Ecosystem: Top 10 Things You Must Know About Ecosystem

Read this article to learn about the following things:

(1) Definition (2) Organization (Components) of Ecosystem consist of Biotic Components and Abiotic Components (3) Ecosystem Structure (4) Functional Aspect of Ecosystem (5) Types of Ecosystem;

(6) Functions of Ecosystem (7) Ecological Pyramids (Eltonian Pyramids) (8) Ecosystem Services (9) Cycling of Mineral Elements and Gases in an Ecosystem (Biogeochemical Cycles) and (10) Man Made Ecosystem.

A biotic community lives in an^environment which provides material and energy to it. Thus, there is an interaction between a biotidcommunity and its environment, and the former cannot live isolated from the latter. A biotic community and its abiotic environment, called biotope (Gr. bios = life, topos = place), together form an ecological system, or ecosystem. The term ecosystem was introduced by Tansley in 1935, but the concept appeared in ecology much later. An ecosystem may be defined as a natural, functional ecologicalspnit comprising living organisms and their nonliving environment that interact to form a stable, self-lkipporting system. The relationship between a biotic community and the nonliving environment is always a mutual one, that is, not only does the r\ environment affect the community but the comrmanity also modifies the environment.

Ecosystem # 1. Definition:

“An ecosystem is a natural unit consisting of biotic communities and their abiotic environment.”

or

“Ecosystem is a self regulatory and self sustaining structure and functional unit of biosphere consisting of community of living beings and the physical environment, both interacting and exchanging materials between them.”

An ecosystem is not an isolated unit as is often held. Instead, materials invariably move from one ecosystem to another —leaves are blown from a forest into a lake, birds migrate between their summer and winter homes.

Examples and Types. Ecosystem may be natural or artificial. Common examples of natural ecosystems are a pond, a lake, a meadow, a desert, k grassland, a forest, a village, a field, a hill side, etc. Even a single log and edge of a pond are also instances of ecosystem. Instances of artificial ecosystems are a manned spaceship, an aquarium, and a pot of houseplants. An ecosystem maybe temporary, such as a rainfed pond and laboratory culture of protozoans, or permanent, such as a forest.

Ecosystem # 2. Organization (Components) of Ecosystem:

To sustain itself and last indefinitely, an ecosystem must have resources for supporting its resident organisms and for disposal of their wastes. The necessary components of an ecosystem are matter (water, minerals, carbon dioxide, oxygen) and several species of organisms. An ecosystem must also receive a continuous supply of energy. The components of an ecosystem may be divided into two main types : biotic and abiotic.

I. Biotic Components:

The living organisms present in an ecosystem form the biotic component. They are classified into three main categories: plants, animals and microorganisms (bacteria and fungi). These are respectively called producers, consumers and reducers or decomposers according to their role in keeping the ecosystem operating as a stable unit (Fig. 3.1).

(i) Producers:

These are the green plants, some protists and certain bacteria. They, with the help of their chlorophyll, entrap the light energy of the sun and change it into the chemical energy of a simple carbohydrate glucose produced by them from simple inorganic compounds, namely, carbon dioxide and water. The process is called photosynthesis.

It may be briefly represented by the following equation:

From the basic organic material (glucose), the plants then form complex organic compounds such as starches, proteins and lipids. Materials and energy- stored by producers are utilized by consumers. As the green plants and other green organisms prepare their organic food themselves, they are known as the photoautotrophs (Gr. phot = liglit, autos = self, trophe = nourishment).

The producers dominate the terrestrial ecosystems, being the most abundant and massive of all groups of organisms there. Some bacteria capture energy released during certain inorganic chemical reactions and prepare organic food with it. They are called chemoautotrophs (Gr. chemeia = alchemy, autos = self, trophikos = nourishing), and the process is termed chemosynthesis.

(ii) Consumers:

These are mainly the animals. They are unable to synthesize their food. Therefore, they consume other organisms or parts of organisms. They are known as the heterotrophs (Gr.heteros = other, trophe = nourishment) or phagotrophs (Gr.phagein = to eat, trophe = nourishment).

The consumers are of 3 or 4 types:

(a) Primary or First Order Consumers:

These are the animals which eat plants or plant products. They are called herbivores. Cattle, deer, goat, rabbit and hare belong to this category. Elton has used the term “key industry animals” for the primary consumers because they convert the plant material into animal material. Plants that are parasites on plants and bacteria and fungi which flourish on living plants are also primary consumers.

(b) Secondary or Second Order Consumers:

These are the animals which eat herbivores. They are called carnivores, cats, dogs, and foxes are examples.

