Food chain, food web and ecological pyramids

Food Chain

A food chain is the series of organisms showing feeding relationships.
It may be defined as “the transfer of energy from the producers, through a series of organisms (herbivores to carnivores to decomposers) with repeated eating and being eaten”. A food chain almost always begins with a green plant (producer) which is eaten by an animal (consumer).

Grazing food chain: This type of food chain starts from living green plants goes to grazing herbivores and on to carnivores. Ecosystem with such type of food chain are directly depend on influx of solar energy. Most of the ecosystem in nature follow this type of food chain. From standpoint these chains are very important. The phytoplanktons–zooplanktons-fish sequence or the grass-rabbit-fox sequence are the examples of grazing food chain.

Detritus food chain: This type of food chin goes from dead organic matter into micro-organism and then to organisms feeding on detritus and their predators. Such ecosystem are thus less dependant on direct solar energy. These depend chiefly on the influx of organic matter produced in another system.

Food Web

Food web is  a network of interrelated food chains in a given area. Under natural conditions, the linear arrangement of food chains, hardly occurs and these remain indeed inter connected with each other through different types of organisms at different trophic levels. For example , in a grazing food chain of a grassland, in the absence of rabbit, grass may also be eaten by mouse. The mouse in turn may be eaten directly by hawk or by snake first which is then eaten by hawk. Thus in nature there are found alternatives, which all together constitute some sort of interlocking pattern-the food web.  

Ecological pyramid

An ecological pyramid (also trophic pyramid or energy pyramid) is a graphical representation designed to show the biomass, biomass productivity or number of individuals at each trophic level in a given ecosystem. Ecological pyramid are of three general types.1. Pyramid of Biomass and 2. Pyramid of energy, and 3. Pyramid of numbers

Pyramid of biomass (g dry weight /m²)

Biomass is the amount of living or organic matter present in an organism. Biomass pyramids show how much biomass is present in the organisms at each trophic level, while productivity pyramids show the production or turnover in biomass.

Ecological pyramids begin with producers on the bottom (such as plants) and proceed through the various trophic levels (such as herbivores that eat plants, then carnivores that eat herbivores, then carnivores that eat those carnivores, and so on). The highest level is the top of the food chain.

An ecological pyramid of biomass shows the relationship between biomass and trophic level by quantifying the amount of biomass present at each trophic level of an ecological community at a particular moment in time. Typical units for a biomass pyramid could be grams per square meter, or calories per square meter.

The pyramid of biomass may be 'inverted'. For example, in a pond ecosystem, the standing crop of phytoplankton, the major producers, at any given point will be lower than the mass of the heterotrophs, such as fish and insects. This is explained as the phytoplankton reproduce very quickly, but have much shorter individual lives.

One problem with biomass pyramids is that they can make a trophic level look like it contains more energy than it actually does. For example, all birds have beaks and skeletons, which despite taking up mass are not eaten by the next trophic level. In a pyramid of biomass the skeletons and beaks would still be quantified even though they do not contribute to the overall flow of energy when ripping and tearing into the next trophic level.

Pyramid of productivity/pyramid of Energy (Biomass or Kilocalories /unit area /unit time)

An ecological pyramid of productivity/pyramid of energy is often more useful, showing the production or turnover of biomass at each trophic level. Instead of showing a single snapshot in time, productivity pyramids show the flow of energy through the food chain. Typical units would be grams per square meter per year or calories per meter per year. As with the others, this graph begins with producers at the bottom and places higher trophic levels on top.

When an ecosystem is healthy, this graph produces a standard ecological pyramid. This is because in order for the ecosystem to sustain itself, there must be more energy at lower trophic levels than there is at higher trophic levels. This allows for organisms on the lower levels to not only maintain a stable population, but to also transfer energy up the pyramid. The exception to this generalization is when portions of a food web are supported by inputs of resources from outside of the local community.

When energy is transferred to the next trophic level, typically only 10% of it is used to build new biomass, becoming stored energy (the rest going to metabolic processes). As such, in a pyramid of productivity each step will be 10% the size of the previous step

The advantages of the pyramid of productivity

• It takes account of the rate of production over a period of time.
• Two species of comparable biomass may have very different life spans. Therefore their relative biomasses is misleading, but their productivity is directly comparable.
• The relative energy chain within an ecosystem can be compared using pyramids of energy; also different ecosystems can be compared.
• There are no inverted pyramids.
• The input of solar energy can be added.
• Productivity pyramids usually provide more insight into an ecological community when the necessary information is available

The disadvantages of the pyramid of productivity

• The rate of biomass production of an organism is required, which involves measuring growth and reproduction through time.
• There is still the difficulty of assigning the organisms to a specific trophic level and there is the problem of assigning the decomposers and detritivores to a particular trophic level.

Pyramid of numbers

An ecological pyramid of numbers shows graphically the population of each level in a food chain.

In grassland the producers, which are mainly grasses are always maximum in number. This number then shows a decrease towards apex as the primary consumers (herbivores) like rabbits rabbits, mice etc., are lesser in number than the grasses. The secondary consumers snakes and lizards are lesser in number than the rabbits and maize etc., Finally the top (Tertiary consumers) hawks and other birds are least in number. Then the pyramid of number is upright. Shapes of pyramid of number in different ecosystem is given below. 

Grassland               : Upright

Pond                       : Upright

Forest                     : Different shape

Parasitic food chain: Always inverted