The functional aspects of ecosystem include the flow of energy and the cycling of nutrients. Inorganic compounds are synthesized into organic structure by the green plants through photosynthesis and solar energy is utilized in this process. Energy enters the ecosystem, when the producers build organic compounds during photosynthesis. The green plants (producers) fix solar energy and with the help of inorganic substances such as carbon dioxide and water taken from soil and aerial environment they build up complex organic matter. The organic matters are carbohydrate, fat, protein, etc. From green plants these organic foods are passed on to the herbivores, which in turn become source of energy for the carnivores. Thus, the animals are known as consumers. All the living organisms whether plants or animals in an ecosystem have a definite life span after which they die. The dead organic body of plants and animals provide food for saprophytic microbes, such as bacteria and fungi. These microbes are known as decomposers, which ultimately decompose the dead organic body into simple organic components into their environment. During the process of decomposition the energy is lost in the form of heat into the environment. Thus in an ecosystem, energy flows in a non-cyclic manner (unidirectional) from the sun to the decomposers via producers and consumers (herbivores and carnivores), whereas the minerals keep on moving in a cyclic manner. Minerals are withdrawn from the substrata, deposited in the tissues of plants and animals, cycled from one feeding group to another, return to the soil, water and air then recycled. These two ecological processes of energy flow and mineral cycling which involve interaction between biotic and abiotic components may be thought as the 'heart' of the ecosystem dynamics.
Trophic Levels:
In an ecosystem the producers and consumers can be arranged into several feeding groups, such groups are known as trophic levels (nutritional levels). The organisms deriving their energy from same source or the organisms that have the same position in the food chain are said to belong to the same trophic level. Photosynthetic organisms (green plants, algae and photosynthetic bacteria) bring energy into the system by capturing and storing light energy. Therefore, they are said to be producers. The producers occupy the first trophic level in any ecosystem. Herbivorous animals which feed on producers, are primary consumers, constitute the second trophic level. Carnivorous animals, which feed on herbivorous, are secondary consumers, constitute the third trophic level. Carnivores which feed on secondary consumers are tertiary consumers, constitute the fourth trophic level. Some organisms are omnivores eating the producers as well as the carnivores of their lower level in the food chain. Such organisms may occupy more than one trophic levels in the food chain. The number of steps in a food chain are always restricted to four or five, since the energy available decreased with each step.
Food Chain: The transfer of food energy from the producers, through a series of organisms (herbivores to primary consumers to secondary consumers to tertiary consumers) with repeated process of eating and being eaten is known as food chain. The food chain form a simple linear chain of eating and being eaten. In the ecosystem, photosynthetic organisms (green plants, algae and photosynthetic bacteria) utilise the radiant energy of sun and transfer to organic compounds, such as carbohydrates, fats and proteins during photosynthesis. Photosynthetic organisms, therefore supply almost all the chemical energy used by living organisms. They are said to be producers. The producers are the only link between the biotic and abiotic components of an ecosystem. The herbivores feed upon the photosynthetic organisms and food energy is transferred from the producers to the primary consumers. Herbivores, in turn are eaten by the carnivores (secondary consumer). The secondary consumers are eaten by tertiary consumers. Some organisms are omnivorous eating the producers as well as the carnivorous of their lower level in the food chain. Microbes (fungi and bacteria) feed on dead organisms or waste material from them and these organisms are called decomposers.
Energy Flow:
As energy is transferred from one organism to another in a food chain, large proportion of it is lost to the surrounding environment as heat. Energy enters a food chain when the producer builds organic compounds. The producer then uses the chemical energy to do work and to build new materials. Energy is used to rebuild molecules, to transport materials, and to do many other kinds of cellular work. Much of the chemical energy used to do work becomes heat and is lost from the food chain. Chemical energy which make up the bodies of producers is available to herbivores. However, the herbivores will not be able to use all of the chemical energy stored in the producers. The herbivores use some of this energy, for processes such as moving and active transport, and once again a high proportion of the energy is lost to the environment as heat. Only a small proportion of the energy is transferred to the carnivores. This pattern continues along the food chain, with approximately 90% of the energy being lost at each transfer between trophic levels.
