Concept of energy and nutrient flow in ecosystems

Concept of energy flow: The sequence of the eaters being eaten is called food chain. The various steps in a food chain are called trophic levels. The transfer of energy from one trophic level to another is called energy flow. The energy enters into an ecosystem in the form of sun light. Only 65% of the sun light can reach the surface of the earth. This solar energy is transformed by green plants through the process of photosynthesis into  stored  chemical energy.

Solar energy +Carbon dioxide + water  ----^{Photosynthesis-}àGlucose (chemical energy)+ oxygen

 This chemical energy is consumed by plant-eating animals (herbivores) which are in turn consumed by meat-eating animals (carnivores). The release of energy from stored sugar is called respiration. Conversion to heat is the ultimate fate of chemical energy. A large amount of energy is lost at each trophic level. No organism ever gets 100 percent of the energy stored in the plant or animal it eats. It is estimated that 80-90 % of the energy is lost, when energy is transferred from one trophic level to another.  On an average of about 10% of the energy available in one trophic level will be transferred to the next. When the nutritional quality of the energy source is high, transfer efficiencies can be much higher than 20 percent. The amount of energy available decreases from step to step. More energy is available at the base of the trophic level. When the food chain is short, the final consumers may get a large amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy.  The energy flows through an ecosystem in one direction (unidirectional). It flows from the producer level to the consumer level and never in the reverse direction. The energy can be used only once in the ecosystem.

Basic principles of energy flow:

1.    The ultimate source of energy for most ecosystems is sun light.

2.    The ultimate fate of energy in an ecosystem is to be lost as heat.

3.    Energy transfers are never 100 percent efficient. Some energy is lost at each step.

4.    Energy and matter are passed from organism to organism through food chain.

5.    Decomposers return the last energy from the organic  remains of dead organisms.

6.    Energy is never cycled but inorganic elements are cycled in an ecosystem.

Trophic structure

Trophic structure (trophe=nourishment) is two layered in an ecosystem. Trophic level is the position that an organism (Greek for food is trophe) occupies in a food chain.

Autotrophs or producers – plants manufacture their own food from simple inorganic substances.

Heterotrophs or consumers - animals feed on autotrophs or other heterotrophs to obtain energy (herbivores, carnivores, omnivores, detrivores and decomposers). Herbivores eat plants and carnivores eat herbivores.

Ecosystems have a hierarchy of feeding relationships.

          Trophic level 1.Primary producers

          Trophic level 2.Herbivores – primary consumers

          Trophic level 3. Carnivores – secondary consumers

          Trophic level 4. Top carnivores – tertiary consumers

Decomposers –micro- consumers – operate at each trophic level.

Since herbivores take their food directly from the producers, they referred as primary consumers. The carnivores feed on other animals (or another type of consumer) are secondary or tertiary consumers. Omnivores feed on both plants and animals (e.g. Human beings, crows). Decomposer are organisms mostly bacteria and fungi that recycle nutrients from decaying organic matter.

Trophic efficiency – refers to the transfer of energy up into the trophic levels e.g. ratio of secondary productivity to primary productivity consumed. Trophic efficiencies generally range from 5 to 20% i.e. only 5 to 20% of primary producer biomass consumed is converted into new consumer biomass. On an average of about 10% of the energy available in one trophic level will be passed on to the next. When the nutritional quality of the energy source is high, transfer efficiencies can be much higher than 20%.

Food chain

Food chain is a straight line sequence of who eats whom. Food chain indicates the transfer of matter and energy from organism to organism. The study of food chains helps in understanding food relationship and interdependence among various organisms in an ecosystem. Food chains maintain ecological balance by regulating population size. In polluted ecosystems food chains biologically magnify toxicity of some chemicals. Long food chains often occur in infertile ecosystems, while short ones are often found in very productive ecosystems.

Types of food chains - there are two types of food chains namely

                             1. Grazing food chains

                             2. Detritus food chains

The two food chains are distinguished by their source of food for the initial consumers. In the grazing food chain, the living plant tissues are the primary source of energy for the initial consumers, herbivores. In the detritus food chain, dead organic matter is the source of energy for the initial consumers like bacteria and fungi.

Grazing food chain – this food chain begins with plants, goes through herbivores and ends in carnivores. Cattle grazing on pastureland, deer browsing in the forest and rabbits feeding in old fields represent the basic consumer groups of grazing food chain.

