Functional Component of an ecosystem: Energy Flow and Nutrient cycling:

What is Energy Flow?

Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR). We know that plants and photosynthetic and chemosynthetic bacteria (autotrophs), fix suns radiant energy to make food from simple inorganic materials. Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world. So, it is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem. All organisms are dependent for their food on producers, either directly or indirectly. Therefore, in nature, we can always find a unidirectional flow of energy from the sun to producers and then to consumers. It is very interesting to note that this unidirectional flow of energy always obeys what we call as the first law of thermodynamics which states that energy can neither be created nor destroyed, but can be converted from one form to another. This is what happens in the plants converting the solar energy into chemical energy of carbohydrates through the process called as photosynthesis.

Further, ecosystems are not exempt from the Second Law of thermodynamics as well. They need a constant supply of energy to synthesize the molecules they require, to counteract the universal tendency toward increasing disorderliness, what we call as entropy. Moreover, in this transformation of energy from one form to another, there always occurs a loss of energy keeping in view the operation of second law of thermodynamics. This explains the reason that there are always a limited number of trophic levels in any given ecosystem.

The green plants in the ecosystem-terminology are called producers. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Likewise, primary producers in an aquatic ecosystem are various species like (phytoplankton), algae and higher plants.

How does this energy flow occur in an ecosystem?  Through food chains/webs!

What is a food chain?

As we already know that in any given ecosystem whether, terrestrial or aquatic, all the living organisms (plants and animals or so called flora and fauna), are linked together in a systematic chain with respect to their mode of manufacturing food/feeding habits such that one organism always becomes the food for the next in the chain (as depicted in the figure below). This linkage of one organism with the next as a source of food in nature is what simply referred to as food chain.

For instance, in a grassland ecosystem, the entire stock of green plants (herbs, shrubs and trees) manufacture food (producers) by utilizing the radiant energy of the sun by the process of photosynthesis. These autotrophs are eaten up by plant eaters i.e. herbivores which, in turn, constitute the food for first level (primary) carnivores. The primary carnivores are subsequently eaten up by middle (secondary) and top level (tertiary) carnivores. Here, one finds that beginning with the producers, onward to herbivores and then to first level carnivores, middle level carnivores and top level carnivores, all organisms are inter-linked in a definite sequence. This sequential inter-linking of organisms involving the transfer of food energy from the producers, through a series of organisms with repeated eating and being eaten is referred to as the food chain. It is important to mention that food chain is always straight and proceeds in a progressive straight line although, it does not happen in nature that a food chain shall invariably be in a straight line because, one organism may be eaten up by several other organisms as well…

But then, it always remains to be a universal truth that the green plants always occupy first level in any given food chain and are commonly termed as the primary producers. The stored energy in primary producers is utilized by the plant eaters i.e. the herbivores which are also known as primary consumers or even as first order carnivores in ecosystem terminology. Herbivores are subsequently eaten by the flesh eating animals or so called the second order carnivores though, also referred to as the secondary consumers.

As noted above, the carnivores in ecological parlance are generally categorized into three categories namely, primary carnivores, secondary carnivores and tertiary carnivores. Some organisms are termed as omnivores as well, as these eat the producers as well as the carnivores at their lower level in the food chain. The classification of various organisms in a food chain definitely relates to their function in an ecosystem so that any two or more distinct species are likely to occupy the same trophic level.

Figure showing: Generalized scheme of nutritional relationship- amongst components of an ecosystem.

Some important characteristics of a Food Chain:

1.   A food chain involves a nutritive interaction between the living organisms (biotic components) of an ecosystem. In a food chain, there occurs repeated eating i.e. each group eats the other and is subsequently eaten by some other group of organisms.

2.   A food chain is always straight and proceeds in a progressive straight line.

3.   In a food chain, there is unidirectional flow of energy from sun to producers and subsequently to series of different types of consumers.

4.   Usually there occur 4 or 5 trophic levels in the food chain.

5.   Generally 80-90% of energy is lost as heat at each transfer in accordance with second law of thermodynamics.

6.   Some organisms occupy different trophic positions in different food chains.

What are trophic Levels in a food chain?