(c) Tertiary or Third Order Consumers:

These are larger carnivores which feed on secondary consumers. For instance, wolves.

(d) Quaternary or fourth Order Consumers:

These are the largest carnivores which take tertiary consumers. They are not eaten by other animals. Tigers and lions are examples.

(iii) Reducers or Decomposers:

These are mainly bacteria and fungi. They obtain their food molecules from the organic materials of dead producers (plants) and consumers (animals) and their waste products.

In the process of extracting nutrients and energy from these materials, they decompose the latter into:

(i) Small organic molecules which they utilize themselves, and

(ii) Into inorganic compounds that are released into the environment for reuse as raw materials by producers. The decomposers are known as the saprotrophs (Gr. sapros = rotton, trophe = nourishment). Some animals also feed on dead organisms. They are called scavengers or detrivores.

Thus, there is a cyclic exchange of materials between a biotic community and its abiotic environment in an ecosystem. In other words, the nutrients are constantly recycled, that is. used again and again in the same small area. If the ecosystem is a balanced one, no materials are ever exhausted. In contrast, energy is not cycled but is continuously lost from an ecosystem. Most organisms would soon die if the sun’s energy is cut off for some time.

Importance (Role) of Decomposers:

Sun is an endless source of energy, but the chemical materials of the environment are not inexhaustible. The decomposers return the chemical nutrients to the environment. They also make space available for new producers. Without this, all life will ultimately cease to exist. Thus, the decomposers have a crucial role in the ecosystem.

The decomposers are found in the soil and at the bottom of ponds, lakes and oceans. The basic requirements for a self-sustaining ecosystem are inorganic nutrients, producers, decomposers, and a continuous

supply of energy. The consumers are not essential, but they do occur in most ecosystems. (Figs. 3.1, 3.2).

II. Abiotic Components:

Abiotic component of an ecosystem consists of non-living substances and factors. The important ones are as follows:

1. Temperature:

Organisms generally live within a narrow range of temperature (5° – 35°C) with the exception of spores, seeds, some prokaryotes and other lowly organized individuals. The latter can be found in hot springs (60°-90°C) or permafrost (- 30° to – 50°C). Temperature range vanes in different parts of the earth.

It has created different life zones — tropical, sub tropical, temperate, arctic or alpine. High or low temperature causes inactivity and death of organisms. It is immediate in case of poikilothermal (= ectothermal = cold blooded) animals and delayed in case of homoiothermal (= endothermal = warm blooded) animals. Therefore, organisms show adaptations to avoid extremes of temperature.

Plants belonging to both hot and cold areas possess adaptations to reduce transpiration and retain water, e.g., tannins, hair, thick covering, mucilage, high solute content, thick leaves. Animals of cold areas possess thick coat of hair, scales, feathers and subcutaneous fat. In warm blooded animals, including humans, pigmentation is little in colder areas, yellow brown to red in arid climates and black in humid hot areas (Gloger’s rule).

2. Light:

It provides solar energy to the ecosystem for heating and photosynthesis. Maximum solar or light energy is available at equator. It decreases towards poles. In a tree more energy is available to upper leaves than the lower ones. Their rate of photosynthesis is accordingly higher. In a forest, trees have higher productivity than shrubs and herbs growing underneath.

Floating hydrophytes have higher photosynthetic rate than the submerged hydrophytes. Besides photosynthesis, light controls morphogenesis (photo-morphogenesis). Photoperiods influence leaf fall, appearance of new leaves and flowering in plants. They control migration and breeding in several animals.

3. Wind:

It controls weather, transpiration, pollination and dissemination of propagates. High speed winds inhibit free growth and flight animals. Unidirectional wind does not allow growth of branches on the wind-ward side.

4. Humidity:

It is the amount of water vapours present in the atmosphere. Humidity controls formation of clouds, dew, fog, etc. Epiphytes grow only in humid areas. Evaporation of water from the body of land organisms in transpiration and perspiration is regulated by humidity. Both plants and animals develop modifications for reducing water loss from their body in arid areas.

5. Precipitation:

It may occur as rainfall, snow, dew, hail, etc. Periodicity and amount of rainfall determines type of forest in an area —evergreen, deciduous, chaparral, grassland, savannah, desert, etc. Animals are also adopted accordingly.

6. Water:

Land plants meet their water requirements from soil. Land animals obtain the same from pools, lakes, rivers, springs, etc. Plants and animals show modifications according to availability of water in the area and requirement of conserving the obtained water. Plants of dry areas are called xerophytes. They develop modifications to increase water absorption, reduce transpiration and at times store absorbed water.

