• Victoria regia has larger (largest leaves) simple undivided leaves.
• Nymphaea has waxy coating on upper leaf surface hence hydrophobic.

 

 

 Ecosystem.                                                                                                                                                            

The word ecosystem was coined by A.G. Tansley in 1935. This term is derived from two words, namely eco and system. Eco refers to environment and system refers to a complex coordinated unit. An ecosystem is a basic functional ecological unit. It consists of living organisms (biotic factors) and non–living substances (abiotic factors). It is an interacting system where the biotic and abiotic factors interact to produce an exchange of materials between the living and non–living factors. An ecosystem is a sum total of living organisms, the environment and the process of interaction between and within all parts of the system (Mathavan, 1974). According to Odum an ecosystem is the basic fundamental unit of ecology which includes both the organisms and the non–living environment each influencing the properties of the other and each is necessary for the maintenance of life. Pond is a suitable example for ecosystem. Lake is another ecosystem. Other examples of ecosystem are river, estuary, ocean, forest, grassland, town etc.

1. Structure : The structure of any ecosystem is formed of two components, namely :
   1. Abiotic factors                      (ii) Biotic factors

1. Abiotic factors : The abiotic factors of an ecosystem include the non–living substances of the environment. eg. Water, soil, air, light, temperature, minerals, climate, pressure etc. The biotic factors of the ecosystem depend on the abiotic factors for their survival.
2. Biotic factors : The biotic factor include the living organisms of the environment. e.g. Plants, animals, bacteria, viruses etc. The biotic factors of an ecosystem are classified into three main groups, namely :

(a) Producers                (b) Consumers            (c) Reducers or Decomposers.

1. Producers : The organisms which carry out photosynthesis constitute the producers of an ecosystem. eg. Plants algae and bacteria. The producers depend on the abiotic factors of the ecosystem for producing energy. They are provided with chlorophyll. Chloropyll is used in the synthesis of energy rich compounds with the utilization of abiotic factors like light, CO₂, water and minerals. A portion of the energy synthesized, is used by the producers for their growth and survival and the remaining energy is stored for future use.
2. Consumers : Consumers are organisms which eat or devour other organisms. The consumers are further divided into three or more types. They are primary consumers, secondary consumers and tertiary consumers.

• Primary consumers : They eat the products like plants, algae and bacteria. The primary consumers are also called herbivores. Elton referred the herbivores as key industry animals. Rabbit, deer, etc., are primary consumers in a terrestrial ecosystem.

 

   Ecosystem

 

Abiotic factors                                                 Biotic factors

Producers                 Consumers           Reducers/Decomposers

 

 

1

Primary (Herbivores)

 

Secondary (Pri. Carnivores)

Tertiary (Sec. Carnivores)

 

 

 

 

 

 

 

 

• Secondary consumers : They kill and eat the herbivores. They are also called carnivores. As these carnivores directly depend on herbivores, they are specifically called primary carnivores. Fox, wolf, etc. are the secondary consumers in a terrestrial ecosystem.
• Tertiary consumers : They kill and eat the secondary consumers. They are also called secondary carnivores. e.g., Lion, tiger, etc.

3. Reducers or Decomposers : The decomposers are heterotrophs organisms that break up the dead bodies of plants and their waste products. They include fungi and certain bacteria. They secrete enzymes. The enzymes digest the dead organisms and the debris into smaller bits or molecules. These molecules are absorbed by the reducers. After taking energy, the reducers release molecules to the environment as chemical to be used again by the producers.
   2. Typical ecosystem (Pond ecosystem) : A pond is a suitable example for ecosystem. It is a lentic fresh– water ecosystem. It contains shallow standing water. The pond ecosystem is formed of abiotic factors and biotic factors.
      1. Abiotic factors : The abiotic factors of the pond ecosystem are water, CO₂, O₂ inorganic compounds, organic compounds, light, temperature, pressure, pH etc.
      2. Biotic factors : The biotic factors of the pond ecosystem are producers, consumers and reducers.

