystem) phytoplanktons → zooplanktons → fish lichen → riendeer → man (Arctic region) 36 (ii) Detritus food chain: It starts with dead organic matter which the detritivore and decomposers consume. Partially decomposed organic matter and even the decomposers are fed upon by the detritivores. A detritivore is an organism that eats dead or decaying plants or animals as food. Detritivores include microorganisms such as bacteria and larger organisms such as fungi, insects, worms, and some crustaceans. Some examples are: Leaf litter → algae → crabs → small carnivorous fish → large fish (mangrove ecosystem) Dead organic matter → fungi → bacteria (forest ecosystem) Dead grass → termite → aardvark (grassland) 37 (iii) Parasitic food chain: Parasites which derive nutrition from other plants and animals also constitute a link in yet another type of food chain which may be designated as Parasitic food chain. It may commence at any level in a trophic structure and may at times result in heavy losses of energy. 38 B. Food web: In natural ecosystems food chains rarely operate as isolated linear sequences. They are found to be interconnected and forming a complex network of several food chains together at the same time. Food web is, thus, a network of food chains where different types of organisms are connected at different trophic levels, so that there are a number of options of eating and being eaten at each trophic level. ✓ For example, in grazing food chain of a grassland, in the absence of rabbit, grass may be eaten by mouse. The mouse in turn may be eaten directly by hawk or by snake which is then eaten by hawk. In such a food web there may be seen as many as five linear food chains. 39  grass → grasshopper → hawk grass → grasshopper → lizard → hawk grass → rabbit → hawk(or vulture or fox or even man , if present) grass → mouse → hawk grass → mouse → snake → hawk ✓ Food webs are very important in maintaining the stability of an ecosystem in nature. 40 C. Ecological pyramids: Graphic representation of trophic structure and function of an ecosystem, starting with producers at the base and successive trophic levels forming the apex is known as an ecological pyramid. The concept of ecological pyramid was developed by Charles Elton after whose name these pyramids are also known as Eltonian pyramids. ✓ There are three types of ecological pyramids i. Pyramid of Numbers: It represents the number of individuals at each trophic level. We may have upright or inverted pyramid depending upon the type of ecosystem and food chain considered. Ecosystems like a grassland or a pond show an upright pyramid of numbers. 41  The producers in a grassland are the grasses and that in a pond are phytoplanktons (algae etc.) which are small in size but very large in number. So they (producers) form a broad base. The herbivores in grassland are insects while tertiary carnivores are hawk or other birds which are lesser and lesser in number and hence the pyramid apex becomes narrower and form an upright pyramid. Similarly in a pond ecosystem, herbivores, carnivores and top carnivores decrease in number at higher trophic levels. Upright pyramid of number as in a grassland or pond 42  Upright pyramid of number as in a pond/aquatic ecosystem Upright pyramid of number as in a grassland ecosystem 43  In a forest ecosystem, big trees are producers, which are less in number and hence form a narrow base. A large number of herbivores including birds, insects and several species of animals feed upon the trees (on leaves, fruits, flowers, bark etc) and form a much broader middle level. The secondary consumers like fox, snakes, lizards, etc. are less in number than herbivores while top carnivores (like lion, tiger, etc) are still lesser in number. So the pyramid is narrow at base, broader at middle and again narrower upwards. Pyramid of number in a forest ecosystem 44  Parasitic food chains show inverted pyramids. The producers like a few big trees harbor fruit eater birds which are large in number. A much higher number of lice, bugs etc grow as parasite on these birds while a still greater number of hyperparasites like bugs, fleas, microbes, etc feed upon them thus making an inverted pyramid. Inverted pyramid as shown in parasitic food chains 45 Inverted pyramid of number in a parasitic food chain 46 47 ii. Pyramid of Biomass: It is based upon the total biomass at each trophic level in a food chain. The pyramid of biomass can also be upright or inverted. The pyramid of biomass in a forest ecosystem is upright in contrast to its pyramid of numbers where it is Dimond shape. This is because the producers accumulate a huge biomass while the consumer’s total biomass declines at higher trophic levels. The pond on the other hand shows an inverted pyramid of biomass. The total biomass of producers is much less than that of herbivores and it goes on increasing towards higher trophic levels. 48 Pyramid of Biomass (up right) 49  Inverted pyramid of Biomass for marine ecosystem 50 51 52 iii. Pyramid of Energy: The amount of energy present at each trophic level is considered for this type of pyramid. This type of pyramid gives the best representation of the trophic relationship and it is always upright. At every successive level there is a huge loss of energy (about 90%) in the form of heat, respiration, etc. thus at each next higher level only 10% of the energy passes on. Hence there is a sharp decline in energy level of each successive trophic level as we move from producers to top carnivores. 53 Upright pyramid of Energy 54 D. Functional aspects of an ecosystem: An ecosystem works as a unit in an efficient and organized way. It receives energy from the sun and passes it on through its components and, in fact, all life depends on this flow of energy. Green plants (including phytoplanktons) alone are able to trap the solar energy in an ecosystem. They make use of this energy for their growth and maintenance. Energy gets stored as chemical bonds of large organic molecules in green plants. Heterotrophs or consumers obtain their energy requirements from this stored energy (in green plants) as food and use it for their development, growth, maintenance or other life activities. All life forms in an ecosystem are linked together by the flow of energy. Besides energy, various nutrients and water, which are also required for life processes, are exchanged by the biotic components within themselves and with their abiotic components. 55  The flow of energy and nutrients in an ecosystem keeps it going on. This mechanism can be studied in a simplified manner as under. Ecosystem Energetics As stated above an ecosystem needs energy inputs from outside. Materials are used from within an ecosystem. Flow of energy in an ecosystem takes place through food chains and it is this energy flow which keeps the ecosystem going on. Most important feature of this flow is that it is unidirectional. Unlike the nutrients which move in a cyclic manner and are reused by the producers after flowing through the food chain, energy is not reused in the food chain. Flow of energy follows the laws of thermodynamics. i. First law of thermodynamics states that the energy can neither be created nor be destroyed but it can be transformed from one form to another. The solar energy captured by the green plants is converted into biochemical energy of plants and latter into that of consumers. 