Certain animals of the dry areas do not drink water at all, e.g., Kangaroo, rat. They use water from food and its metabolism to run their body machinery. Animals of dry areas often reduce water loss by producing solid faeces and excreting solid urine.

Water is abundant in aquatic habitats. Plants of aquatic habitats are called hydrophytes. Hydrophytes possess aerenchyma or air storing parenchyma to support themselves in water. Clarity of water, salt content, depth and water waves or speed determine the growth and distribution of plants and animals. In rivers and streams, animals obtain most of their food from organic materials coming from outside the water.

In ponds and lakes producers grow in sufficient strength. Organisms found in fresh water have a problem of excess internal water because of endosmosis. Organisms found in ocean or saltish water have a problem of low internal water content due to exosmosis. Some have problem of excreting excess salts obtained from outside. In oceans at a depth of more than 200 m, producers do not occur. Only consumers are found there. Deep sea animals do not possess air sacs. Many of them are luminescent.

Water currents restrict distribution of organisms in streams and intertidal areas of oceans. In streams only attached plants grow. They have dissected or ribbon shaped leaves. Animals found here are either strong swimmers, have attaching organs or live under stones, in burrows, crevices etc. Similarly in intertidal area of ocean attached plants (Fucus, Laminaria), sessile animals (Sea anemone and limpets), burrowing animals (e.g., Nereis, tube worms) or very strong swimmers are met with.

7. Background:

Most animals have an adaptation to have colour, pattern and general texture similar to that of the background in which they operate. This allows them to camouflage themselves so that they do not become conspicuous to their preys or predators. For example, elephants and rhinos have colour similar to that of tree trunks and mud. Lions and camels are sand coloured. Praying mantis, bee, frog, Hyla and grasshopper are green in colour. Jelly-fishes and sea-cucumbers are glassy. Chameleon is able to change its colour according to the back ground.

8. Topography:

It is the surface behaviour of the earth like altitude, slope, exposure, mountain chains, valleys, plains, etc. Topography influences other environmental factors, atmospheric pressure, winds, rainfall, light intensity and light duration, temperature, water currents or wave action.

It is also a causal agent for isolation, formation of new species and geographical distribution of organisms. North and south faces of hill possess different types of flora and fauna because they differ in their humidity, rainfall, light intensity, light duration and temperature regimes. It is because the two faces of the hill receive different amounts of solar radiations and wind action.

Similarly, the centre and edge of a pond possess different depths of water and different wave action. Top side of a rock is exposed to wave action and light while the underside of the rock has little wave action and light. Therefore, different parts of the same area may possess different species of organisms.

9. Gases:

Nitrogen is present in abundance (4/5th of atmosphere) but is itself chemically inert. It forms useful salts through electrochemical, photochemical and biological fixation. Carbon dioxide concentration of the atmosphere is always a limiting factor for photosynthesis.

However, excess of carbon dioxide concentration is harmful to animals as well as climate. Its-concentration increases during night but decreases during day. In water it occurs as bicarbonate and carbonate ions.Oxygen concentration is supra-optimal for C₃ plants, optimal for C₄ plants and animals except at high altitudes.

In water oxygen concentration determines distribution of organisms. In the middle or intermediate stratum photosynthesis increases oxygen concentration during day but it becomes little during night depending upon population, pollution and decomposition. In deep waters, animals are faced with very low oxygen concentration.

10. Soil:

It determines vegetation growth and pattern, under-ground flora and fauna through its constitution, origin, temperature range, water retentively, aeration, minerals, etc. Soil present on the slopes as well as the one which is uncovered are liable to be eroded by water and air respectively.

11. pH (Hydrogen ion Concentration):

There is very little change in pH in oceans. Terrestrial animals are also not much influenced by pH of the substratum. However, distribution of land plants and soil organisms is determined by pH of soil. A similar control on distribution is found in fresh water habitats. Snails and earthworms do not occur in acidic soils. At this pH, Euglena and other flagellates are quite abundant. Animals having calcareous shells live in media having neutral or alkaline pH.

12. Mineral Elements:

A large number of minerals, also called biogenic or biogenetic nutrients, are required by organisms for their proper growth. Deficiency or absence of any one results in abnormal growth which may lead to death. Excess minerals are equally harmful. Abundance of some minerals favour the growth of some tolerant species.

Snails occur in soils rich in calcium content. Soils deficient in nitrogen salts often possess nitrogen fixing bacteria and cyanobacteria. Plants having symbiotic relationship with these bacteria also abound in the soils. Carnivorous plants meet their requirement of nitrogen by catching small insects, worms, etc. Salinity of ocean is overcome by many animals through salt secreting glands. Similar glands occur in halophytes or plants growing in saline soils and marshes.