1. Producers : The producers synthesize the energy from abiotic substances. The producers of a pond include phytoplankton like diatoms, blue green algae (Oscillatoria), green algae, green flagellates (Volvox, Euglena, Chlamydomonas), rooted plants, submerged plants and floating plants.
2. Consumers : Consumers eat other organisms. The organisms which depend on producers are called primary consumers or herbivores. e.g., Zooplankton (Cyclops, Daphnia, larvae of Chironomus etc), Dysticus (insect), Lymnaea (snail) etc. The

primary consumers are eaten by the secondary consumers of carnivores. These carnivores are called primary carnivores because they are the first carnivores in the food chain. e.g., Small fishes, frogs etc. The secondary consumers are eaten by the tertiary consumers or secondary carnivores. e.g., large fish.

3. Reducers or Decomposers : The decomposers are organisms that break up the dead bodies of

organisms and their waste products.

Pond ecosystem

 

 

They include microbes like bacteria. They secrete enzymes. The enzymes digest the dead organisms and the debris into smaller bits or molecules. These molecules are absorbed by the reducers. After taking energy, the reducers release molecules to the environment as chemical to be used again by the producers.

3. Types of ecosystem : The ecosystem may be large, as large as the world or small, as small as a cow dung ecosystem. The biosphere of (The total life content of the world) is the major ecosystem. It comprises all other ecosystems.

(i) Mega ecosystem : The biosphere is formed of four mega ecosystems. They are as follows :

1. Marine ecosystem : It is the largest ecosystem of earth. Fresh water ecosystem are two types :

• Lotic : Runing water ecosystem as river.
• Lentic : Still water ecosystem such as pond or lake. It includes saline-water ecosystems like oceans, seas, estuaries, brackish waters, etc.

2. Limnic ecosystem : It includes all fresh water ecosystems like ponds, pools, lakes, rivers, streams, etc.
3. Terresrial ecosystem : It includes the ecosystems of air, forests, grasslands, deserts, etc.
4. Industrial or Artificial ecosystems : These are man made ecosystems. e.g., Crop land, city, town, etc.
5. Macro ecosystems : The four mega ecosystem is further divided into sub units called macro ecosystems. e.g., Forests. The terrestrial macro ecosystem is formed of many forest ecosystems.
6. Meso ecosystem : The macro ecosystem is further divided into meso ecosystem. For example, the forest ecosystem if formed of many meso ecosystems like deciduous forest, coniferous forests, etc.
7. Micro ecosystem : The meso ecosystem is further divided into micro ecosystems, e.g., A low land in a forest, a mountain in a forest, etc. All ecosystems in the world are further divided into natural and artificial ecosystems.
8. Natural ecosystems : These are self–regulating systems without much direct human interference and manipulations. e.g., Ponds, lakes, rivers, seas, oceans, grasslands, deserts, etc.
   4. Dynamics of ecosystem : The various components of the ecosystem constitute an interacting system. They are connected by energy, nutrients and minerals. The nutrients and minerals circulate and recirculate between the abiotic and biotic factors of the ecosystem several times. The flow of energy, on the other hand, is one way, once used by the ecosystem, it is lost. The continuous survival of the ecosystem depends on the flow of energy and the circulation of nutrients and minerals in the ecosystem. Thus the dynamics (functions) of the ecosystem includes the following :

 

(i) Energy	(ii) Primary production	(iii) Secondary production (iv) Food chain
(v) Food web	(vi) Trophic levels	(vii) Energy flow             (viii) Ecological pyramids.

1. Energy : Energy is the ability of do work. The flow of energy is unidirectional in the ecosystem. The main source of energy for an ecosystem is the radiant energy or light energy derived from the sun. The amount of solar radiation reaching the surface of the earth is 2 Cals/sq.cm/min. It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. Plants utilize only 0.02% of the energy reaching earth. The light energy is converted into chemical energy in the form of sugar by photosynthesis.

6H₂O + 6CO₂ + Light ® 6C₆H₁₂O₆ + 6O₂

The sugar synthesized is utilized for many purposes :

• It can be converted into starch and stored.
• It combines with other sugars to form cellulose.