56  ii. Second law of thermodynamics states that energy dissipates as it is used or in other words, it gets converted from concentrated to dispersed form. As energy flows through the food chains, there occurs dissipation of energy at every trophic level. At each trophic level, about 90% of energy gets lost and only 10% of it gets transferred to the next level. Energy Flow in Ecosystem As I mentioned above the sun is the only source of energy for the entire biosphere. Solar energy travels in electromagnetic waves form. It consists of a wide range of wavelengths and various types of radiations (infra red, visible, ultra violet, etc.). Only a specific portion of sun’s electromagnetic spectrum is utilized by the producers. The amount of solar energy reaching a surface perpendicular to the sun rays at outer atmosphere is called solar constant. This is 2.00 calories per per sq cm per minute. Of this quantity about 1.00cal/sq.cm/min reaches the earth’s surface. The flow of energy through various trophic levels in an ecosystem can be explained with the help of various energy flow models. 57 Universal energy-flow-model: As the energy enters and flows through the ecosystem there is a gradual loss of it at every level, thereby resulting in less energy available at every next trophic level. Universal Energy Flow Model This is indicated by narrower (E.P. Odum) pipes(energy flow) and smaller boxes(stored energy in biomass) in Figure beside. The loss of energy is the energy not utilized (NU). This is the energy lost in locomotion, excretion, other life activities etc. or it is the energy lost in respiration(R). The rest of energy is used for production (P). 58 a) Single channel energy-flow-model: This model depicts the flow of energy in ecosystem an ecosystem through a single channel or linear sequence. Energy enters as sunlight in an ecosystem and flows from green plants or producers to herbivores and carnivores. During this energy flow, there is a gradual decline in energy level due to loss of energy at each successive trophic level in a grazing food chain. Single channel Energy Flow Model 59 60 61 b) Double channel or Y-shaped energy-flow-model: This model is more realistic as it considers both types of food chains found in natural ecosystems. In nature both grazing food chain and detritus food chains operate in the same ecosystem. In a forest ecosystem a huge quantity of biomass produced cannot be all consumed by herbivores. A large proportion of the live biomass enters into the detritus (dead) component of ecosystem in the form of litter. Hence the detritus food chain is equally important. In marine ecosystems, however, a major portion primary production is eaten by the herbivorous marine animals. Therefore, very little primary production is left to be passed on to the dead or detritus component. The Y-shaped model of energy flow shows the passage of energy through ecosystem where both grazing and detritus food chains operate together. 62 63 64 65 66 Material/Nutrient Cycling in Ecosystem: Besides energy flow the other important functional attribute of an ecosystem is nutrient cycling. All organisms require two types of nutrients: Macro-nutrients and Micro-nutrients (Macro-nutrients: Required in large amounts e.g. C, N, O, H, S, P, Ca, Mg etc.; Micronutrients: required in small amounts e.g. Fe, Mn, Cu, Zn, B, Co, Cl, Na, etc.). Nutrients like carbon, nitrogen, sulphur, oxygen, hydrogen, phosphorus, etc. move in circular paths through biotic and abiotic components and are therefore known as biogeochemical cycles. There are two types of biogeochemical cycles - (i) gaseous; and (ii) sedimentary Gaseous-Reservoir lies in atmosphere e.g. C, N, O cycle etc.; and, Sedimentary- Reservoir lies in the earth’s crust e.g. P, S, Ca, etc. Water also moves in a cycle known as hydrological cycle. 67  The term “biogeochemical” indicates that in these cycling of nutrients biological, geological and chemical factors are all involved in the process. The circulation of chemical nutrients and water takes place through the biological as well as physical world. In effect, the elements are recycled, although in some cycles there may be places (called reservoirs) where the elements are accumulated or held for a long period of time (such as an ocean or lake for water) The nutrients move through the food chains and ultimately reach the detritus component (containing dead organic matter) where various microorganisms carry out the process of decomposition. Various organically bound nutrients of dead animal and plants are converted into inorganic substances by microbial decomposition and are again used up by plants and the cycles start afresh. Some of the important biogeochemical cycles we will discuss briefly here. 68 Nitrogen cycle: Nitrogen is present in the atmosphere as N2 in large amount (78%) and it is fixed either by the physical process of lightening or biologically by some bacteria and/or cyanobacteria (blue green algae). 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. After death of the plants and animals, the organic nitrogen the organic nitrogen in dead tissues is decomposed by several groups of ammonifying and nitrifying bacteria which convert them into ammonia, nitrites and nitrates, which are again used by plants. Some bacteria convert nitrates, into molecular nitrogen or N2 which is released back into the atmosphere and the cycle goes on. 69 Nitrogen cycle 70 Nitrogen cycle 71 Carbon cycle: Carbon is taken up by green plants in the form of carbon dioxide as a raw material for photosynthesis. In the process a variety of carbohydrates and other organic substances are produced. So it moves through the food chains and ultimately organic carbon present in the dead matter is returned to the atmosphere as carbon dioxide by microorganisms. Respiration by all organisms produces carbon dioxide which is released in the atmosphere from where is used up by plants. In the recent years carbon dioxide levels have increased in the atmosphere due to burning of fossil fuels etc. It has caused an imbalance in the natural cycle and the world today is facing the serious problem of global warming due to enhanced carbon dioxide emissions in the atmosphere. 72 Carbon cycle 73 74 Phosphorus Cycle: Reservoir of phosphorus lies in the rocks, fossils etc. which is excavated by man for using it as a fertilizer. Farmers use the phosphate fertilizers indiscriminately and as a result excess phosphates are lost as run-off, which causes the problem of eutrophication of lakes leading to algal blooms. A good proportion of phosphates moving with surface runoff reaches the oceans and lost into the deep sediments. Our limited supply of phosphorus lying in the phosphate rocks of this earth are thus over-exploited by man and a large part is taken out of the normal cycle due to loss into oceans. So human beings are making the phosphorous cycle acyclic. 75  Sea birds, on the other hand, are playing an important role in phosphorus cycling. They eat sea-fishes which are phosphorus rich and the droppings or excreta of the birds return the phosphorus on the land. The Guano deposits on the coasts of Peru are very rich sources of phosphorus. Phosphorus Cycle 76 77 Oxygen Cycle: Oxygen is taken up by plants and animals from the air during respiration. The plants return oxygen to the atmosphere during photosynthesis. The main source of atmospheric free oxygen is photosynthesis, which produces sugars and free oxygen from carbon dioxide and water: 78  Photosynthesizing organisms include the plant life of the land areas as well as the phytoplankton of the oceans. The tiny marine Cyanobacterium prochlorococcus accounts for more than half of the photosynthesis of the open ocean. An additional source of atmospheric free oxygen comes from photolysis, whereby high-energy ultraviolet radiation breaks down atmospheric water and nitrous oxide into component atoms. The free H and N atoms escape into space, leaving O2 in the atmosphere: The main way free oxygen is lost from the atmosphere is via respiration and decay, mechanisms in which animal life and bacteria consume oxygen and release carbon dioxide. The lithosphere also consumes free oxygen by chemical weathering and surface reactions. An example of surface weathering chemistry is formation of iron oxides (rust): 79  Oxygen is also cycled between the biosphere and lithosphere. Marine organisms in the biosphere create calcium carbonate shell material (CaCO3) that is rich in oxygen. When the organism dies, its shell is deposited on the shallow sea floor and buried over time to create the limestone sedimentary rock of the lithosphere. Weathering processes initiated by organisms can also free oxygen from the lithosphere. Plants and animals extract nutrient minerals from rocks and release oxygen in the process. 