Special adaptations are found in animals inhabiting estuaries where there are wide fluctuations in salt content. Areas having very high salt content are usually devoid of much vegetation, e.g., Dead Sea, Great Salt Lake.

Thing # 3. Ecosystem Structure:

1. Species Composition:

It differs from one ecosystem to another depending upon geography, topography and climate. Maximum species composition occurs in tropical rain forests and coral reefs. Minimum occurs in deserts and arctic regions.

2. Stratification:

It is formation of vertical layers where vegetation is dense, e.g., 5-7 strata in tropical rain forests. Stratification is absent or rare in deserts.

3. Trophic Structure:

Each ecosystem has specific food chains and food webs, e.g. grazing food chain in grassland.

4. Standing Crop:

It is the amount of living biomass present in an ecosystem. Dry weight preferred over fresh weight because the latter is liable to be influenced by seasonal moist differences.

5. Standing State:

It is the amount of inorganic nutrients present any time in the soil water of ecosystem, it tends to vary from season to season and ecosystem to ecosystem.

Ecosystem # 4. Functional Aspect of Ecosystem:

This is dynamic aspect of ecosystem that states how its structural components interact and are interconnected by nutrients, respiratory gases, minerals and energy. The following dynamic states maintain an ecosystem.

Predator Food Chain:

(i) Grassland:

(ii) Forest:

Green plants → Goat → Man → Lion

(iii) Pond:

Phytoplankton’s → Zooplanktons → Small fishes

(Bacteria, algae) (Daphnia, Crustacea)

Ecosystem # 5. Types of Ecosystem:

Ecosystems are classified into many types on the basis of climate, habitat and biotic communities. The classification gives an idea of form and function of ecosystem.

Ecosystems are of the following types:

Study of a Specific Ecosystem: A Fresh Water Ecosystem:

Pond Ecosystem:

Pond is an example of fresh water ecosystem (Fig. 3.3). It is limited to a small area of shallow standing water containing abundant vegetation and aquatic animals. It is an example of lentic habitat and completely self maintaining and self regulating ecosystem. It is composed of the following components.

1. Abiotic Components:

The non-living substances of pond are its standing water and the sediments below water. Sunlight penetrates into it during daytime. Oxygen and CO₂ from atmosphere dissolve into water. Various organic and inorganic compounds of calcium, phosphorus and nitrogen, amino acids, humic acids are available in the sediment or to some extent in dissolved state.

Biotic Factors:

2. Producers:

The chlorophyll bearing phytoplankton’s and hydrophytes are the producers. The phytoplankton’s are minute floating algae such as Eudorina, Volvox, Clostridium, Oscillatoria, Euglena and Ceratium etc. The hydrophytes are filamentous algae floating and rooted plants and submerged plants. They synthesize carbohydrates for the nutrition of the biotic components.

3. Consumers:

The first order of consumers are Zooplanktons (herbivores) are ciliates, flagellates, rotifers, crustaceans, tadpole larvae of frog etc. These are fed by insect larvae, adult insects, small fishes which are primary carnivores making the second order of consumers. The secondary carnivores are large carnivorous fishes and frogs. Frogs and snakes temporarily come in for food and shelter. Birds like herons, cranes, king fishers depend upon pond for food. The bottom dwelling forms called benthos are molluscs and annelids.

4. Decomposers:

These are saprophytes like bacteria, flagellates and fungi which are abundant in mud at the bottom of the pond. They decompose dead and decaying organisms and derive their nutrition. Simultaneously, they release the abiotic raw materials which are reused by the producers for photosynthesis and nutrition.

Terrestrial Ecosystem:

Forest ecosystem is given as an example here. Forests are natural plant communities with dominance of phanerogams. In India, forests occupy about 1/10 of the land area.

Indian forests can be divided into the following four categories:

1. Tropical Forests (Wet evergreen, semi-evergreen, moist deciduous and dry deciduous)

2. Subtropical Forests

3. Temperate Forests

4. Alpine Forests

1. Abiotic Components:

This includes inorganic and organic substances present in the atmosphere and soil. The climate (temperature, light, rainfall etc.) and soil (minerals) vary from forest to forest. In addition to minerals, the occurrence of litter is the characteristic feature of majority of forests. Through litter decomposition approx. 90% energy trapped in the ecosystem by autotrophs dissipates into space as heat energy. The litter fall increases with decreasing latitudes.

2. Biotic Components:

(A) Producers:

They are represented mainly by trees but shrubs and ground flora are also found. Depending upon the kinds of forests the flora varies.