The distinct sequential steps in the straight food chains are referred to as different trophic (nutritional) levels. For instance, green plants (producers) stand for the first trophic level i.e. the producer level; the plant eaters (herbivores) also called, primary consumers belong to second trophic level i.e. the primary consumer level and the flesh eaters (carnivores) also called secondary consumers represent the third trophic level i.e. the secondary consumer level.

It is worthwhile to mention that the quantum of available energy in a food chain successively gets decreased at each trophic level as a result of waste of energy into unavailable heat. This can be explained on the basis of second law of thermodynamics according to which transformation of energy from one form to the other is inefficient and involves dissipation of unavailable energy. This phenomenon (loss of energy at successive levels) restricts the size of food chain in any ecosystem to maximum of 4 or 5 steps as shown in the figure below:

What can be the types of Food Chains? 

In nature, generally two types of food chains are distinguished namely, grazing food chain and detritus food chain.

A.) Grazing food chain: The common examples of the grazing food chain are those of grassland (terrestrial) ecosystem starting from grasses-herbivores-carnivores (primary, secondary, tertiary) and aquatic ecosystems starting from phytoplanktons - zooplanktons - small fishes - large fishes. In a grazing food chain (e.g. grassland ecosystem), green plants (producers) constitute the first step. The plant biomass, in turn, is eaten by the herbivores (e.g. insects, rats and mice, grazing cattle) which are subsequently consumed by a variety of carnivores. The grazing food chains are directly dependent on an influx of solar radiations. Green plants, with the help of solar radiations, manufacture food (plant biomass) by the process of photosynthesis. These are longer food chains and these always end at decomposer level. From energy point of view, these grazing type of food chains are very important.

B.)  Detritus food chain: Unlike grazing food chains, the detritus food chains depend upon the dead organic matter either in the form of fallen leaves etc. or dead animal bodies. Hence, these are not directly dependent on solar energy.

The starting point, in a detritus food chain, is the dead bodies of animals or fallen leaves etc. commonly termed as detritus. This dead organic matter is first eaten by decomposers e.g. bacteria, fungi and protozoans etc. These decomposing organisms have saprotrophic mode of feeding and are designated as primary consumers in this type of Chain. These are also termed as detritivores which means (detritus eating). The detritivores, in turn, are eaten by their predators like insect larvae, nematodes etc. The main function of the detritivores is to release the inorganic elements into the ecosystem by decomposing the dead organic matter. The recycling of inorganic elements in ecosystem helps in keeping them fit.

As compared to grazing food chains, the detritus food chains are generally shorter.

Noted that a food chain having consumers in it is also called as a predator food chain. The decomposers may be excluded from the food chain because they act at all the trophic levels of a food chain. The producers are also called energy transducers as they change the radiant energy of the sun into chemical energy stored in the form of their organic compounds principally, the glucose.

Now having understood the concept of a food chain, it is convenient to see it in live action with the help of following two examples:

 (i) Food Chain in a Pond: In a pond, rooted or floating plants and floating algae synthesize food materials from dissolved nutrients by photosynthesis. They, thus, act as the producers. Unicellular algae are eaten by protozoans (amoebae, paramecia) and water fleas. Similarly, the snails, mosquito larvae and tadpoles etc. also take in aquatic plants. All these herbivore organisms are primary consumers. These are then eaten by Hydra, dragonfly larvae, water insects (giant water bugs, water scorpions, water beetles including small fish etc.) These carnivore organisms are secondary consumers. They are fed upon by large fishes and frogs. These animals are tertiary consumers. Frogs and fishes are eaten by snakes and birds, which are quaternary consumers. The snakes and birds and also the organisms of other trophic levels, when dead, serve as food for the bacteria and fungi of decay. The latter change the dead organisms into simple inorganic materials for reuse by the producers. This completes the circuit of both the grazing and detritus food chain in nature.

(ii)  Food Chain on Land: On land, grass grows by manufacturing food from carbon dioxide of air and water and minerals of soil with the help of chlorophyll and sunlight. Grass, thus, acts as the producer on land. Grass is eaten by rabbits, the primary consumers. The latter are preyed upon by cats, the secondary consumers. The cats may be taken by wolves, the tertiary consumers, and the tigers may capture the wolves. The tigers represent the quaternary or terminal consumers. They are not eaten by any predators. The tigers as well as any other trophic level organism in the chain, on death, are reduced or so called decomposed by bacteria and fungi of decay to simple inorganic materials. The latter are reused by grass. The plants/trees and subsequent organisms in the food chain illustrates another example of a food chain on land…

What is a Food Web?