80 Hydrological cycle: The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice), and gas (vapor). The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. The sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapour into the air. Ice, rain and snow can sublimate directly into water vapour. Evapotranspiration is water transpired from plants and evaporated from the soil. 81  Rising air currents take the vapour up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move water vapour around the globe, cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow or hail, sleet, and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with stream flow moving water towards the oceans. Runoff and water emerging from the ground (groundwater) may be stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground as infiltration. 82  Some water infiltrates deep into the ground and replenishes aquifers, which can store freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface- water bodies (and the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. In river valleys and flood-plains there is often continuous water exchange between surface water and ground water in the hydrospheric zone. Over time, the water returns to the ocean, to continue the water cycle. The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate. 83 Hydrological cycle 84  The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet. Biogeochemical cycles are also the links between different components of the environment such as lithosphere, hydrosphere, atmosphere and biosphere. They portray the movements of substances on the entire globe. Together in a systematic manner these cycles are responsible for maintaining life on earth. If man, through his excessive interference, disturbs these cycles beyond the limits that nature can sustain, they will eventually break down and lead to a degraded earth on which man will not be able to survive. 85 E. Ecosystem Productivity: Production, in ecology, is related with the generation of biomass in an ecosystem. The productivity of an ecosystem thus refers to the rate of production i.e. the amount of organic matter, which is accumulated in any unit time. It is usually expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day (g m–2 d–1). Productivity of autotrophs such as plants is called primary productivity, while that of heterotrophs such as animals is called secondary productivity. ❖ Primary production: Primary production is the synthesis of new organic material from inorganic molecules such as H2O and CO2. It is dominated by the process of photosynthesis which uses sunlight to synthesise organic molecules such as sugars, although chemosynthesis represents a small fraction of primary production. 86  These are the green plants, higher saprophytes as well as lower forms, the phytoplanktons and some photosynthetic bacteria. We can define Primary productivity as “the rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producers.” Primary production of an ecosystem depends upon the solar radiations, availability of water and nutrients and upon the type of the plants and their chlorophyll content. Productivity of tropical forests and estuaries are the highest. This is because tropical forests have abundant rainfall, warm temperature congenial for growth, abundant sunlight and a rich diversity of species. Primary productivity is of two types. A. Gross primary productivity: ✓ it is the total rate of photosynthesis including the organic matter used up in respiration. It is also called as Total assimilation. Primary productivity is estimated either in terms of chlorophyll content as Chl/g dry weight per unit area or amount of CO2 fixed /g Chl/hour. 87  B. Net primary productivity: ✓ is the rate of storage of organic matter in plant tissues in excess of the respiratory utilization by plants during the measurement period. This is, thus, the rate of increases of biomass and is also known as Net Assimilation. In this way, net primary productivity refers to balance between gross photosynthesis and respiration and other plant losses as death etc. ❖ Secondary production Secondary production is the generation of biomass of heterotrophic (consumer) organisms in a system. This is driven by the transfer of organic material between trophic levels, and represents the quantity of new tissue created through the use of assimilated food. Organisms responsible for secondary production include animals, protists, fungi and many bacteria. 88  Secondary productivity is thus rate of energy storage at consumers’ level. Since consumers only utilize food materials (already produced) in their respiration, simply covering the food matters to different tissues by an overall process. The secondary productivity is not divided into ‘gross’ and ‘net’ amount. Net Productivity: Net productivity refers to the rate of storage of organic matter not used by the heterotrophs (consumer) i.e. equivalent to net primary production minus consumption by the heterotrophs during the unit period. It is thus the rate of increase of biomass of the primary producers, which has been left over by the consumers. 89  5. Ecological Succession Succession is a gradual process in which structure of an ecosystem gets changed over time. Ecosystems are not static in nature. They are always in a state of change and dynamism. It is actually the structure of biotic community that evolves in the process. They change themselves in accordance with the prevalent environmental conditions. These changes are very orderly and predictable. It is seen that at a particular place a particular community of organisms is totally replaced by another over a period of time. ✓ Ecological succession can be defined as an orderly process of changes in the structure and function of a community in ecosystem with time mediated by modifications in environmental complex. 90 ✓ Succession takes place because through the processes of living, growing and reproducing, the organisms interact with and affect their environment and gradually change it. ✓ 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 outcompeted by a different set of species which are better adapted to the new conditions. ✓ Change in the plant species present in an area is one of the driving forces behind changes in animal species. This is because each plant species will have associated animal species which feed on it. ✓ The presence of these herbivore species will then dictate which particular carnivores should be present. Likewise, the microbial communities are also influenced by the plant and animal communities present at a particular place. ✓ So the ºengineº of succession, the cause of ecosystem change, is the impact of established species upon their own environments. Hence the process of ecological succession is mediated by the interaction between the biotic communities and their environment. 