(B) Consumers:

(i) Primary Consumers (Herbivores) are small animals feeding on tree leaves, include ants, beetles, flies, bugs, spiders, leaf hoppers etc., deer, elephants, mole, squirrels and fruit bats are large animals which feed upon shoots and/or fruits.

(ii) Secondary Consumers are different kinds of birds, snakes, lizards, feeding on primary consumers.

(iii) Tertiary Consumers are tiger and lion are the top carnivores.

C. Decomposers:

Streptomyces (Sps. of Angiococus, Bacillus and Pseudomonas) and fungi (Aspergillus, Polyporus, Alternaria, Fusarium, Trichoderma) are helpful in decomposing the litter. Further, litter decomposition found to be slow in cooler and drier areas, therefore, sometimes its accumulation on the soil surface makes a thick layer.

Ecosystem # 6. Functions of Ecosystem:

The ecosystem is a dynamic unit. It shows continuous interactions, such as flow of energy, transfer of food etc. The biotic and abiotic components of an ecosystem are closely linked with one another to carry out these functions. An such processes are called functions of an ecosystem.

Some of the characteristic functions are given below:

(I) Flow of Energy:

All the potential energy of plant material eaten is not converted into flesh of herbivores. A part of it is excreted as undigested food and another part is lost by respiration i.e., dissipates in the form of heat. Therefore, only a small part of energy is fixed in the form of potential chemical energy in the protoplasm.

The same process is repeated at the secondary consumer level and so on. Thus, at each step of transfer of energy, large amount is degraded and dissipates and never returns to ecosystem. The flow of energy from one to another trophic level takes place according to second law of thermodynamics which states that “whenever energy is transformed from one kind to another, there is an increase in entropy (relative disorder) and decrease in amount of useful energy.”

The 10% Law:

If the net primary production is taken to be 100 units in producers, only 10 units of potential energy of plant material is actually assimilated by the herbivores. Similarly only 1 unit of potential energy of herbivores is assimilated in carnivores. Thus, during energy flow in ecosystem, the energy fixed in one level is only 10% of its previous level.

Thus, in an ecosystem, there is:

(i) A constant flow or transfer of energy from sunlight through plants and plant-eating animals to flesh-eating animals in the form of food.

(ii) A decrease in useful energy at each successive level of nutrition due to loss of some energy as heat at each transformation of energy, and

(iii) Return of entire solar energy that entered the living systems back to the nonliving world as heat but not as light.

Every food chain or web is essentially a system of energy transfer. In fact, energy transfer is the very basis of life. Food is the means of transfer of both matter and chemical energy in the living world.

(II) Food Chains and Trophic Levels:

The number of organisms of each species, or more precisely their total mass, is determined by the rate of flow of energy through the biological part of the ecosystem that includes them. The transfer of energy from its ultimate source in plants, through a series of organisms each of which eats the preceding and is eaten by the following, is known as food chain (Fig. 3.5).

The number of steps (i.e., trophic levels) in the series is limited to perhaps four or five because of the great decrease in available energy at each step. The percentage of food energy consumed that is converted to new cellular material, and thus is available as food energy for the next animal in the food chain, is known as the percentage efficiency of energy transfer.

The flow of energy in ecosystems, from sunlight through photosynthesis in autotrophic producers, through the tissues of herbivorous primary consumers and the tissues of carnivorous secondary consumers, determines the number and total weight (biomass) of organisms at each level in the ecosystem. The flow of energy is greatly reduced at each successive level of nutrition because of the heat losses at each transformation of energy and this decreases the biomass in each level.

Some animals eat only one kind of food and, therefore, are members of a single food chain. Other animals eat many different kinds of food and not only are members of different food chains but also may occupy different positions in different food chains. An animal may be a primary consumer in one chain, eating green plants, but a secondary or tertiary consumer in other chains, eating herbivorous animals or other carnivores.

Humans are at the end of a number of food chains; for example, man eats a fish such as a black bass, which ate little fish, which ate small invertebrates, which ate algae. The ultimate size of the human population (or the population of any animal) is limited by the length of the food chain, the per cent efficiency of energy transfer at each step in the chain, and the amount of light energy falling on the earth.

Since humans can do nothing about increasing the amount of incident light energy and very little about the per cent efficiency of energy transfer, they can increase their food energy only by shortening their food chain, i.e., by eating the primary producers, plants, rather than animals. In overcrowded countries such as India and China, people are largely vegetarians because this food chain is shortest and a given area of land can in this way support the greatest number of people.