 

• It combines with inorganic substances (N₂, P, S) to form amino acids, protein, nucleic acids, pigments, hormones.
• Some amount of sugar is oxidised during respiration and the energy is released to do various functions.

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + Energy.

2. Primary production : Plants convert light energy into chemical energy in the form of sugar by photosynthesis. The total amount of sugar and other organic materials produced in plants per unit area per unit time is called gross primary production. During photosynthesis respiration is also going on side by side. During respiration some amount of sugar is oxidised. Hence it is not easy to measure gross primary production. The total organic material actually present (biomass) in plants is called net primary production.

Net primary production = Gross primary production – Respiration. i.e.

Pn = Pg – R

Where, Pg = Gross primary production

            Pg = Pn + R      Pn = Net primary production

R = Respiration

Thus the amount of organic material produced during a given period of time per unit area is called primary production. The productivity is generally expressed in terms of grams or kilocalories per square meter per day or per year. If Pg equals R no storage of energy occurs. When Pg is less than R, productivity decreases. When Pg is greater then R, productivity increases.

Measurement of primary production : Primary production refers to the amount and the rate of energy produced by autotrophs. There are many methods to measure the primary production. They are the following :

1. Harvest method : In this method the plants grown on a particular area are harvested at ground level and their weight is taken. They are dried and again weighed. This is done at regular intervals. The primary production is expressed in terms of biomass or mass per unit area per unit time.
2. Carbon-dioxide assimilation method : Plants utilize carbon dioxide for photosynthesis. So the rate of photosynthesis can be calculated by calculating the amount of carbon dioxide utilized by plants per unit time. The incorporation of carbon dioxide in photosynthesis can be determined by using infrared gas analyzer. With the help of this analyzer, it is possible to measure the amount of carbon dioxide entering or leaving an air tight chamber of known volume.
3. Oxygen production method : This method is used to measure primary production in aquatic ecosytem. In this method the amount of oxygen produced per unit time is taken as an index to measure the rate of photosynthesis. For this, light and dark bottle technique is used. Samples of water containing the autotrophs are collected in a light bottle (transparent bottle) and in a dark bottle. The light bottle allows light to enter in the same depth from which the sample is collected. After a certain period of time the amount of oxygen present in the two bottles is calculated by titration using sodium thiosulphate. (Winkler's method). In dark bottle photosynthesis does not occur but respiration occurs. In light bottle both respiration and photosynthesis occur. The rate of photosynthesis is calculated by calculating the amount of oxygen present in the two bottles.
4. Radio isotope method : This method is similar to the oxygen producing method. In this method a known quantity of C¹⁴ is introduced into the light and dark bottles along with the sample and the bottles are

 

 

 

suspended for six hours. During this period the C¹⁴ is incorporated into the protoplasm of the autotrophs. The autotrophs are filtered and dried. After drying the radioactivity is measured. The amount of radioactivity is proportionate to the amount of carbohydrate produced.

3. Secondary production : The energy trapped by the producers (primary production) is utilized by the consumers. The producers are directly consumed by the herbivores that are eaten by the primary carnivores that in turn are consumed by the secondary carnivores. The consumers store some amount of energy in their tissues. This energy, stored by the consumers, is called secondary production. Only about 10 to 20% of the primary production is converted into secondary production. The remaining 80 to 90% is lost by the consumers in the form of faeces.
4. Food chain : The biotic factors of the ecosystem are linked together by food. For example, the producers form the food for the herbivores. The herbivores form the food for the carnivores. The sequence of the eaters being eaten is called food chain to another trophic level.

Producers ® Herbivores ® Carnivores

The various steps in a food chain are called trophic levels. Owing to repeated eating being eaten, the energy is transferred from one trophic level.