91  ✓ In very simple terms ecological succession is a natural process by which different groups or biological communities colonize the same area over a period of time in a sequence. ✓ The first ever living organisms that colonize a place and start the process of succession are known as pioneers. ✓ The communities which follow pioneers are known as the seres or seral communities and the final community of plants and animals that establishes itself after the process of succession is known as climax community. 5.1. Types of succession: A. Primary succession: ✓ It occurs in essentially lifeless areas—regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. 92 B. Secondary succession: ✓ It occurs in areas where a community that previously existed has been removed; it is typified by smaller-scale disturbances that do not eliminate all life and nutrients from the environment. C. Autogenic Succession: ✓ If the existing community itself causes its replacement by some other community it is said to be autogenic succession. D. Allogenic succession: ✓ If the existing community is replaced by another community due to some external force it is called allogenic succession. E. Autotrophic succession: ✓ Here the early and continued dominants are autotrophs. There is gradual increase in the organic matter content supported by energy flow. 93  F. Heterotrophic succession: ✓ Here early dominants are heterotrophs. There is progressive decline in energy content. Depending upon the environment where the process of succession takes place, it is denoted with different terms such as: a) Hydrosere: Succession takes place in water such as in ponds, lakes and stream. b) Xerosere: Process of succession begins in xerophytic or desert like conditions. c) Lihosere: Succession starts on rocks or rocky background. d) Halosere: Succession takes place in saline water or soil e ) Psammosere: Succession, here, takes place in sand. 94 5.2. Causes of Succession: There are three major causes of succession: A. Initial or initiating causes: These are climatic as well as biotic. Climatic factors include erosion and deposits, wind, fire etc caused by lightening or volcanic activity and biotic include various activities of organisms. These causes produce bare areas or destroy the existing populations in an area. B. Ecesis (continuing) causes: These are the processes as migration, ecesis, aggregation, competition reaction etc which cause successive waves of populations as a result of changes chiefly in the edaphic features of the area. C. Stabilizing causes: These cause the stabilization of the community. Climate of the area is the chief cause of stabilization. 95 5.3. General Mechanism of succession: The whole process of a primary succession is completed through a number of sequential steps, which follow one another. These steps in sequence are as follows: A. Nudation: This is the development of a bare area without any form of life due to several causes such as landslide, erosion, deposition etc. The cause of nudation may be: i. Topographic: ✓ Due to soil erosion by gravity, water or wind, the existing vegetation may disappear. Other causes may be deposition of sand etc., landslide, volcanic activity and other factors. ii. Climatic: ✓ Glaciers, dry period, hails and storm, frost, fire etc may also destroy the vegetation. 96  iii. Biotic: ✓ Man is responsible for destruction of forests, grasslands etc. for industry, agriculture, housing etc. Other factors are disease epidemics due to fungi, viruses etc which destroy the whole population. B. Invasion: It is the successful establishment of a species in a bare area. The process is completed in following 3 successive stages: i. Migration (dispersal): ✓ The seeds, spores or other propagules of the species reach the bare area. This process is known as migration, and is generally brought about by air, water etc. 97  ii. Ecesis (establishment): ✓ After reaching to new area, the process of successful establishment of the species starts and is known as ecesis. iii. Aggregation: ✓ After ecesis, as a result of reproduction, the individuals of the species increase in number and they come close to each other. This process is known as aggregation. C. Competition and Co-action: ✓ After aggregation of a large number of individuals of the species at the limited place, there develops competition mainly for space and nutrition. Individuals of a species affect each other’s life in various ways and this is called co-action. 98 D. Reaction: ✓ This is the most important stage in succession. The mechanism of modification of the environment through the influence of living organisms on it is known as reaction. ✓ It is a result of reactions, changes take place in the environment and as a result it gets modified, becoming unsuitable for existing community which sooner or later replaced by another community. ✓ The whole sequence of communities that replaces one another in the given area is called a sere and various communities constituting the sere are known as seral communities, seral stages or developmental stages. 99  E. Stabilization (climax): ✓ Finally, there occurs a stage when the final terminal community becomes more or less stabilized for a longer period of time and it can maintain itself in equilibrium with the climate of the area. ✓ This final community is not replaced and is known as climax community and the stage as climax stage. Theories about climax There are three schools of interpretations explaining the climax concept: i. Monoclimax or Climatic Climax Theory: ✓ It was advanced by Clements (1916) and recognizes only one climax whose characteristics are determined solely by climate (climatic climax). 100  ✓ The processes of succession and modification of environment overcome the effects of differences in topography, parent material of the soil, and other factors. ✓ The whole area would be covered with uniform plant community. Communities other than the climax are related to it, and are recognized as subclimax, postclimax and disclimax. ii. Polyclimax Theory: ✓ It was advanced by Tansley (1935). It proposes that the climax vegetation of a region consists of more than one vegetation climaxes controlled by soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity. iii. Climax Pattern Theory. ✓ It was proposed by Whittaker (1953). The climax pattern theory recognizes a variety of climaxes governed by responses of species populations to biotic and abiotic conditions. 101 ✓ According to this theory the total environment of the ecosystem determines the composition, species structure, and balance of a climax community. ✓ The environment includes the species responses to moisture, temperature, and nutrients, their biotic relationships, availability of flora and fauna to colonize the area, chance dispersal of seeds and animals, soils, climate, and disturbance such as fire and wind. ✓ The nature of climax vegetation will change as the environment changes. The climax community represents a pattern of populations that corresponds to and changes with the pattern of environment. The central and most widespread community is the climatic climax. 102  6. Ecosystem-Examples Owing to the great diversity in the physical environment of the earth there is also a great diversity in the ecosystems of the world. There are terrestrial and aquatic ecosystems ranging from lakes and oceans to forests and deserts. All ecosystems, however, exhibit similar general structural and functional framework. Some examples of the main ecosystems are briefly illustrated here. A. Terrestrial ecosystems include forests, grasslands, deserts, etc. 103 6.1. Forest Ecosystem: A forest is a natural terrestrial ecosystem where the trees, shrubs, climbers and ground flora in plants and several groups of mammals, birds, reptiles and microorganisms in animals predominantly form the structure the biotic community. Each forest type forms a habitat for a specific community of animals that are adapted to live in it. The types of forests present in a particular geographic region are determined by the environmental conditions prevalent in that region. Forests on the mountains and hills differ from those along the river valleys. Similarly in the type of vegetation and the animal communities vary from forest to forest. In India, for instance, the coniferous tree specis occur in the Himalayas, mangrove trees in river deltas and the thorn trees and bushes grow in the arid regions. Likewise among animals, the snow leopard, wild sheep and goats live in the Himalayas while the leopard and tiger are found in the forests of the rest of India. 