Types of Food Chain:
In nature three types of food chains are recognised:
1) Grazing food chain (prey predator food chain)
2) Detritus food chain (saprophytic food chain)
3) Parasitic food chain
1) Grazing Food Chain: THis type of food chain starts from the living green plants, goes to grazing herbivores and on to the carnivores. Ecosystems with such type of food chain are directly dependent on an influx of solar radiation. This type of chain thus depends on autotrophic enrgy capture and the movement of this captured energy to herbivores. Most of the ecosystems in nature follow this type of food chain. These chains are very important from energy standpoint. The general sequences are as follows:
Producer ------> Primary consumers ------> Secondary consumers ------> Tertiary consumers
Example:
In a pond Ecosystem:
Phytoplanktons------> Zooplanktons------> Small fishes ------> Large fishes
In a grassland Ecosystem:
Grasses------> Grass hopper------> Frog------> Snake------> Hawk
2) Detritus Food Chain: PArticles of organic material derived from dead and decomposingorganisms of the grazing food chain are generally called detritus. Organic matters are decomposed into detritus by the activities of microorganisms like bacteria and fungi. The energy contained in this detritus is not totally lost in the ecosystem, but it serves as the source of energy for a group of organisms detritivores, that are separate from the grazing food chain. There are certain animals (consumers) which feed on these detritus and such a food chain is called detritus food chain. These depend chiefly on the influx of organic matter produced in another system. A good example of a detritus food chain is based on mangrove leaves described by Heald (1969) and W.E. Odum 1970. The mangrove leaves fall into the warm shallow water of the sea and are decomposed by microorganisms like bacteria nad fungi. The detritus in turn is eaten by a group of animals. These animals include crabs, copepods, insect larvae, grass shrimps, mysids, nematods, amphipods, bivalve molluscs etc. All these animals are detritus consumer (detritivores). They feed upon partially decomposed organic matter. These animals are in turn eaten by somesmall fishes (i.e. the small carnivores), which in turn are eaten by larger fishes and fish eating birds (i.e. the top carnivores). In fact this type of chain (detritus type) is simply a sub-component of another ecosystem. And, the above described two typers of food chain in nature are indeed linked together belonging to the same ecosystem. The general sequence of food chain is as follows:
Dead organic material------> Decomposer------> Primary consumer ------> Secondary consumer ------> Tertiary consumer
3) Parasitic Food Chain: A third type of food chain also exists in nature, in which smaller organisms consume larger ones. This is food chain in which parasites live on or inside the body of the host deriving benefit from the host. The parasites get nutritional benefit and the host is harmed. For example: a sheep feeds on green plants and in this process food energy flows from the green plants to sheep. The liverfluke lives inside the bile ducts of sheep and takes food from the sheep. In this way food enrgy flows from the sheep to liverfluke. The parasitic food chain may be represented as follows:
Green plants ------> Sheep ------> Liver fluke
Food Web:
Trophic relation between organisms is not always in simple chain but forms complicated network. A food web is a diagram of all the food chains, with their interconnections, within an ecosystem. The feeding patterns in a community are much more complex than simple linear chains. A consumer may derive its food from more than one source. Even the same organism may be eaten by several organisms of a higher trophic level. Thus, in a given ecosystem various food chains are linked together and intersect each other to form a complex network called the food web. For example in grassland five linear food chains may be seen in following sequences:1) Grass ---> Grasshopper ---> Hawk
2) Grass ---> Grasshopper ---> Lizard ---> Hawk
3) Grass ---> Rabbit ---> Hawk
4) Grass ---> Mouse ---> Hawk
5) Grass ---> Mouse ---> Snake ---> Hawk
Productivity:
The amount of energy from a trophic level passes on to the next trophic level is called production. Productivity is a measure of the amount of energy incorporated into the organisms in a trophic level, in a given area, over a certain period of time. The area used is normally one square meter, and the time is usually one year, so productivity is measured in units of kilo-joules per square meter per year, that is KJM^-2 per year. Green plants fix solar energy and accumulate it in organic forms as chemical energy. The rate at which producers accumulate energy or convert light energy into chemical energy is called primary productivity. Whereas the total amount of chemical energy incorporated into the producers is called gross primary product (GPP). A good fraction of gross primary production is utilized in respiration of green plants. The amount of energy-bound organic matter created per unit area and time that is left after respiration of these plants if the net primary products (NPP) or plant growth. Only the net primary productivity is available for harvest by man and other animals.
NPP = GPP - energy used in respiration
The rate of conversion of light energy into chemical energy is affected by many environmental factors. The factors include:
1) The amount of sunlight and the temperature.