Phytoplankton ® Zooplankton ® Fishes ® Snakes Tr. L₁                          Tr. L₂ Tr. L₃ Tr. L₄

Plants ® Mouse ® Snake ® Hawk = Grassland Plants ® Goat ® Man ® Lion = Forest

This transfer of energy from one trophic level to another is called energy flow. A typical food chain can be seen in a pond ecosystem. The algae and phytoplakton are eaten by the zooplankton. The zooplankton are eaten by fishes which are eaten by snakes.

Types of food chains : The food chains are of two types, namely :

1. Grazing food chain

2. Detritus food chain

A food chain – 1. Producer grass 2. Primary consumer-grasshopper 3- Secondary consumer-Frog 4- Tertiary consumer-snake 5- Quaternary Top consumer-Owl

1. Grazing food chain : This food chain starts from plants, goes through herbivores and ends in carnivores. Plants ® Herbivores ® Primary Carnivores ® Sec. Carnivores

This type of food chain depends on the autotrophs which capture the energy from solar radiation. A few chains are given below :

Grass ® Grasshopper ® Lizard ® Hawk Grass ® Mouse ® Snake ® Hawk

 

Phytoplankton ® Zooplankton ® Fish ® Snake.

The grazing food chain is further divided into two types, namely :

• Predator chain : In predator food chain one animal captures and devours another animal. The animal which is eaten is called prey and the animal which eats other animals is called predator. The predator food chain is formed of plants, herbivores, primary carnivores, secondary carnivores and so on.
• Parasitic chain : The plants and animals of the grazing food chain is infected by parasites. The parasites derive their energy from their hosts. Thus the parasitic chain within the grazing food chain is formed.

2. Detritus food chain : It starts from dead organic matter and ends in inorganic compounds. There are certain groups of organisms which feed exclusively on the dead bodies of animals and plants. These organisms are called detritivores. The detritivores include algae, bacteria, fungi, protozoans, insects, millipeds, centipeds, crustaceans, mussels, clams, annelid worms, nematodes, ducks, etc. These organisms ingest and digest the dead organic materials. Some amount of energy is trapped and the remainder is excreted in the form of simple organic compounds. These are again used by another set of detritivores until the organic compounds are converted into CO₂ and water.

Dead organic materials ® Detritivores ® CO₂ + H₂O

Linking of grazing and detritus food chains – The two main food chains can not operate independently. They are interconnected at various levels. According to Wilson and Bossert (1971) the stability of the ecosystem is directly proportional to the number of such links. The detritus feeders obtain energy from the dead bodies of animals and plants which are components of the grazing food chain. Again some of the detritus feeders are eaten by the consumers of the grazing food chain. For example, in a pond ecosystem earthworms belonging to the detritus food chain are eaten by fishes belonging to the grazing food chain.

5. Food web : In an ecosystem the various food chains are interconnected with each other to form a net work called food web. The interlocking of many food chains is called food web. Simple food chains are very rare in nature. This is because each organism may obtain food from more than one tropic level. In other words, one organism forms food for more than one organisms of the higher trophic level.

Examples : In a grassland ecosystem, grass is eaten by grasshopper, rabbit and mouse. Grasshopper is eaten by lizard which is eaten by hawk. Rabbit is eaten by hawk. Mouse is eaten by snake which is eaten by hawk. In addition hawk also directly eats grasshopper and mouse. Thus there are five linear food chains which are inter connected to form a food web.

• Grass ® Grasshopper ® Hawk
• Grass ® Grasshopper ® Lizard ® Hawk
• Grass ® Rabbit ® Hawk
• Grass ® Mouse ® Hawk
• Grass ® Mouse ® Snake ® Hawk

This is a very simple food web. But in any ecosystem the food web is more complex. For example, in the grassland itself, in addition to hawk, there are many other carnivores such as vulture, crow, fox, man, etc.

 

 

Significance of food web : Food webs are very important in maintaining the stability of an ecosystem. For example, the deleterious growth of grasses is controlled by the herbivores. When one type of herbivores increase in number and control the vegetation. Similarly, when one type of herbivorous animal becomes extinct, the carnivore predating on this type may eat another type of herbivore.