104  Like any other ecosystem a forest ecosystem consists structurally of two components. a. Abiotic component: ✓ It consists of the physical environment of a forest including climatic and edaphic (soil) conditions. Climatic conditions such as precipitation, temperature, etc. differ from place to place and so do the forest types. Forest soil is very rich in humus or organic matter and it differs from other types of soil. b. Biotic component: ✓ It includes various groups of plants, animals and microorganisms. Plants include the trees, shrubs, climbers, grasses, and herbs in the forest. These include species that flower (angiosperms), and non-flowering species (gymnosperms) such as ferns, bryophytes, fungi and algae. ✓ Trees are the dominant vegetation group in a forest. The animals include species of mammals, birds, reptiles, amphibians, insects and other invertebrates. 105  Depending upon the prevailing climatic conditions forests can be of various types: i. Tropical Rain Forests: They are evergreen broadleaf forests found near the equator. They are characterized by high temperature, high humidity and high rainfall, all of which favour the growth of trees. They are the richest in biodiversity. Different forms of life occupy specialized areas (niches) within different layers and spaces of the ecosystem depending upon their needs for food, sunlight, water, nutrient etc. We come across different types and layers of plants and animals in the tropical rain forests. e.g. the emergent layer is the topmost layer of the tallest broad-leaf evergreen trees, below which lies the canopy where top branches of shorter trees form an umbrella like cover. Below this is present the understory of still smaller trees. On the tree trunks some woody climbers are found to grow which are known as Lianas. 106  There are some other plants like Orchids which are epiphytes i.e. they are attached to the trunks or branches of big trees and they take up water and nutrients falling from above. The orchids have special type of leaves to capture and hold the water. Some large epiphytes can hold as much as 4 litres of water, equivalent to a small bucket! Thus, these epiphytes almost act like mini-ponds suspended up in the air, in the forest crown. That is the reason why a large variety of birds, insects and animals like monkeys have made their natural homes (habitats) in these forests. The under storey trees usually receive very dim sunlight. They usually develop dark green leaves with high chlorophyll content so that they can use the diffused sunlight for photosynthesis. The shrub layer receives even less sunlight and the ground layer commonly known as forest floor receives almost no sunlight and is a dark layer. Most of the animals like bats, birds, insects etc. occupy the bright canopy layer while monkeys, toads, snakes, chameleons etc. keep on moving up and down in sunny and darker layers. 107  Termites, fungi, mushrooms etc. grow on the ground layer. Warm temperature and high availability of moisture facilitate rapid breakdown (decomposition) of the dropped leaves, twigs etc releasing the nutrients rapidly. ✓ The Silent Valley in Kerala is the only tropical rain forest lying in India which is the natural habitat for a wide variety of species. Being the store-house of biodiversity, the forests provide us with an array of commercial goods like timber, fuel wood, drugs, resins, gums etc. ii. Tropical deciduous forests: They are found a little away from the equator and are characterized by a warm climate the year round. Rain occurs only during monsoon. A large part of the year remains dry and therefore different types of deciduous trees are found here, which lose their leaves during dry season. 108 iii. Tropical scrub forests: They are found in areas where the dry season is even longer. Here there are small deciduous trees and shrubs. iv. Temperate rain forests: They are found in temperate areas with adequate rainfall. These are dominated by coniferous trees like pines, firs, redwoods etc. They also consist of some evergreen broad-leaf trees. v. Temperate deciduous forests: They are found in areas with moderate temperatures. There is a marked seasonality with long summers, cold but not too severe winter and abundant rainfall throughout the year. The major trees include broad leaf deciduous trees like oak, hickory, poplar etc. 109 vi. Evergreen coniferous forests (Boreal Forests): They are found just south of arctic tundra. Here winters are long, cold and dry. Sunlight is available for a few hours only. In summer the temperature is mild, sun-shines for long hours but the season is quite short. The major trees include pines, spruce, fir, cedar etc. which have tiny, needle- shaped leaves having a waxy coating so that they can withstand severe cold and drought. The soil is found to get frozen during winter when few species can survive. The leaves, also known as needles, fall on the forest floor and cover the nutrient poor soil. These soils are acidic and prevent other plants from growing. Species diversity is rather low in these forests. 110  Forest types in India: Forests in India can broadly be divided into two main categories viz., Coniferous forests and Broadleaved forests. ✓ They can also be classified according to the nature of their tree species – evergreen, deciduous, xerophytic or thorn trees, mangroves, etc. ✓ They, sometimes, may also be classified according to the most abundant species of trees such as Sal, Teak, Oak, Pine, Deodar or Chinar forests. Coniferous forests grow in the Himalayan mountain region, where the temperatures are low. These forests have tall stately trees with needle like leaves and downward sloping branches so that the snow can slip off the branches. Broadleaved forests have several types, such as evergreen forests, deciduous forests, thorn forests, and mangrove forests. Broadleaved forests have large leaves of various shapes. 111 6.2. Grassland ecosystem: Grassland ecosystem the grasses and shrubs form the dominant part of vegetation. It grows in areas where rainfall is usually low and the soil depth and quality is poor. A variety of grasses, herbs, and several species of insects, birds and mammals have evolved so that they are adapted to these wide-open grass covered areas. Three types of grasslands are found to occur in different climatic regions: i. Tropical grasslands: They occur near the borders of tropical rain forests in regions of high average temperature and low to moderate rainfall. In Africa, these are typically known as Savannas, which have tall grasses with scattered shrubs and stunted trees. The Savannas have a wide diversity of animals including zebras, giraffes, gazelle, antelopes etc. 112  Fires are quite common during dry season. Termite mounds are very common here. Tropical savannas have a highly efficient system of photosynthesis. Most of the carbon assimilated by them in the form of carbohydrates is in the perennating bulbs, rhizomes, runners etc. which are present underground. Deliberate burning of these grasslands can release huge quantities of carbon dioxide, a green house gas, responsible for global warming. ii. Temperate grasslands: They are usually found on flat, gentle sloped hills, winters are very cold but summers are hot and dry. Intense grazing and summer fires do not allow shrubs or trees to grow. In the United States and Canada these grasslands are known as prairies, in South America as Pampas, in Africa as Velds and in central Europe and Asia they are known as Steppes. Winds keep blowing and evaporation rate is very high. It also favours rapid fires in summer. The soils are quite fertile and therefore, very often these grasslands are cleared for agriculture. 