The transfer of food energy from the producers, through a series of organisms (herbivores to carnivores to decomposers) with repeated eating and being eaten is termed as a food chain. A Copepod consumes a diatom. Then a small fish consumes the Copepod . Now the small fish forms the food source of large fish and so on. For example;
Scenedesmus boligues (Phytoplankton) → Brachionus falcatus (zooplankton) Amblypharyngodon sp (a small fish) → Wallago attu (a large fish) → Homo sapiens (man). This is a food chain of an Indian river.
The following is an example of food chain which occurs in an Indian pasture.
Cynodon dactylon (a grass) → Melanoplus differentialis (a grasshopper) → Bufo melanostictus (a toad) → Zamenis mucosus (a snake).
In the above two examples, we find that a plant forms the base of food chain . Herbivores consume the plant. The carnivores then consume the herbivores which may be eaten by another carnivore. Thus, the arrangement is as follows:
Plant → Herbivore → Carnivore 1 → Carnivore 2
Producers utilize the radiant energy of sun which transforms to chemical form, ATP during photosynthesis. Thus, green plants occupy in any food chain, the first trophic level i.e., the producers level and are termed as the primary producers.
The energy, as stored in food matter, manufactured by green plants is then utilized by plant eaters – the herbivores which constitute the second trophic level i.e., the primary consumers level and are termed as primary consumers (herbivores).
Herbivores, in turn, are eaten by the carnivores which constitute the third trophic level i.e., the secondary consumers level and are termed as secondary consumers (carnivores). These may be eaten by other carnivores at tertiary consumers level i.e., by the tertiary consumers.
In any food chain, energy flows from primary producers to primary consumers (herbivores), from primary consumers to secondary consumers (carnivores) and from secondary consumers to tertiary consumers (Carnivores/omnivores) and so on. This simple chain of eating and being eaten away is termed as food chain.
A food chain in grassland ecosystem starts with grasses and goes through grasshoppers, the frogs, the snake, the hawk whereas in a pond the order would start with phytoplanktons, going through zooplanktons, smaller fishes, bigger fishes, birds, larger animal and so on.
Types of Food Chain
In nature, we generally distinguish two general types of food chains i.e., grazing food chain and detritus food chain.
Grazing food chain:
This type of food chain starts from the living green plants, goes to grazing herbivores (that feed on living plant materials with their predators) and onto carnivores (animal eaters). Ecosystems with such type of food chains are directly dependent on an influx of solar radiation. This type of chain, thus, depends on autotrophic energy capture and the movement of this captured energy to herbivores.
Most of the ecosystem in nature follows this type of food chain. The phytoplankton → zoo plankton→ fish sequence or the grasses → rabbit → fox sequence are the examples of grazing food chain.
Detritus food chain:
This type of food chain goes from dead organic matter into microorganisms and then to organisms feeding on detritus (detritivores) and their predators. Such ecosystems are, thus, less dependent on direct solar energy. These depend chiefly on the influx of organic matter produced in another system. For example, such type of food chain operates in the decomposing litter in a temperate forest.
A good example of detritus is based on the mangrove leaves. In the brackish zone of Southern Florida, leaves of red mangrove fall into the warmest swallow waters. The grazing insects consume only 5% of the leaf material before leaf fall. A group of small animals consume and re-consume the fallen leaf fragments (acted on by such saprotrophs as fungi, bacteria, protozoa etc. and colonized mainly by phytoplankton and benthic algae ). These animals include Copepods, insect larvae, grass shrimps, nematodes, amphipods, bivalve molluscs etc. All these animals are detritus consumers.
These detritivores ingest large amount of the vascular plant detritus. These animals are, in turn, eaten by some minnows and small game fish, i.e., the small carnivores which in turn serve as the main food for larger game fish and fish eating bird which are top carnivores.
Thus we see that the detritus food chain ends up in a manner similar to the grazing food chain (i.e., big fish eats little fish) but the way in which two chains begin is quite different.
Food Web- The interlocking pattern of organisms
However, food chains in natural conditions never operate as isolate sequences, but interconnect with each other forming some sort of interlocking pattern, which is termed as food web.
Food web in grassland and pond ecosystems
Under natural conditions, the linear arrangements of food chains, hardly occurs and these interconnect with each other through different types of organisms at different trophic levels. For example; in grazing food chain of grassland, in the absence of rabbit, a mouse consumes the grass. A hawk in turn consumes the mouse directly or a snake consumes it first. Hawk may again consume the snake. Thus, in nature there are alternatives, which altogether constitute some sort of interlocking pattern; the food web.
In such a food web in grassland, as shown in the figure, there may be seen as many as five following linear food chains, which in sequences are:
1. Grass→ Grasshopper → Hawk
2. Grass → Grasshopper→ Lizard→ Hawk
3. Grass → Rabbit → Hawk (or vulture or fox or man)
4. Grass → Mouse→ Hawk
5. Grass → Mouse→ Snake → Hawk
Besides those shown in the figure there may also be present some other consumers as vultures, fox and man in grasslands, and if so, the food web may be even more complex than shown here. However, all these five chains interlink with each other at different points forming food web. Similarly the population in an African grassland may be characterized by their feeding relationships in the following two food chains.
Live grass (Producer) → Wild beast (herbivore)→ Lion (Primary carnivore)
Dead grass (dead organic matter)→ Termites (decomposers)→ Aardvark (Primary carnivore)
The broken arrows show how the two chains link into a food web. Real food webs usually hundreds of species interlinked by their feeding habits. Food webs are basic units of ecosystem. A similar food web in a pond, with different interlinked food chains is in the following figure.
Importance of food web
The food webs are very important in maintaining the stability of an ecosystem in nature. For example, decrease in the population of rabbit would naturally cause an increase in the population of alternate herbivores, the mouse. This may decrease the population of the consumer (carnivore) that prefers to eat rabbit.
Thus, alternatives (i.e., substitutes) serve for maintenance of stability of the ecosystem, moreover, a balanced ecosystem is essential for the survival of the living organisms of the system. For instance, had the primary consumers (herbivores) not been in nature, the producers would have perished due to overcrowding and competition. Similarly, the survival of primary consumers links with the secondary consumers (carnivore) and so on. Thus, each species of any ecosystem is indeed kept under some sort of a natural check to balance the system.
Complexity of food web:
The complexity of any food web depends upon the diversity of organisms in the system. It would accordingly depend upon the following two main points.
Length of the food chain:
Diversity in the organisms based upon their food habits would determine the length of food chain. More diverse the organisms in the food habits, more longer would be the food chain.
Alternatives at differing points of consumers in the chain: More the alternatives, more would be the interlocking pattern. In deep oceans, seas, etc. where we find a variety of organisms, the food webs are much complex.