The grazing food chain is further divided in to (1) Predator food chain and (2) Parasitic food chain.

Predator food chain – In predator chain, one animal captures and devours another animal. The animal which is eaten is called prey and the animal who eats other animals is called predators.

    Prey ------àPredator 1 ------àPredator 2

Parasitic food chain -  The plants and animals are infected by parasites. The parasites derive their energy from their hosts.

Detritus food chain – It starts from dead organic matter and ends in inorganic compounds.

Dead organic matter àDetritivores  à carbon dioxide and water

Detritivores are animals which feed upon dead bodies of animals and plants.  Some amount of energy is trapped and the remainder is excreted in the form simple organic compounds.

Functional significance of food chains

1.    Food chains are the vehicles of transfer of energy from one trophic level to another. They maintain energy flow and nutrient cycling.

2.    Food chains maintain ecological balance by regulating population size.

3.    Food chains biologically magnify toxicity of some chemicals e.g. DDT, metals, called biological magnification.

4.    In many cases the food chains of the ecosystem overlap and interconnect forming a food web.

5.    Long food chains are found in infertile ecosystems and short food chains occur in productive ecosystems.

Food webs

Food webs are networks of feeding interactions among species in an ecological community. The animal Ecologist Charles Elton (1927) introduced the concept of food web (cycle). In all ecosystems, the grazing and detritus food chains are interconnected. Simple food chains are very rare in nature.  This is because each organism may obtain food from more than one trophic level.

In a grassland ecosystem, grass is eaten up by grass hoppers, rabbit and mouse. Grass hopper is eaten up lizard, which is eaten by hawk.

Rabbit is eaten by hawk. Mouse is eaten by snake, which is eaten by hawk.

A food web differs from a food chain in that the food chain shows only a portion of a food web involving a simple linear series of species connected by feeding links. A food web aims to show a more complete picture of the feeding relationships. A food web is a bundle of many interconnected food chains occurring within the community.

The members of the food webs are connected by arrows that point from the consumed to the consumers. These arrows can be interpreted as the direction in which energy flows during feeding. Grazing food webs are more important in grass lands, whereas detritus food webs are more important in forests.

Food webs are very important in maintaining the stability of an ecosystem. For example, the weed grasses are controlled by the herbivores.  When one type of herbivore becomes extinct, the other herbivore increases in number and control vegetation.

Ecological pyramids

The trophic levels in a food chain can be shown graphically through ecological pyramids, with producers at the base and successive levels of consumers forming the higher layers.  Charles Elton (1927) arranged the community of organisms by trophic levels and he found that the arrangement of trophic levels formed a pyramid. Ecological pyramids are of three basic types.

1.    Pyramid of energy

2.    Pyramid of biomass

3.    Pyramid of numbers

Pyramid of energy – An energy pyramid illustrates the amounts of energy available at each successive trophic level. The energy pyramid always shows a decrease of size moving up trophic levels because:

 (1) Only a certain amount of food is captured and eaten by organisms on the next trophic level.

(2) Some of food that is eaten cannot be digested and exits digestive tract as undigested waste.

(3) Only a portion of digested food becomes part of the organism’s body.

(4) Only about 10% of the energy available at a particular trophic level is incorporated into tissues at the next trophic level.

Pyramid of biomass – The pyramid of biomass is calculated by multiplying the average weight for organisms times the number of organisms at each trophic level. The total biomass decreases at each successive trophic level. The biomass is generally more reliable index of energy flow in an ecosystem.

Pyramid of numbers – A pyramid of numbers is based on how many organisms occupy at each trophic level. The number of organisms at each successive level is less. The number over emphasize the importance of small organisms.

 Inverted pyramids – In most of the ecosystems, the number and biomass of producers are more than the consumers. In this type, the pyramid is upright in structure. In some ecosystems, the number and biomass of producers are less and that of consumers are more. In this type, the apex of the pyramid is pointed downwards. This type of pyramid is called inverted pyramid. The inverted pyramid of numbers occurs in a tree ecosystem. A single tree (producer) contain many fruit eating birds (primary consumers). The birds contain many parasites (secondary consumers).   

  

Key points

1.    The ultimate energy source of ecosystem is the sun.

2.    Energy and matter are transferred from organism to organism through the food chain.

3.    Decomposers remove the last traces of energy from the remains of organisms.

4.    Inorganic nutrients are cycled through an ecosystem but energy is not cycled.