113 iii. Polar grasslands (Arctic Tundra): They are found in arctic polar region where severe cold and strong, frigid winds along with ice and snow create too harsh a climate for trees to grow. In summers the sun-shines almost round the clock and hence several small annual plants grow in the summer. The animals include arctic wolf, weasel, arctic fox, reindeer etc. A thick layer of ice remains frozen under the soil surface throughout the year and is known as permafrost. In summer, the tundra shows the appearance of shallow lakes, bogs etc. where mosquitoes, different type of insects and migratory birds appear. Grassland Types in India Grasslands form a variety of ecosystems that are located in different climatic conditions ranging from near desert conditions to moist conditions. 114  The Himalayan pasture belt extends up to the snowline. These Himalayan pastures have a large variety of grasses and herbs. Himalayan hill slopes are covered with thousands of colourful flowering plants. There are also a large number of medicinal plants. The patches of tall elephant grass, which grows to a height of about five meters, are located in the low-lying waterlogged areas. Himalayan wildlife requires both the forest and the grassland ecosystem as important parts of their habitat. The animals migrate up into the high altitude grasslands in summer and move down into the forest in winter when the snow covers the grassland. The Semi-arid plains of Western India, Central India and the Deccan are covered by grassland tracts with patches of thorn forest. Several mammals such as the wolf, the blackbuck, the chinkara, and birds such as the bustards and floricans are adapted to these arid conditions. 115  The Scrublands of the Deccan Plateau are covered with seasonal grasses and herbs on which its fauna is dependent. It is teaming with insect life on which the insectivorous birds feed. The grasses are the major producers of biomass in these regions. Each grassland ecosystem has a wide variety of species of grasses and herbs. 6.3. Desert Ecosystem: A desert is an arid or semi arid area with very low annual rainfall and sparse patches of vegetation. Desert ecosystems witness very extreme climatic conditions, either too hot as in Thar desert or too cold as in Ladakh. Deserts occupy one-fifth of the Earth’s land surface. These ecosystems occur in regions where evaporation exceeds precipitation (rainfall, snow etc.). 116  The precipitation is less than 25 cm per year. Deserts have little species diversity and consist of drought resistant or drought avoiding plants. The atmosphere is very dry and hence it is a poor insulator. That is why in deserts the soil gets cooled up quickly, making the nights cool. Deserts are of three major types, based on climatic conditions: i. Tropical deserts like Sahara and Namib in Africa and Thar desert, Rajasthan, India are the driest of all with only a few species. ii. Temperate deserts like Mojave in Southern California where day time temperatures are very hot in summer but cool in winters. iii. Cold deserts like the Gobi desert in China and High altitude cold desert in Ladakh have cold winters and warm summers. 117  Desert plants and animals show most typical adaptations for conservation of water. Many desert plants are found to have reduced, scaly leaves so as to cut down loss of water due to transpiration or have succulent leaves to store water. Many a times their stems get flattened and develop chlorophyll so that they can take up the function of photosynthesis. Some plants show very deep roots to tap the groundwater. Many plants have a waxy, thick cuticle over the leaf to reduce loss of water through transpiration. Desert animals like insects and reptiles have thick outer coverings to minimize loss of water. They usually live inside burrows where humidity is better and heat is less. Desert and semi arid regions have a number of highly specialized insects and reptiles. The rare animals include the Indian wolf, desert cat, desert fox and birds such as the Great Indian Bustard and the Florican. Some of the commoner birds include partridges, quails and sand grouse. 118  Desert soil is rich in nutrients but deficient in water. Due to low species diversity, shortage of water and slow growth rate, the desert plant communities, if faced with a severe stress take a long time to recover. The Thar Desert in Rajasthan is most typical desert landscape in india. This has sand dunes. There are also areas covered with sparse grasses and a few shrubs, which grow if it rains. In most areas of the Thar the rainfall is scanty and sporadic. In an area it may rain only once every few years. 119 B. Aquatic ecosystems: Ecosystems where water is the dominant environmental factor in controlling abiotic characteristics and the floral and faunal makeup are known as the aquatic ecosystems. They include oceans, seas, estuaries, rivers, lakes, ponds, etc. Abiotic characteristics: Some of the important abiotic environmental factors of aquatic ecosystems include substrate type, water depth, nutrient levels, temperature, salinity, and flow. It is often difficult to determine the relative importance of these factors without rather large experiments. The amount of dissolved oxygen in a water body is frequently the key substance in determining the extent and kinds of organic life in the water body. Fish need dissolved oxygen to survive, although their tolerance to low oxygen varies among species; in extreme cases of low oxygen some fish even resort to air gulping. Plants often have to produce aerenchyma, while the shape and size of leaves may also be altered. Conversely, oxygen is fatal to many kinds of anaerobic bacteria.120  Nutrient levels are important in controlling the abundance of many species of algae. The relative abundance of nitrogen and phosphorus can in effect determine which species of algae come to dominate. Algae are a very important source of food for aquatic life, but at the same time, if they become over-abundant, they can cause declines in fish when they decay. The salinity of the water body is also a determining factor in the kinds of species found in the water body. Organisms in marine ecosystems tolerate salinity, while many freshwater organisms are intolerant of salt. The degree of salinity in an estuary or delta is an important control upon the type of wetland (fresh, intermediate, or brackish), and the associated animal species. Dams built upstream may reduce spring flooding, and reduce sediment accretion, and may therefore lead to saltwater intrusion in coastal wetlands. 121 Biotic characteristics: The biotic characteristics are mainly determined by the organisms that occur. For example, wetland plants may produce dense canopies that cover large areas of sediment—or snails or geese may graze the vegetation leaving large mud flats. Aquatic environments have relatively low oxygen levels, forcing adaptation by the organisms found there. For example, many wetland plants must produce aerenchyma to carry oxygen to roots. Other biotic characteristics are more subtle and difficult to measure, such as the relative importance of competition, mutualism or predation. There are a growing number of cases where predation by coastal herbivores including snails, geese and mammals appears to be a dominant biotic factor. 