The aforementioned food chain which starts from a plant base, then goes to grazing or browsing herbivores and on to the predator carnivores is known as predator food chain or grazing & browsing food chain. In these food chains the size of the predators increases at each trophic level.

In addition to the predator food chains, there are parasite food chains. For example, mammals and birds are parasitized by fleas; in the fleas live protozoa, which are in turn the hosts of bacteria. Since the bacteria might be parasitized by viruses, there could be a five-step parasite food chain. A parasite food chain also starts from a plant base, then goes to herbivores which may be the host of a large number of small animals (parasites).

Decay or detritus food chain:

A third type of food chain is one in which plant material is converted into dead organic matter, detritus, before being eaten by animals such as millipedes and earthworms on land, by marine worms and mollusks, or by bacteria and fungi.

In a community of organisms in the shallow sea, about 30 per cent of the total energy flows via detritus chains, but in a forest community, with a large biomass of plants and relatively small biomass of animals, as much as 90 per cent of energy flow may be via detritus pathways. In an intertidal salt marsh, where most of the animals—shellfish, snails and crabs—are detritus eaters, 90 per cent or more of the energy flow is via detritus chains.

In a generalized form, a food chain may be represented as under:

Photosynthetic Organisms → Herbivores → Carnivores → Microorganisms of decay.

The energy originally derived from the sun by plants thus passes in material form through the various trophic levels of a food chain.

Examples:

The food relations are very complex, even in a small community, but may be illustrated by two simplified examples.

(i) Food Chain in a Pond (Fig. 3.3):

In a pond, rooted or floating plants and floating algae synthesize food materials from dissolved nutrients. Unicellular algae are eaten by protozoans, which are in turn taken by small crustaceans. The latter are fed upon by water insects, which are devoured by small fishes. The small fishes are taken by large fishes. The large fishes, or any intermediate organisms, when dead, serve as food for the bacteria and fungi of decay. This completes the circuit.

(ii) Food Chain on Land (Figs. 3.6. & 3.7):

On land, grass grows by synthesizing food from carbon dioxide of air and water and minerals of soil with the help of chlorophyll and sunlight. Grass is eaten by rabbits, which are preyed upon by cats. The latter may be taken by wolves, and the tigers may capture the wolves. The tigers as well as the other participants of the chain, on death, are reduced by bacteria and fungi of decay to simple inorganic materials. The latter are reused by grass.

Another land food chain is grass → grass shopper → frog → snake → peacock → Hawk (Fig. 3.7).

There is perhaps no living thing that does not serve as a trophic level in some food chain. Some animals may form a link in more than one food chain. In any food chain, the successive members are larger in size but fewer in number.

Food chains may be longer or shorter than those cited above, but usually there are only 4 or 5 successive trophic levels. An organism cannot always be assigned to just one trophic level. The insectivorous plants, such as Venus flytrap and pitcher plant, are producers as well as consumers. Frog is herbivorous in the larval and carnivorous in the adult stage. Many mammals, such as fox, bear and man, are omnivorous and belong to many trophic levels.

Three important ecological principles emerge from the study of food chains:

(i) To be complete and self-containing, a food chain must always begin with photosynthesis and end with decay. A food chain must get energy from outside to keep going,

(ii) The shorter a food chain, the more efficient it is. The more steps it has, the greater wastage of energy.

(iii) The size of any population is ultimately determined by the number of trophic levels in the food chain. With the decrease in useful energy at each step, very little energy is available for a population of quaternary consumers. The size of a population of quaternary consumers is less than that of tertiary consumers, a population of tertiary consumers is smaller than one of secondary consumers, and so on.

(III) Food Web:

Food chains are not strictly linear. They may have branches that may link one food chain with another. Thus, there may be several interlinked food chains in a community, and one animal may be a link in more than one food chain. The various interlinked food chains in a community constitute a food web, or food cycle. A food web includes all the feeding relationships in an ecosystem.

Composition (Figs. 3.8, 3.9 & 3.10). Normally a food web operates according to taste and food preferences of the organisms at each trophic level. However, availability of food source and other compulsions are equally important. In Sunder-bans, the tigers eat fish and crab in the absence of their natural preys. Some organisms normally operate at more than one trophic level. Thus human beings are not only herbivores but also carnivores of various levels.

Jackals are both carnivores and scavengers. Snakes feed on mice (herbivores) as well as frogs (carnivores). Wild cats prey upon mice as well as birds and squirrels. A wolf eats not only fox but also rabbit and deer. Therefore, the concept of food web appears more real ecologically than the concept of a simple food chain.