We must very well understand that in nature, food chains do not operate in isolation which means that in nature, it does not happen or rather inconceivable to imagine that a particular organism shall be eaten by a particular consumer only. There thus always result that the food chains in nature always appear to be interconnected with each other at several points thereby, forming a sort of an interlocking pattern of food chains. This interlocking or networking of several food chains in nature is actually referred to as a food web.

Thus in simplest terms, a food Web is a network of food chains which become inter-connected at various trophic levels so as to form a number of feeding connections amongst different organisms of a biotic community.

It is thus very clear that unlike food chains, food webs are never straight. Interlocking pattern in a food web is promoted by the fact that a single living organism may participate at different trophic levels in different food chains. For instance, in a grassland ecosystem, the grass may be eaten by the grazing cattle. In the absence of grazing cattle in the biotic community, grass may be eaten by a rabbit or in the absence of both, rat or mouse may also consume the grass. Similarly, rat or mouse may be eaten by predatory birds or snakes. Snakes may also be eaten by predatory birds etc.

 To sum up, we can scientifically argue that the alternatives are always available in nature which result in a sort of inter-locking pattern of food chains and hence, the formation of food webs. Thus, in food webs, any given species may operate simultaneously at more than one trophic level.

Some important characteristics of a Food Web:

1.   Unlike food chains, food webs are never straight.

2.   Food web is formed by interlinking of food chains.

3.   A food web provides alternative pathways of food availability. For example, if a particular species of producer is destroyed by a disease in the ecosystem, the herbivores of that area can feed on other species of producers. Similarly, secondary consumers e.g. predatory birds may feed on rats or mice in the event of decrease in population of rabbits in that area on which they have commonly been feeding. Refer to the figure below for more clarity as to the formation of food webs in an aquatic (pond) ecosystem:

Figure showing: Food web in a pond.

4.   In a food web, greater are the alternative pathways more stable is the ecosystem.

5.   Food webs also help in checking the over-populations of highly fecundive species (species which are highly prolific in producing offsprings) of organisms (both animals and plants).

6.   Food webs also help in ecosystem development.

What is the significance of food webs in ecology? Food webs provide stability to the ecosystem. Since, most animals are polyphagous in nature which means that they feed on more than one kind of organisms. If the population of a particular species decreases to an endangered level, its predators shift to another prey and the endangered species may get a chance to recover its population. Even otherwise, if there happens a complete loss of a particular type of organism or species, it would not produce large fluctuations in the populations of other organisms which use it as food just because, they would have alternative food organisms available to feed upon.

Thus, from the above, we can easily conclude that it is through the mechanism of these food chains and webs that the third most essential component of an ecosystem called as energy flow is accomplished although, more than that it also justifies the ecological significance of flora and fauna because, without them neither food chains or webs are possible nor do the flow of energy from one trophic level to another.

In other words, for us, the flow of energy in an ecosystem drives home the universal truth that no energy that is trapped into an organism remains in it for ever. The energy trapped by the producers, hence, is either passed on to a consumer or the organisms dies. Death of an organism is the beginning of the detritus food chain/web.

The important point to note is however that the amount of energy decreases at successive trophic levels. When any organism dies, it is converted to detritus or dead biomass that serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands.

Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight. Measurement of biomass in terms of dry weight however, is more accurate.

Connecting concepts: What is 10% law of ecology?

As we have noted somewhere earlier that if the ecosystems obey the first law of thermodynamics, then they also are not exempted from the second law. As such, there always occurs a loss of energy as it flows through one trophic level to another either in a food chain or web. That is why; the number of trophic levels in the grazing food chain (GFC) is restricted as the transfer of energy follows 10 per cent law. According to this law, only 10 per cent of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain. Ironically, due to the operation of this ten percent law, the number of trophic levels in a detritus food chain is always less and restricted and even in an ordinary food chain say, on land, this number does not exceed beyond four or five for obvious  reasons.