122 Autotrophic organisms: Autotrophic organisms are producers that generate organic compounds from inorganic material. Algae use solar energy to generate biomass from carbon dioxide and are possibly the most important autotrophic organisms in aquatic environments. In the shallow waters the biomass contribution from rooted and floating vascular plants is greater. These two sources combine to produce the extraordinary production of estuaries and wetlands, as this autotrophic biomass is converted into fish, birds, amphibians and other aquatic species. Chemosynthetic bacteria are found in benthic marine ecosystems. These organisms are able to feed on hydrogen sulfide in water that comes from volcanic vents. Great concentrations of animals that feed on these bacteria are found around volcanic vents. Heterotrophic organisms: Heterotrophic organisms consume autotrophic organisms and use the organic compounds in their bodies as energy sources and as raw materials to create their own biomass. Euryhaline organisms are salt tolerant and can survive in marine ecosystems, while stenohaline or salt intolerant species can only live in freshwater environments. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. These major types can further be divided into many categories depending upon various environmental factors. 124 6.4. Marine Ecosystem: Marine ecosystems cover approximately 71% of the Earth’s surface and contain approximately 97% of the planet’s water. They generate 32% of the world’s net primary production. They are distinguished from freshwater ecosystems due to the presence of dissolved compounds, especially salts in the water in high concentrations. Approximately 85% of the dissolved materials in seawater are sodium and chlorine though the salinity varies among different marine ecosystems, seawater has an average salinity of 35 parts per thousand (ppt) of water. Various classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fishes caught in marine ecosystems are the biggest source of commercial foods obtained from wild populations 125  Marine ecosystems can be divided into many zones depending upon water depth and shoreline features. The oceanic zone is the vast open part of the ocean where animals such as whales, sharks, and tuna live. The benthic zone consists of substrates below water where many invertebrates live. The intertidal zone is the area between high and low tides; in figure it is termed the littoral zone. Other near-shore (neritic) zones can include estuaries, salt marshes, coral reefs, lagoons and mangrove swamps. In the deep water, hydrothermal vents may occur where chemosynthetic sulfur bacteria form the base of the food web. Oceans are the major sinks of carbon dioxide and play an important role in regulating many biogeochemical cycles and hydrological cycle, thereby regulating the earth’s climate. 126  The oceans have two major life zones: Coastal zone: this is relatively warm, nutrient rich shallow water. Due to high nutrients and ample sunlight this is the zone of high primary productivity. Open sea: It is the deeper part of the ocean, away from the continental shelf (The submerged part of the continent). It is vertically divided into three regions: (i) Euphotic zone which receives abundant light and shows high photosynthetic activity. (ii) Bathyal zone receives dim light and is usually geologically active. (iii) Abyssal zone is the dark zone, 2000 to 5000 metres deep. The abyssal zone has no primary source of energy i.e. solar energy. It is the world’s largest ecological unit but it is an incomplete ecosystem. Environmental problems concerning marine ecosystems include unsustainable exploitation of marine resources (for example overfishing of certain species), marine pollution, climate change, and building on coastal areas. 127 128 6.5. Freshwater Ecosystem Freshwater ecosystems cover 0.80% of the Earth’s surface andconsist 0.009% of its total water. They generate nearly 3% of its net primary production. There are three basic types of freshwater ecosystems: Lentic: Standing water, including pools, ponds, and lakes. Lotic: Moving water, for example streams and rivers. Wetlands: Areas where the soil is saturated or inundated for at least part of the time. Lentic Water Ecosystems: The three primary zones of a lake 129 Lakes Lake ecosystems can be divided into zones. The first, the littoral zone, is the shallow zone near the shore where rooted wetland plants occur. The offshore is divided into two further zones, an open water zone and a deep water zone. In the open water zone (or photic zone) sunlight supports photosynthetic algae, and the species that feed upon them. In the deep water zone(aphotic), sunlight is not available and the food web is based on detritus entering from the littoral and photic zones. Some systems use other names. The off shore areas may be called the pelagic zone, and the aphotic zone may be called the profundal zone. 130  Towards inland from the littoral zone one can also frequently identify a riparian zone which has plants still affected by the presence of the lake—this can include effects from windfalls, spring flooding, and winter ice damage. The production of the lake as a whole is the result of production from plants growing in the littoral zone, combined with production from plankton growing in the open water. Lakes have several types of organisms: a) Planktons that float on the surface of waters e.g. phytoplanktons like algae and zooplanktons like rotifers. b) Nektons that swim e.g. fishes. c) Neustons that rest or swim on the surface. d) Benthos that are attached to bottom sediments e.g. snails. e) Periphytons that are attached or clinging to other plants or any other surface e.g. crustaceans. 131  Small/ Big/ Average lake Great Lakes Fresh water Lake Ecosystem 132 ✓ Stratification: This is an important feature of temperate region lakes which show vertical zonation of its water based on temperature differences. During summer, the top waters become warmer than the bottom waters. Therefore, only the warm top layer circulates without mixing with the colder layers below. Different layers exhibit different physical, chemical and biological characteristics. Following zones are generally described in lakes which show stratification or zonation. i. Epilimnion : Warm, lighter, circulating surface layer ii. Hypolimnion : Cold, viscous, non-circulating bottom layer. iii. Thermocline: In between the two -warmer and colder- layers lies the region of sharp drop in temperature which is known as thermocline. 133  Types of Lakes: Some important types of lakes are: 1. On the basis of nutrient status a) Oligotrophic lakes which have low nutrient concentrations. b) Eutrophic lakes which are over nourished by nutrients like nitrogen and phosphorus, usually as a result of agricultural run-off or municipal sewage discharge. They are covered with algal blooms. e.g. Dal Lake. c) Dystrophic lakes that have low pH, high humic acid content and brown waters e.g. bog lakes. 2. On the basis of origin d) Volcanic lakes that receive water from magma after volcanic eruptions e.g. many lakes in Japan. They have highly restricted biota. e) Artificial lakes or impoundments that are created due to construction of dams e.g. Govind sagar lake at Bhakra-Nangal, Bagliar lake near Ramban. 134 3. On the basis of salt content f) Fresh water lakes such as Wular lake in Kashmir g) Saltwater lakes eg Pangong lake in Leh h) Meromictic lakes that are rich in salts and are permanently stratified e.g. lake Nevada. i) Desert salt lakes that occur in arid regions and have developed high salt concentrations as a result of high evaporation. e.g. Sambhar lake in Rajasthan. Others j) Endemic lakes that are very ancient, deep and have endemic fauna which are restricted only to that lake e.g. the Lake Baikal in Russia; the deepest lake, which is now suffering a threat due to industrial pollution. 