2) The amount of water available.
3) The amount of mineral nutrition such as nitrates and phosphates.
In natural ecosystems, primary productivity is higher in tropical ecosystems than in high altitudes, because plants get more light and higher temperature in the tropics. There are three fundamental concepts of productivity:
i) Standing Crop: The abundance of the organisms existing in the area at any time is referred to as standing crop. It may be expressed in terms of number of individuals, as biomass of organisms, as energy content or in some other suitable terms.
ii) The Materials Removed: It includes the yield to man, organisms removed from the ecosystem.
iii) The Production Rate: The amount of material formed by each link in the food chain per unit of time per unit area or volume is the production rate.
Ecological Pyramids:
Graphical representation of an ecological parameter, like energy, biomass or number at different trophic levels in a food chain in an ecosystem is called ecological pyramid. Pyramids can be drawn to represent the proportions of energy which are lost in a food web. A considerable portion of the energy is lost as heat at each step in the food chain. As a result, organisms in each trophic level pass on lesser energy to the next trophic level than they actually receive. Due to the gradual decrease of energy in the food chain a pyramid is formed that is known as ecological pyramid. The ecological pyramids represent the trophic structure and also trophic function of the ecosystem. In each ecological pyramid, producer level forms the base and successive levels make up the apex. The ecological pyramid are of three types: (1) Pyramid of number, (2) pyramid of biomass , (3) Pyramid of energy.
1) Pyramid of Numbers: The graphic representation of the numbers of producers and consumers of different trophic levels in an ecosystem is called pyramid of numbers. The shape of pyramids may be upright, inverted or any other shape. In grassland ecosystem the pyramid is upright. Here the producers are mainly grass, which are always maximum in number. The primary consumers (herbivores) like rabbits, mice are lesser in number than the grass; the secondary consumers, snakes and lizards are still lesser in number than the primary consumers. Finally the top consumers hawks are least in number. The producers represent the base of pyramid, which are the most abundant in number and the number of individual rapidly decreases in the successive levels of consumers until there are very few carnivores at the top. In a pond ecosystem also, the pyramid of number is upright. The producers are mainly algae and hydrophytes, which are maximum in number. The primary consumers like zooplankton and benthos are lesser in number than the producers. This is followed by secondary consumers (carnivores) like insects, fishes, frogs and water snakes that are still less in number than the primary consumers. Finally, the top consumers like large fish and water birds are the least in number. However, the pyramid of numbers in a forest ecosystem appears somewhat different in shape. Here, the producers, mainly large sized trees, are lesser in number. They represent the base of pyramids. The primary consumers; birds and deer are most in number than producers. Then there is a gradual decrease in the number of successive carnivores, thus making the pyramid upright again. In a parasitic food chain the pyramid of numbers is always inverted. Here, a single plant or tree may support the growth of many herbivores like the fruit eating birds. These herbivores in turn may support varieties of parasites like lice and bugs. Subsequently each parasite might support a number of hyper-parasites like bacteria and fungi. Hence, there is a gradual increase in the number of organisms occur from the producer level to the consumers in the parasitic food chain. As a result, the pyramid becomes inverted.
2) Pyramid of Biomass: The graphic representation of the total biomass (dry weight) of organisms at each trophic level in an ecosystem is known as pyramid of biomass. In forest ecosystem there is a gradual decrease of biomass in each trophic level from the producers to the top carnivores. The biomass of producers is always higher than the total biomass of the herbivores in an ecosystem. Likewise, the total biomass of the primary consumer is more than the secondary consumer. The least amount of biomass is present at the level of tertiary consumers. Thus, the pyramid of biomass is upright in forest ecosystem. In an aquatic ecosystem, the producers are small organisms which weigh less than the consumers that feed on producers. This value gradually show an increase towards the apex of the pyramid. Gradual increase in weight of biomass makes the inverted pyramid.
3) Pyramid of Energy: The graphic representation of amount of energy in different trophic levels of food chain in an ecosystem is called pyramid of energy. Out of the three types of ecological pyramids, the energy pyramid give the best picture of overall nature of the ecosystem. It is always upright because less energy is transferred from each level. At the producer level the total energy is much greater than the energy at the successive higher trophic level. In other words there is decrease in energy flow from autotrophs onwards at successive trophic levels. In the course of energy flow from one organism to the other, there is considerable loss of energy in the form of heat.
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