Flow of energy in an ecosystem lakes place through the food chain and it is this energy which keeps the ecosystem going. The flow of energy through various trophic levels in an ecosystem can be explained with the help of various energy flow models.
- Single Channel Energy Flow Model: The flow of energy takes place in an unidirectional manner through a single channel of green plants or producers to herbvivores and carnivores. From the energy flow model shown in Figure 1, two things are clear :
(i) There is unidirectional flow of energy. The energy captured by autotrophs does not revert back to solar input but passes to herbivores; and that which passes to herbivores does not go back to the autotrophs but passes to consumers. Due to one way flow of energy, the system would collapse if the primary sources of energy (i.e., sun) were cut off.
(ii) At each trophic level, there occurs progressive decrease in energy. This is accounted largely by the energy lost as heal in metabolic reactions (respiration) coupled with unutilized energy.
Figure 2 depicts a simplified energy flow model of three trophic levels. One can clearly note that the energy flow is greatly decreased at each successive trophic level starling from producers (autotrophs) to herbivores and then to carnivores. In the Figure, boxes represent the trophic levels and pipes represent the energy flow in and out of each level. Working of both the laws of thermodynamics is clearly seen as energy inflows balance outflows at each trophic level (as per first law of thermodynamics) and energy transfer is accompanied by dissipation of energy into unavailable heat i.e., respiration as per the second law of thermodynamics. Thus, of the total 3,000 kcal of light falling upon green plants, 1,500 kcal (50%) is absorbed level (first trophic level). 1% (15 kcal) is converted at autotroph level (first trophic level). Thus, net production is mearly 15 kcal. Secondary productivity (shown as P2 and P3 in Figure 2) tends to be about 10% at successive consumer levels i.e., at herbivore level and carnivore level. As has earlier been mentioned, there is successive decrease in energy flow at successive trophic levels. Therefore, shorter the food chain, greater would be the available food energy.
- Y-Shaped or Double Channel Energy Flow Model.
Figure, 3 describes Y-shaped energy flow models as pioneered by H.T. Odum in 1956. This model shows a common boundary, light and heat flows as well as the import, export and storage of organic matter. Decomposers is placed in a separate box as a means of partially separating the grazing and detritus food chains. In terms of energy levels, decomposers are, in fact, a mixed group. The significant part in Y-shaped model is that the two food chains are not isolated from each other.
Y-shaped energy flow is more realisitc and practical than the single-channel energy flow model because of following points :
(i) It conforms to the basic stratified structure of ecosystems.
(ii) It separates the two chains i.e., grazing food chain and detritus food chain in both time and space.
(iii) Microconsumers (e.g.. bacteria, fungi) and the macroconsumers (animals) differ greatly in size-metabolism relations in two models.
- Universal Energy Flow Model.
E.P. Odum (1983) gave a generalized model by combining both single channel model and Y-shaped models which are both applicable to terrestrial and aquatic ecosystem and this combined model is known as Universal energy flow model. In this model I- Incident solar rays; A- Assimilated energy; P-net production; G-Growth; B-Biomass; R-Respiration; S-Stored energy; E-Excreted energy; NU-Unutilized energy.
This model can be used in two ways as:
- It can represent a species population in which case the appropriate energy inputs and links with other species would be shown as a conventional species oriented food levels and,
- The model can represent a discrete energy level in which case the biomass and energy channels represent many populations supported by the same energy source.
Fig. 3 : The relationship between flow of energy through the grazing food chain and detritus pathway (After H.T. Odum. 1956.