Ecological Pyramids

 

In an ecosystem the arrangement of biotic components of the food chain according to their size, metabolic relationship is known as ‘Trophic Structure’. This is specific to each ecosystem. The relationship between the various trophic levels of a food chain i.e., Producer – herbivores – carnivores, may be shown graphically by Ecological Pyramids.

Ecological pyramids were first proposed by Charles Elton and can also be called as ‘Elton pyramids’. In ecological pyramid, the producer level constitutes the base of the pyramid and consumers occupy the successive levels up to the apex.

Ecological pyramids are of three types – 1. Pyramid of numbers, 2. Pyramid of biomass and 3. Pyramid of energy.

Pyramid of Numbers:

In this type the relationship between producers, herbivores and carnivores at successive trophic levels are shown in term of their numbers.

In a grassland ecosystem the producers, which are mainly grasses, are always maximum in number. This number gradually reduces at every trophic level from producers to primary, secondary, tertiary consumers and decomposers, i.e., from the base of the pyramid to the tip.

The primary consumers or herbivores like rabbits, mice etc., are lesser in number than the grasses; the secondary consumers like snakes, lizards are lesser in number than the rabbits and mice. Finally, the top or tertiary consumers like hawks or other birds, are least in number.

Thus, the pyramid becomes upright.

Similarly, in pond ecosystem the pyramid is upright. Here the producers, which are mainly phyto-planktons as algae, bacteria, etc., are maximum in number. The herbivores, which are smaller fish, rotifers, etc., are lesser in number than the producers. The secondary consumers or carnivores such as small fish eating each other, water beetles, etc. are lesser in number than the herbivores. Finally, the top or tertiary consumers, the bigger fish are least in number.

In a forest ecosystem, however, the pyramid of number is somewhat different in shape. The producers, which are mainly large-sized tress, are lesser in number, and form the base of the pyramid. The, which are fruit-eating birds, like elephants, deers, etc. are more in number than the producers. Then there is a gradual herbivores decrease in the number of successive carnivores, thus, making the pyramid again upright.

However, in a parasitic food chain the pyramid is always inverted. This is due to the fact that a single plant may support the growth of many herbivores and each herbivore in turn may provide nutrition to several parasites, which support many hyper parasities. Thus, from producer towards consumers, there is a reverse position, i.e. the number of organisms gradually shows an increase, making the pyramid inverted in shape. Herbivores, T₂ ˂ parasites on herbivores, T₃ ˂ hyperparasites, T₄.

The Pyramid of Biomass:

The amount of living material in an organism is called biomass. Pyramid of biomass shows quantitative relationship existing at various trophic levels.

In the grassland ecosystem, the biomass of producers i.e., grasses is high. Primary consumers or herbivores like rat and rabbit have lesser biomass than the producers. Secondary consumers or carnivoers like lizard, snake have lesser biomass than the primary consumers. Top carnivores like eagle has still lower biomass than the secondary consumers.

Thus, the pyramid of biomass of grassland ecosystem is upright.

Similarly, in the forest ecosystem, the biomass of producers i.e., large trees is always high. Herbivores like deer, elephant, forming the primary consumers have lower biomass than that of the producers. Secondary consumers like lizards, snakes, fox exhibit lower biomass than the primary consumers. Top carnivores like lion, tiger have lower biomass than the secondary consumers.

Thus, the pyramid of biomass of forest ecosystem is also upright.

However, in a pond ecosystem as the producers are small organisms i.e., phytoplanktons, their biomass is least. Herbiovores like small fish, forming the primary consumers, weigh more than the producers. Large fish, the secondary consumers, weigh more than the primary consumers.

Thus, the biomass gradually shows an increase towards the apex of the pyramid, thus making the pyramid of pond ecosystem inverted in shape.

 

Pyramid of Energy:

In an ecosystem, the pyramid of energy shows the amount of total energy trapped by the organism at each trophic level in a unit area and time and is expressed as kcal/m²/year. Energy pyramids provide the best picture of overall nature of the ecosystem.

In an ecosystem the producers prepare food material through photosynthesis with the help of solar energy and is converted into chemical energy. This energy is transferred from producers to primary, to secondary, to tertiary consumers, where energy is lost at every trophic level.

As energy is converted to heat during transfer of energy from one trophic level to the other, it is imperative that the consumers have lesser energy than the producers. Hence, the energy pyramid is always upright in all ecosystems. This is due to the gradual decrease in the energy content at successive trophic levels.

Energy Flow in Ecosystem

Energy is the most essential requirement of all living organisms which help in their metabolic activities. The main source of energy in an ecosystem is the solar energy or solar radiation. Green plants or producers trap this solar radiation and convert it into chemical form such as carbohydrates, protein, lipids, etc. This energy flows from one trophic level to another. During this process good amount of energy is lost or dispersed as heat and some of it is converted into other forms of energy. Energy transfer is always unidirectional and is governed by the laws of thermodynamics.

Energy enters the ecosystem through solar radiation. The existence of living world depends on the flow of energy and the circulation of materials through the ecosystem. Energy flow is a basic functional characteristic of the ecosystem.

Solar energy is fixed by the green plants or autotrophic component of the ecosystem as primary production. Out of solar energy reaching the plants, only 1% is used by the plants in photosynthesis. The plants convert the solar energy into chemical energy for their metabolic activities and for their growth.

The major part is used up in respiration. Some of the energy is not utilized and released as heat. The plants are eaten up the herbivores which are the primary consumers. The herbivores, in turn, become a food for carnivores. At each step of transfer of energy in food chain, a considerable amount of potential energy is lost as heat, till the available energy is so small that few organisms can be supported at that source alone. This limits the number of steps in a food chain to four or five. The longer the food chain, the less is the energy available to the final member.

1.     There is unidirectional energy flow.

2.     Energy is neither created nor destroyed but simply converted from one form to another. This is why we should call green plants converters or transducers.

3.     Energy transfer from one trophic level to another trophic level will not occur unless there is a loss of energy at each transfer

4.     Energy lost as heat cannot be used by plants photosynthesis.

5.     There is a continuous transfer of energy from on trophic level to another.

6.     The various pathways of loss is equivalent to and account for the total energy capture of the green plants i.e., gross production.

7.     The rest of the energy i.e, decomposition, non-utilized and herbivory is equivalent to net production.