135 Ponds Ponds are small bodies of freshwater with shallow and still water, marsh, and aquatic plants. They can be further divided into four zones: vegetation zone, open water, bottom mud and surface film. The size and depth of ponds often varies greatly with the time of year; many ponds are produced by spring flooding from rivers. Food webs are based both on free-floating algae and upon aquatic plants. There is usually a diverse array of aquatic life, with a few examples including algae, snails, fish, beetles, water bugs, frogs, turtles, otters and muskrats. Top predators may include large fish, herons, or alligators. Since fish are a major predator upon amphibian larvae, ponds that dry up each year, thereby killing resident fish, provide important refugia for amphibian breeding. 136  Ponds that dry up completely each year are often known as vernal pools. Some ponds are produced by animal activity, including alligator holes and beaver ponds, and these add important diversity to landscapes. Pond Ecosystem 137  Lotic Water Ecosystems: River Ecosystem: The major zones in river ecosystems are determined by the river bed’s gradient or by the velocity of the current. Faster moving turbulent water typically contains greater concentrations of dissolved oxygen, which supports greater biodiversity than the slow moving water of pools. These distinctions form the basis for the division of rivers into upland and lowland rivers. The food base of streams within riparian forests is mostly derived from the trees, but wider streams and those that lack a canopy derive the majority of their food base from algae. Environmental threats to rivers include loss of water, dams, chemical pollution and introduced species. 138 River Ecosystem 139 6.5. Wetland Ecosystems Wetlands are dominated by vascular plants that have adapted to saturated soil. There are four main types of wetlands: swamp, marsh, fen and bog (both fens and bogs are types of mire). Wetlands are the most productive natural ecosystems in the world because of the proximity of water and soil. Hence they support large numbers of plant and animal species. Due to their productivity, wetlands are often converted into dry land with dykes and drains and used for agricultural purposes. 140 Wetland Ecosystem 141 6.6. Estuarine ecosystem Though a type aquatic ecosystem, an estuary is a transitional zone between marine and fresh water ecosystems and hence exhibits some unique characteristics in addition to those common with marine or fresh waters. Estuaries are places where rivers meet the sea and may be defined as areas where salt water is measurably diluted with fresh water. On average, estuaries are biologically more productive than either the adjacent river or the sea because they have a special kind of water circulation that traps plant nutrients and stimulates primary production. Fresh water, being lighter than salt water, tends to form a distinct layer that floats at the surface of the estuary. At the boundary between fresh and salt water, there is a certain amount of mixing caused by the flow of fresh water over salt and by the ebb and flow of tides. Additional mixing may be caused from time to time by strong winds and by internal waves that are propagated along the interface between fresh and salt water. 142  Three types of estuary are recognized according to the degree of mixing: salt wedge estuaries, partially mixed estuaries and vertically homogeneous estuaries. A salt wedge estuary has minimal mixing and the salt water forms a wedge, thickest at the seaward end, tapering to a very thin layer at the landward limit. Organic and inorganic particles carried by rivers tend to flocculate (aggregate into a mass) and sediment out when they encounter salt water. When the organic matter decomposes, it adds still more nutrients to the estuary. The inorganic matter settles on the bottom and provides enriched sediment for flowering plants adapted to salt water. Between the tide marks, mangrove forests flourish in tropical conditions, while salt marshes form in temperate and subarctic conditions. Below low tide, sea grasses form dense beds on muddy substrates. 143  In a partially mixed estuary, the vigorous rise and fall of the tide generates strong turbulence and causes partial mixing between the fresh water above and the salt water below. In a vertically homogeneous estuary the river flow is weak and the tidal flow is strong. Consequently, all stratification is broken down and salinity is almost the same from top to bottom at any given place. The salinity is lowest where the river enters the estuary and highest near the sea. The high level of plant production in estuaries supports a correspondingly high level of production of invertebrate animals and fish. Estuaries often contain beds of shellfish such as mussels and oysters and large populations of shrimps and crabs. Fish such as plaice and flounders are common. Other species use the estuaries as nursery grounds. Organisms in early stages of development enter the salt wedge at the seaward end and are carried up the estuary by the bottom currents. 144  Juveniles find abundant food as well as protection from predators in the mangrove forests, salt marshes, or sea-grass beds that line the estuary. Later, they may migrate to the open ocean to continue their growth and development. Other species pass through the estuaries in the course of their migrations. For example, salmon migrate from the sea to the rivers to spawn, while the young fish later migrate back to the sea. Eels migrate in the opposite direction, breeding in the sea but returning to fresh water as juveniles. Ecosystem Services Ecosystem services refer to the benefits mankind obtains from natural ecosystems present on the earth. There is a multitude of ways in which humans get benefitted from ecosystems. These benefits are collectively known as ecosystem services. Ecosystem services are classified into four categories viz. provisioning services, supporting services, regulating services and cultural services. 145  Provisioning services It refers the material products obtained from ecosystems such as: Food including seafood, crops, wild foods, and spices Raw materials including lumber, skins, fuel wood, organic matter, fodder, and fertilizer. Genetic resources including crop improvement genes, and health care Water resources Minerals resources Medicinal resources including pharmaceuticals, chemical models, and test and assay organisms Energy resources including hydropower, biomass fuels, etc. 146  Supporting Services It refers to the benefits which we get due to supportive role of ecosystems that are necessary for the production of all other ecosystem services. These include services such as nutrient recycling, primary production and soil formation. Regulating services It refers to those benefits which are there due to the regulation of ecosystem processes such as: Carbon sequestration and climate regulation Waste decomposition and detoxification Purification of water and air Pest and disease control 147  Cultural services It refers to the nonmaterial benefits people obtain from ecosystems through spiritual enrichment, cognitive development, reflection, recreation, and aesthetic experiences. Cultural services include: Spiritual and historical (including use of nature for religious or heritage value or natural) Recreational experiences (including ecotourism, outdoor sports, and recreation) Science and education (in