 

 

Fig : Food web in a grassland ecosystem (Producers, Herbivores, Carnivores)

6. Trophic levels : Each food chain contains many steps like producers, herbivores, primary carnivores and so on. Each step of the food chain is called trophic level. The number of trophic levels in a food chain in restricted to 5 or 6. Green plants make first trophic level.

T₁ ® Producers – (Trees, Plants, Grass)

C₁ or T₂ – Herbivorous – (Cow, Grass hopper, Zooplankton) C₂ or T₃ – Primary carnivorous (Dog, Frog, Lizard)

 

 

C₃ or T₄ – Secondary carnivorous (Hawk, Fox, Snake)

C₄ or T₅ – Tertiary carnivorous or Top carnivorous (Tiger, Lion, Man) Phytoplankton ® Zooplankton ® Fishes ® Snakes

Tr. L₁                  Tr. L₂             Tr. L₃      Tr. L₄

7. Energy flow : The transfer of energy from one trophic level to another trophic level is called energy flow. The flow of energy in an ecosystem is unidirectional. That is, it flows from the producer level to the consumer level and never in the reverse direction. Hence energy can be used only once in the ecosystem. But the minerals circulate and recirculate many times in the ecosystem. A large amount of enregy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. Hence the amount of energy available decreases from step to step. Only about 10% of the biomass is transferred from one trophic level to the next one is a food chain. And only about 10% chemical energy is retained at each

trophic level. This is called 10% law of Lindeman (1942). When the food chain is short, the final consumers may get a large amount of energy. But when the food chain

Energy flow diagram: L = Total light, La = Absorbed light, PG = Gross production, PN = Net production, I = Energy intake, A = Assinilated energy, NA = Non assimilated energy, NV = Unused energy, R = Respiratiory energy loss. Bottom line shows the order of magnitude of energy losses expected or major transfer points starting with the solar in put of 3000 Kcal per square meter per day (ODUM 1963)

is long, the final consumer may get a lesser amount of energy. As shown in the figure. about 3000 K cal of light falls

on the green plants. Of this approximately 50% (1500 K cal) is absorbed. Of the 50% only 1% (15 K cal) is converted at the first trophic level. Thus the net primary production is merely 15K cals. Secondary productivity (P₂ and P₃) is 10% (1.5 K cal & 0.3 K cal) at the herbivores and carnivores level.

8. Ecological pyramids : The number, biomass and energy of organisms gradually decrease from the producer level to the consumer level. This can be represented in the form of a pyramid called ecological pyramid. Ecological pyramid is the graphic representation of the number, biomass, and energy of the successive trophic levels of an ecosystem. The use of ecological pyramid was first described by Charles Elton in 1927. In the ecological

 

 

pyramid, the producer forms the base and the final consumer occupies the apex. There are three types of ecological pyramids, namely :

(a) The pyramid of number : The number of individuals at the trophic level decreases from the producer level to the consumer level. That is, in an ecosystem the number of producers is far high. The number of herbivores is lesser than the producers. Similarly, the number of carnivores is lesser than the herbivores.

 

 

 

Fig : Pyramid of numbers in a cropland ecosystem                       Fig :Pyramid of numbers in a pond ecosystem

 

 

• In a cropland ecosystem : In croplands the crops are more in numbers. The grasshoppers feeding on crop plants are lesser in number. The frogs feeding on grasshopper are still lesser in number. The snakes feeding on frogs are fewer in number.

Crop ® Grasshopper ® Frogs ® Snakes ® Hawks

• In a grassland ecosystem : In a grassland the grasses are there in large numbers. The consumers decrease in the following order.

Grass ® Grasshopper ® Lizard ® Hawk Grass ® Rabbit ® Fox ® Lion

• In a pond ecosystem : The number in a pond ecosystem decreases in the following order.

Phytoplankton ® Zooplankton ® Fishes ® Snakes

 

 

Top Carnivores

T₄                                                e.g. Tigers, Lions

 

T3                                                                                     e.g. Wolfs

T2                                                                                                     e.g. Birds, Dears

 

Producers

T1                                                                                        e.g. Trees