ENERGY FLOW- Energy is the basic force responsible for all metabolic activities. The flow of energy from producer to top consumers is called energy flow which is unidirectional.
Energy flows through the trophic levels: from producers to subsequent trophic levels. There is a loss of some energy in the form of unusable heat at each trophic level.
The trophic level interaction involves three concepts namely :-
- Food Chain
- Food Web
- Ecological Pyramids
- FOOD CHAIN- A food chain starts with producers and ends with top carnivores. The sequence of eaten and being eaten, produces transfer of food energy and it is known as food chain.
Grazing food chain-The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.
This food chain begins from green plants at the base and the primary consumer is herbivore
For example, In terestrial ecosystem, grass is eaten up by caterpillar, which is eaten by lizard and lizard is eaten by snake.
In Aquatic ecosystem phytoplanktons (primary producers) is eaten by zoo planktons which is eaten by fishes and fishes are eaten by pelicans
Detritus food chain- The food chain starts from dead organic matter of decaying animals and plant bodies to the micro-organisms and then to detritus feeding organism called detrivores or decomposer and to other predators.
Litter —■Earthworms —■Chicken—■Hawk
Detritus food chain
The distinction between these two food chains is the source of energy for the first level consumers.
- FOOD WEB
“A food web illustrates, all possible transfers of energy and nutrients among the organisms in an ecosystem, whereas a food chain traces only one pathway of the food”.
- ECOLOGICAL PYRAMIDS
The steps of trophic levels expressed in a diagrammatic way are referred as
ecological pyramids.
The food producer forms the base of the pyramid and the top carnivore forms the tip. Other consumer trophic levels are in between.
The pyramid consists of a number of horizontal bars depicting specific trophic levels which are arranged sequentially from primary producer level through herbivore, carnivore onwards. 
The length of each bar represents the total number of individuals at each trophic level in an ecosystem.
The ecological pyramids are of three categories-
1.Pyramid of numbers,
2.Pyramid of biomass, and
3.Pyramid of energy or productivity
- Pyramid of Numbers
This deals with the relationship between the numbers of primary producers and consumers of different levels. Depending upon the size and biomass, the pyramid of numbers may not always be upright, and may even be completely inverted.
(a) Pyramid of numbers – upright
In this pyramid, the number of individuals is decreased from lower level to higher trophic level.
This type of pyramid can be seen in grassland ecosystem.
(b) Pyramid of numbers – inverted
In this pyramid, the number of individuals is increased from lower level to higher trophic level.
A count in a forest would have a small number of large producers, for e.g. few number of big trees. This is because the tree (primary producer) being
few in number and would represent the base of the pyramid and the dependent herbivores (Example – Birds) in the next higher trophic level and it is followed by parasites in the next trophic level. Hyper parasites being at higher trophic level represents higher in number.
A pyramid of numbers does not take into account the fact that the size of organisms being counted in each trophic level can vary
the pyramid of number does not completely define the trophic structure for an ecosystem.
- Pyramid of Biomass
In this approach individuals in each trophic level are weighed instead of being counted. This gives us a pyramid of biomass, i.e., the total dry weight of all organisms at each trophic level at a particular time.
Biomass is measured in g/m2.
(a) Upward -pyramid For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller trophic level perched on top
(b) Inverted pyramid-In contrast, in many aquatic ecosystems, the pyramid of biomass may assume an inverted form
- Pyramid of Energy
To compare the functional roles of the trophic levels in an ecosystem, an energy pyramid is most suitable.
An energy pyramid, reflects the laws of thermodynamics, with conversion of solar energy to chemical energy and heat energy at each trophic level and with loss of energy being depicted at each transfer to another trophic level.
Hence the pyramid is always upward, with a large energy base at the bottom.
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