The mechanism of operation of food web in maintaining stability of ecosystem is given below:

A herbivore, like rabbit, does not get starved if its preferred plant species is reduced in quantity due to some problems. It begins feeding on alternate plant species. The preferred gets chance to recover. So is the case with predators, who too switch over to another alternative organisms if the prey of their first choice is not available.

In a food web there can be 3 types of food chains:

(i) Predator chains:

It begins with plants and proceeds from small to large animals.

(ii) Parasitic chains:

Which proceeds from large to small organisms.

(iii) Saprophytic Chains:

That proceeds from dead animals to microorganisms.

Ecosystem # 7. Ecological Pyramids (Eltonian Pyramids):

An ecological pyramid is a graphic representation of a specific parameter (aspect) of a food chain developed by Charles Elton (1927). Since in any food chain there is a loss of energy at each step, it follows that there is a smaller biomass in each successive step.

A food chain may be visualized as a pyramid, each step in the pyramid is much smaller than the one on which it feeds. There are three important parameters of each trophic level in a food chain, namely, number of individuals, amount of biomass and amount of energy.

Accordingly, three types of ecological or food pyramids are recognized:

(1) Pyramid of numbers,

(2) Pyramid of biomass and

(3) Pyramid of energy.

In a food pyramid, the first trophic level forms the base and the last forms the apex.

1. Pyramid of Numbers:

The pyramid of numbers represents numerical relationship between different trophic levels of a food chain. Starting from the base of the pyramid and moving towards the apex, one finds a gradual decrease in the number of organisms and an increase in the body size (Fig. 3.11).

In a lake ecosystem, the base of the pyramid is occupied by producers, such as diatoms and algae, whose number is maximum. The second trophic level is represented by zooplanktons, which are primary consumers and are less abundant than the producers. The third trophic level is represented by medium-sized fishes which feed upon primary consumers, these are still smaller in number. The apex or fourth trophic level is represented by large fishes which are very few in number.

Similarly in a grassland ecosystem, the base of the pyramid (i.e., first trophic level) is occupied by grasses (producers), and the apex by large carnivores, such as tigers (Fig. 3.11a). In parasitic food chains the pyramid of numbers is reversed (Fig. 3.12). For instance, a tree supports a large number of fruit or seed-eating birds, which in turn are infested by a large number of ecto- and endoparasites.

However, the pyramids of numbers is not an ideal device to illustrate the food chain because of its various drawbacks and limitations.

2. Pyramid of Biomass:

As the name itself indicates, the pyramid of biomass represents total weight of living matter present at each trophic level of the food chain. As one moves from base to apex, one finds gradual loss of biomass at each trophic level. For example, in a water ecosystem, the first trophic level (i.e. base) is occupied by a huge mass of phytoplankton; the second trophic level is occupied by zooplankton; the third trophic level is occupied by the primary carnivores, such as worms, mollusks and small fishes; the apex or fourth trophic level is occupied by large fishes (Fig 3.13).

Here also, in a parasitic food chain the pyramid of biomass is inverted (Fig. 3.14).

The pyramid of biomass is relatively more illustrative than the pyramid of numbers because it represents quantitative relationship of the standing crop biomass.

3. Pyramid of Energy:

The pyramids of number and biomass do not take into consideration the rate of energy flow (i.e., rate of passage of food mass), while a pyramid of energy illustrates the total available energy at each trophic level of the food chain. Here also, as one moves from base to apex there is a gradual loss of energy (Fig. 3.15).

It will be seen that the pyramid of energy is the best way to illustrate the functional nature of communities because:

(i) The number and weight of organisms at any trophic level depends on the rate at which food is being synthesized.

(ii) The shape of the pyramid of energy is not affected by variations in size and metabolic rate of individuals.

(iii) The number and biomass do not determine the role of decomposers in the dynamics of a community. Only measurements of actual energy utilization, as shown in the pyramid of energy, places the microorganisms in true relationship with the larger biotic components, and

(iv) It not only provides a means of comparing different ecosystems, it also helps in evaluating the relative importance of populations.

Ecosystem # 8. Ecosystem Services:

Humankind benefits from a multitude of resources and processes that are supplied by natural ecosystems. Collectively these benefits are known as ecosystem services and include products like clean drinking water and processes such as the decomposition of wastes. Detrivores like dung beetle help to turn animal wastes into organic material that can be reused by primary producers.

The Millennium Ecosystem Assessment (MEA) report 2005 defines Ecosystem services as benefits people obtain from ecosystems and distinguishes four categories of ecosystem services (supporting services, provisioning services, regulating services. Cultural services) where supporting services are regarded as the basis for the services of the other three categories.

The following lists represent samples of each categories of ecosystem services:

1. Supporting Services:

(i) Nutrient dispersal and cycling

(ii) Seed dispersal

(iii) Primary production

2. Provisioning Services:

(i) Food, crops, wild foods, and spices

(ii) Water & minerals

(iii) Pharmaceuticals, bio-chemicals, and industrial products

(iv) Energy (hydropower, biomass fuels)

3. Regulating Services:

(i) Carbon sequestration and climate regulation

(ii) Waste decomposition and detoxification

(iii) Purification of water and air

(iv) Crop pollination

(v) Pest and disease control

4. Cultural Services:

(i) Cultural, intellectual and spiritual inspiration

(ii) Recreational experiences (including ecotourism)

(iii) Scientific discovery

Ecosystem # 9. Cycling of Mineral Elements and Gases in an Ecosystem (Biogeochemical Cycles):

Life on the earth depends upon the availability of energy and circulation of essential elements needed for growth and development of plants and animals. These essential elements (nutrients) flow from nonliving to living world and back to nonliving in a more or less circular fashion and are called biogeochemical cycles.

These cycles are of three types such as:

(i) Hydrological cycles

(ii) Gaseous cycles, such as carbon, hydrogen, oxygen and nitrogen cycles and

(iii) Sedimentary cycles such as phosphorus, sulphur and calcium cycles.

(i) Hydrological Cycle:

This is otherwise known as water cycle. Land plants absorb water from soil and land animals consume water through food and drink. Plants and animals loose water through transpiration and excretion respectively. The remaining water returns to environment after death and decay. The water lost as vapour merges into global water cycle and again is made available for soil and water bodies.

(ii) Gaseous Cycles:

1. Carbon Cycle:

The carbon source is carbon dioxide gas of atmosphere and hydrosphere. The plants fix CO₂ through photosynthesis and the total amount of carbon fixed per annum is approximately 1.30 x 10⁹ tons. Movement of carbon takes place along the food chain. Some of the CO₂ is returned to atmosphere through expiration and waste products of animals and plants. The rest is given out by decomposition of dead organisms (Fig. 3.16).

 2. Oxygen Cycle:

The atmosphere contains 21% of oxygen. It is essential for biological oxidation and release of energy from food. It is consumed by animals and plants in respiration and is released by plants only in photosynthesis (Fig. 3.17).

3. Hydrogen Cycle:

Free hydrogen is not available but it enters into biotic components in the form of water molecules. It is fixed through photosynthesis. It is released as water by oxidation of organic compounds (Fig. 3.18).

4. Nitrogen Cycle:

Air has 78% of nitrogen. It is fixed by blue green algae and nitrogen fixing bacteria in the form of nitrate. These are absorbed by green plants by root and are reduced into ammonia and finally incorporated into protoplasm as amino acids and proteins.

Animals obtain protein through food chain. These are excreted as nitrogenous wastes and are converted into ammonia by ammonifying bacteria. Soil bacteria Nitrosomonas convert ammonia into nitrites and Pseudomonas bacteria reduce nitrate into nitrogen which returns to atmosphere (Fig. 3.19).

(iii) Sedimentary Cycle:

1. Phosphorus Cycle:

The major source of phosphorus is phosphates of fossil bone deposits and crystalline rocks. Plants absorb phosphates through their root and convert them into phosphates of nucleic acids and phospholipids. Animals get them through food chain. The phosphorus is released when dead animals and plants and their wastes are decomposed (Fig. 3.20).

2. Sulphur Cycle:

The main source of sulphur is inorganic sulphates. Plants absorb them through roots and incorporate into protein molecules. Animals get them through food chain. It is returned to lithosphere after death and decay of organisms.

Ecosystem # 10. Man Made Ecosystem:

Artificial ecosystems are man-made ecosystems made by human beings for their own benefit. Such ecosystems are variable in their stability and durability. Examples of some man-made ecosystems are villages, towns, cities, parks and gardens, orchards, plantations and crop fields. Similarly, some manmade aquatic ecosystems are dams, reservoirs, lakes, ponds, canals, fishery tanks and aquaria.

The rise of human civilization is linked with the knowledge of use of fire, cultivation of land and domestication of animals. The man made modifications in the biotic community are brought about primarily by cultivation. Agriculture related ecosystems are called agro ecosystems and are highly significant man-made ecosystems.

Balance of nature refers to establishing balance between abiotic and biotic components between producers, consumers and decomposers. It also refers to keep the food chains and food webs intact. Man plays vital role in keeping the balance of nature intact. He has to take care of two fundamental aspects such as reduction of human population growth and minimizing the growing pollution in order to establish the balance of nature.